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					Main Linac                                                                                 1


6. Main Linac
Baseline: Modulator

To power the 10 MW MBK’s, the baseline choice is to use the bouncer-compensated pulse
transformer modulator that was developed initially by FNAL and industrialized by DESY
(with seven units built). Although the units perform well, they are expensive, require multi-
ton, oil-filled transformers, and are susceptible to single-point failures.

FNAL recently designed a lower-cost, more-reliable pulse-transformer modulator that has 5
ms pulse capability (desired by the Proton Driver project). Two units are being built and will
be commissioned in 2006. They use the latest generation of high power transistors for the
switches, which SLAC has designed and will provide for these units. In addition, DESY
continues to work with industry to improve the reliability of their modulator design in
preparation for the XFEL, which requires 35 units (these units will likely be run at about 80%
of the peak power level required for ILC).

Alternatives

The main alternative is a Marx-style generator, which is being developed at SLAC and
through the DOE-funded Small Business Innovative Research (SBIR) Program. It has a
modular design with built-in redundancy and should be easier to mass produce and repair
compared with the baseline modulator (in part, because it has no transformer and is forced-air
cooled). Preliminary cost estimates suggest that it may be up to 50% less expensive than the
baseline choice. The first full-scale prototype is expected to be tested near the end of 2006.
Other approaches being considered include a direct-switch (DTI will build a prototype
through SBIR funding) and a DC-to-DC converter (LANL is simulating operation of a higher
power version of their SNS design).

For these alternatives, if a TDR-like tunnel layout were adopted with the modulator separated
from the klystron by up to 2.5 km, the transport and impedance matching (cable to klystron)
of the 120 kV pulses would require further development. The Marx approach would be the
easiest to operate with an impedance mismatch as its turn-on can be ramped step-wise.

Baseline: Klystron

The 10 MW Multi-Beam Klystrons (MBK’s) being developed by Thales, CPI and Toshiba are
the baseline choice. The basic tube design appears to be robust while alternative approaches
have not been fully designed nor are currently funded to be developed. At worst, if the MBK’s
do not meet availability requirements, the commercial, single-beam, 5 MW tube from Thales
could be used (it has been the ‘work-horse’ for L-band testing at DESY and FNAL). Although
it is less efficient (42% vs 60-65%), this tube has been in service for over 30 years with good
availability.




Rev. July 13, 2006
Main Linac                                                                                 2

Alternatives

The three alternatives discussed were a 10 MW Sheet-Beam Klystron (proposed by SLAC to
reduce cost), a 5 MW Inductive Output Tube (proposed by CPI to improve efficiency) and a
10 MW, 12 beam MBK (proposed by KEK to reduce the modulator voltage, and the
modulator plus klystron cost).

Cavity Shapes

Overview

The cavity shape determines the fundamental mode as well as the higher order modes
properties. The aperture of cavity cells determines the cell-to-cell coupling in the fundamental
mode, the loss factor of wakefields, and the higher order mode propagation. There are several
options under consideration for ILC BCD and ACD. These options differ in terms of the
following cavity parameters,

       The ratio of the peak magnetic field to the accelerating gradient (Hpk/Eacc).
       The ratio of the peak electric field to the accelerating gradient (Epk/Eacc).
       The product of the geometry factor G and R/Q (G x R/Q).
       The cell-to-cell coupling factor (kc).
       The loss factors of longitudinal (k_l) and transverse (k_t) wakefields.
       The Lorentz detuning factor (K_L).

The choice of a specific shape has profound impact on the cavity performance, beam quality,
beam stability and manufacturability. The mature TESLA shape has a favorable low Epk/Eacc,
acceptable cell-to-cell coupling and wakefield loss factors. It has lower risk of field emission
and dark current.

Two major new shapes, the Cornell re-entrant shape and the DESY/KEK low-loss shape, are
under initial developments. Both new shapes have a lower Hpk/Eacc and a higher G x R/Q.
They have a higher ultimate gradient reach since Hpk is the fundamental limit, and lower
cryogenic losses. Both shapes carry higher risk of field emission and dark current since
Epk/Eacc are 20% higher than the TESLA shape. The iris aperture is a major geometrical
difference between the two new shapes. The DESY/KEK low-loss shape has a smaller iris
aperture by about 15%, whereas the Cornell re-entrant shape has the same aperture as that of
the TESLA shape.

The ‘superstructure’ concept improves the cavity packing fraction in the cryomodule by a few
per cent and reduces the number of input couplers by a factor of two. The idea can be applied
to any of the cavity shapes besides TESLA.

Baseline: TESLA Shape

Description

The TESLA cavity is the benchmark that all other designs must be compared to.

Pros:
Rev. July 13, 2006
Main Linac                                                                                 3

       It has low Epk/Eacc, large cell-to-cell coupling and small wakefields.
       It has been studied and tested extensively:
            o Single-cell cavities achieved up to 43 MV/m
            o It has achieved ~ 35MV/m gradient at Q = 10^10 for a number of multi-cell
                 cavities, best achieved 40 MV/m at 10^10.
            o High power tests were done as well as a test of one cavity in a module with
                 beam.
            o Over 100 cavities have been constructed. Several companies have fabricated
                 successful cavities showing that the procedures are well understood.
       Wake fields and HOM damping have been thoroughly investigated.
       Cavities and modules have operated in TTF for considerable time.
       Cavity Data base, processing and test history is extensive.
       The cost basis for limited quantity is well established and a number of vendors have
        produced these cavities.
       Industrialization studies of fabrication and processing have been carried out.
       These cavities are planned for the XFEL.
       These cavities are closest to meeting ILC requirements at this time.

Pro & Con:

35MV/m is close to the ultimate gradient (42MV/m ?) for this cavity shape. This points to the
maturity of the R&D program.

Cons:

       This shape has a higher Hpk/Eacc (than the alternatives under development) and a
        higher risk of premature quench induced by a higher surface magnetic field for
        gradients > 40 MV/m.
       These cavities do not have the ultimate gradient potential that some of the other ACD
        designs have. However other designs reduce Hpk/Eacc at the expense of Epk and/or
        iris diameter. These new shape cavities have not yet been proven as a module
        operating at 35MV/m with a beam.

Required R&D

Most critical R&D is the establishment of proof of principle of 35MV/m modules with
acceptable dark currents, and long-term operation at 35 MV/m with acceptable trip rates.

Topics relevant to cavities listed under R1 to R4 in the TRC should be addressed with existing
or planned test facilities.

Further work is necessary to establish an adequate gradient safety margin and to get
reproducible high gradient results from cavity to cavity, and from processing to processing.

There needs to be a measurement cryogenic power deposited by HOMs to be sure that this is
less than 20% as required to keep the overall cryo load under control.

Potential Mods to BCD

Rev. July 13, 2006
Main Linac                                                                                 4


A number of minor modifications and improvements could be implemented without impact to
the basic cavity design. These include:

       Slight modifications to the HOM coupler, and pickup design for ease of fabrication,
        fundamental power rejection, and thermal stability.
       Design modification to the helium vessel end walls for more strength.
       Shortening the beam tube lengths to their acceptable minimum to improve the packing
        fraction.
       Review of the overall mechanical design, including flanges, sealing gaskets, number
        of nuts and bolts, and end group fabrication, with an eye toward industrial production.

Technical advantages, increased tech potential:

Savings in cavity length (and interconnect) will shorten the tunnel required. HOMs would
have better power margin.

Potential cost impacts:

If a 5% cavity slot length reduction could be realized, this would impact the tunnel length and
cost (but probably less than or ~ 1% of total cost.) Greatest cost impact is probably in the
design for industrial production if good ideas emerge.

Risk and Reliability impacts:

Better design with more margins should decrease risk and improve reliability. This is
especially true if a reliable and simple flange design (or weld) can be developed.

Cost Estimation

Lattice Files

Parameter Tables

Supporting Documentation

Original Snowmass document by R. Geng

TESLA TDR Cavity Chapter

P. Schmueser et al.; The Superconducting TESLA Cavities; PRST-AB 3 (9) 092001

Alternatives

    1. A number of different cavity shapes are being proposed. These shapes tend to decrease
       Hpk/Eacc (Pro) and increase G x R/Q (Pro), but increase Epk/Eacc and may have
       smaller iris diameters (Cons). The most work to date has been done on the
       DESY/KEK low-loss.

Rev. July 13, 2006
Main Linac                                                                                   5

    2. The ’superstructure’ concept improves the packing fraction and reduces the number of
       couplers. The idea can be applied to any of the cavity shapes.

Alternative cavity Shapes

Technical advantages, increased tech potential

       Reduced risk of premature quench due to lower Hpk/Eacc for gradient > 35 MV/m.
       Higher gradient possibly up to 45-50 MV/m.
       Has higher G x R/Q so lower cryogenic power loss.
       Needs shorter tunnels.
       Gradient improvement could be used for operating margin.

Risk and Reliability impacts

       Has higher Epk/Eacc.
       Dark current (exponential with Epk) may be a greater problem.
       Operating at higher gradient implies greater reliability issues, and greater risk,
        especially during commissioning and early operation.

DESY/KEK low-loss shape

Pros:

       Most work done to date, several single cells tested, some multicells fabricated.
       Successful tests of 45-47 MV/m with 1.3 GHz single cell cavities
           o (45 MV/m with 2.2 GHz single cell cavity).
       Computational analysis of wakefields underway.
       Test of 9 cell cavity underway.
       Lorentz force detuning analysis underway.

Cons:

       Has smaller iris than TESLA.
       May have less mechanical strength.
       Manufacturabillity (danger of making it re-entrant)
       Needs much development and testing to reach maturity of TESLA.

Cornell Re-entrant shape

Pros:

       Has expectation of higher gradient with TESLA like iris diameter.
       Single cells at 1.3 GHz tested up to 52 MV/m.
       Successful HPR with single cell done to give record Epk (> 100 MV/m).
       First order HOM analysis of multicells complete.
       Lorentz force detuning analysis underway.
       9 cell cavity fabrication underway.

Rev. July 13, 2006
Main Linac                                                                                  6

Cons:

       Weaker mechanical strength.
       HPR is problematic because of re-entrant design. This is a critical complication, as the
        Epeak/Eacc ratio is also enhanced.
       Needs much development and testing to reach maturity of TESLA.

Modifications/ variants of re-entrant:

       smaller aperture re-entrant
       half-re-entrant.

Required R&D

Considerable R&D will be required and different check points:

       Wake fields:
           o The allowed iris diameter must be specified from theoretical analysis. This is a
               trade off between allowable emittance growth (luminosity) and cost.
           o Complete wake field analysis must be carried out computationally and checked
               with measurements.
           o Cold tests of wake fields must be carried out on two or more adjacent cavities.
           o Wake fields must be checked in modules with beam.
       Gradient and Q:
           o Gradient and Q expectations up to at least 35MV/m must be achieved first in
               9-cell cavity tests then in modules with beam.
       Time scales for R&D
           o Full program to bring one of these cavity ideas to the state of understanding of
               the TESLA cavity may be of order several years with substantial funding.
           o The rules for when the ACD would be considered to replace the present BCD
               should be proposed. Such a point might be when ~6 cavities have achieved
               gradients in excess of 35MV/m with Q >10^10, and when HOM damping has
               been checked in at least a two-cavity (9-cells each) string without beam.

Superstructure

Pros:

       Superstructure has possibilities for significant cost savings through the use of only one
        input coupler per two cavities.
       Significant design work has been carried out.
       A two superstructure module (with two pairs of 7-cell structures) has been tested with
        beam at DESY.
       Wake fields have been investigated and the mode analysis understood.

Cons:




Rev. July 13, 2006
Main Linac                                                                                 7

       A main drawback of the superstructure is how to process and test such a long assembly,
        either with BCP or EP processing. This would take significant infrastructure
        development beyond that needed for single cavity structures.
       Alternatively a superconducting joint might be developed to join the superstructure
        pair after processing. This has been attempted recently at DESY without success. Jlab
        has a program to continue superconducting joint work.

Technical advantages, increased tech potential

       The main technical advantage would be the reduction in the number of input couplers
        by a factor of two (elimination of 8000 couplers). These couplers would need to carry
        double power.
       Wake fields are less

Potential cost impacts

The cost saving might be the cost of 1/2 the couplers. However if coupler fabrication cost is
reduced significantly then the impact would be less.

Supporting Documentation

Original Snowmass document by R. Geng

Superconducting superstructure for the TESLA collider: A concept, J. Sekutowicz et al., Phys.
Rev. ST Accel. Beams, 2, 062001 (1999)

Test of two Nb superstructure prototypes, J. Sekutowicz et al., Phys. Rev. ST Accel. Beams 7,
012002 (2004)

Cavity Materials

Overview

Specifications for high purity niobium (RRR) sheet, used for fabrication of cavity cells and
auxiliary components such as HOM- and FP-couplers and beam pipes have been developed
over the years. Material produced to these specifications by various companies in Japan
(Tokyo Denkai), Germany (W.C.Heraeus) and the US (Wah Chang) and used for cavity
fabrication has resulted in high performance prototype and production cavities, when
combined with appropriate QA measures during sheet production such as e.g. clean rollers or
eddy current or squid scanning for defects prior to deep drawing of cavity cells.

Recently, single cell and multi-cell cavities have been produced at JLab from large grain ingot
material or from single crystal, cut directly from the billet by either wire EDM or saw cutting.
This is an exciting new development, and has the potential of simplifying the production
sequence and consequently the cost. Initial experience indicates that very smooth surfaces can
be obtained with the single crystal material or even the large grain material using the BCP
(chemical) etch process only, thus avoiding the necessity for using the more complex electro-
polishing (EP) processing. This might be related to less defects, a reduced intrinsic strain in
the single crystal material and a significantly reduced number of grain boundaries.
Rev. July 13, 2006
Main Linac                                                                                 8

Nb/Cu laminated material has been successfully used to produce high gradient single cell
cavities at DESY and KEK from Nb/Cu tubes by hydro-forming. Explosion bonding, HIP
bonding, back-extrusion and hot rolling techniques were successfully used to produce the
composite tubes. The laminated Nb/Cu approach takes advantages of the bulk Nb
performance (Nb layer ~ 0.5 mm thick) combined with the increased thermal conductivity and
stiffness of the copper backing resulting in possible significant cost savings. Welding presents
a difficulty in that the Cu must be removed at the weld joints before e-beam welding and that
there are risks of contamination/leaky joints. This material is probably best suited when used
with hydro-formed multi cell assemblies.

Low frequency (< 500 MHz), lower gradient cavities historically use a thin layer of Nb
deposited on Cu. Cavities made from deposited Nb suffer from very strong Q slope and do not
appear suitable for high gradient (>15 MV/m), high Q ILC application. Research continues
with different deposition techniques (e.g. vacuum- arc and ecr plasma-deposition) to try to
understand and improve the SRF properties.

Other types of superconductors, such as Nb3Sn, NbN and MgB2 are experimental and
material property evaluation stage, and far from being useful for project application. There are
also fundamental questions related to the limiting RF field and its dependence on kappa in
these high kappa materials.

Options under consideration

       Nb RRR fine grain sheet
       Nb single crystal or large grain
       Nb/Cu lamination
       Nb deposited on Cu
       Other superconductor

Please note that the second part of the original Snowmass document by P. Kneisel contains
some general R&D topics which are not solely related to one of the technologies described
here. Please see also section Fundamental Material R&D.

Baseline

Nb RRR fine grain sheet

Description

Pro:

       Specifications exist and are - with some exceptions - met by industry.
       This material is best known. Measurements of the thermal conductivity, Kapitza
        resistance (under different surface conditions), mechanical properties and mechanical
        anisotropy, texture and formability have been published. Post purification with Ti at
        different temperatures and durations has been studied.
       Many examples of high performance cavities (single+multi-cell) made from this
        material exist.
       Studies of possible cost-savings in a mass-production scheme were performed.
Rev. July 13, 2006
Main Linac                                                                                   9

Con:

       For accelerating gradients > 28 MV/m EP for final surface treatment is necessary to
        give a smooth surface finish, even though there exist examples of cavity performances
        beyond this level after BCP treatment (rougher surface) only. In any case, ‘in situ’
        baking is necessary to remove the ‘Q-drop’, typically starting at gradients > 22MV/m,
        corresponding to peak magnetic surface fields > 80 mT (G.Ciovati,SRF2005). In-situ
        baking seems to be more effective in electropolished cavities to remove the Q-slope at
        high fields.
       At this time it is statistically unclear, if titanization at 1200°C-1400°C is required for
        best performance of this material. The titanization with subsequent etching increases
        the RRR from ~300 to ~600, providing better thermal stability of the material.
        However, the mechanical properties degrade significantly (yielding!) and the process
        adds cost.
       The process of producing sheet from ingot material is inherently expensive because of
        rolling, cleaning and annealing steps and loss of material (edges, etc.) Also, in
        comparison to large grain ingot material the sheet manufacturing process appears more
        prone to introduction of defects. Reproducibility of mechanical properties from sheet
        to sheet is still an issue although the process is well advanced. The issue of skin rolls,
        affecting texture and micro-structure still needs to be addressed. Other related issues
        are microyielding (Myneni, SRF2005), spring back (half cells formed from different
        sheets/heats of material end up with different frequencies), grain size distribution and
        texture. Interesting work is ongoing on Equal Channel Angular Extrusion (ECAE),
        consisting of extruding the niobium through an angled, narrow channel. ECAE
        promises to produce even smaller grain size, better uniformity and better formability.
        It is unknown, whether this process introduces unwanted impurities.

Potential Mods to BCD with impact (tech, cost, difficulty/time scale)

       The effect of impurities in the Nb is being investigated (e.g. Tantalum). It may be
        possible to relax specifications on impurity content without compromising the cavity
        performance. Such a possibility is indicated by recent prototype cavities made from
        higher Ta content material at JLab (800 and 1500 wt ppm), which reached high fields
        of 34 – 36 MV/m.
       Understanding and optimizing the industrial production process, e.g. number of melts
        to reach the specified RRR/impurity content, should lead to high quality material at a
        cost savings. ECAE and other procedural steps, for instance, could yield material with
        better formability. Benefits from this research are also expected for the alternate cavity
        fabrication technologies, such as for hydro-forming.

Required R&D

       The impact of the impurity content on the cavity performance (e.g. Tantalum) should
        be studied further; the starting point of the existing study (JLab, DESY, Reference
        Metals) was the claim of cost reduction benefits by the participating material supplier.
        It is not clear how many melts are needed to achieve the specified RRR value;
        however, the impurity level of interstitial impurities such as H,C,N,O affect RRR
        significantly and possibly improved vacuum conditions during EBM would reduce the

Rev. July 13, 2006
Main Linac                                                                                   10

        number of melts and therefore cost. In this context new measurements of the content
        of light as well as heavy impurities in bulk and surface (and their effect on RRR) need
        to be conducted. This work would also produce improved specifications for the fine
        grain Nb sheet.
       The mechanical properties of fine grain rolled sheet material need further exploration
        to study: best crystal orientation for forming, Reproducibility, Grain size control, Yield
        strength, Spring back, Texture, Equal Angle Extrusion, best grain size. This is
        especially important for alternate production technologies such as hydroforming.
       Current sheet quality control measures (eddy current scanning, optical inspection)
        allow detection of ~100 um size defects. Thermal model calculations indicate that
        detection of much smaller normal inclusions is needed to guarantee ultimate cavity
        performance for the specified RRR-value of > 300. DESY, the University of Giessen,
        Heraeus, Amac Int. and Fermilab are developing a Squid Scanner aiming at 10 um
        resolution
       Better understanding of different surface treatments, i.e. chemical polishing and
        electro-polishing on the cavity performance, surface roughness, oxide thickness and
        composition in sheet material is also a R&D priority (this has at various levels been
        done in the past with little success and is again at various levels going on now, e.g.
        CARE for EP, L.Philipps - SRF2005, Cr.Boffo SRF2005, Geng SRF2005)
       Need to establish best value of RRR needed; high performance cavities have been
        made with Nb from RRR = 200 – 400. Nominal specification is 300.

Cost Estimation

       Potential cost savings in Nb are currently expected from the single-crystal (or large
        grain) or the Nb/Cu laminate approaches. Another possible avenue of cost reduction is
        via a relaxation of the impurity content specifications, in particular for Tantalum,
        provided Eacc > 35 MV/m can be reliably reached.
       Cost savings can also be generated as a result of mass-production for an ILC size
        project (back-flow of ‘scrap-material’, economy of scale). The cost impact of large
        production quantities needs to be better understood, however.

Parameter Tables

Supporting Documentation

Original Snowmass document by P. Kneisel

TESLA TDR Cavity Chapter

P. Schmueser et al.; The Superconducting TESLA Cavities; PRST-AB 3 (9) 092001

Alternatives

    1. The most exciting new idea is the large grain/single crystal material, (started at Jlab)
       mainly because it opens the possibility of ’streamlining’ the procedures at comparable
       performance, which could result in significant cost savings.



Rev. July 13, 2006
Main Linac                                                                                   11

    2. Nb/Cu laminate has the potential for significant cost savings. Single cell cavities have
       been made with good performance.

Single crystal and large grain

Description

Jlab has made single cell 1.3 – 1.5 GHz cavities with large grain Nb from various suppliers
reaching Eacc = 34 - 36 MV/m. JLab has purchased and received a 500 kg ingot from CBMM
with Ta =800 ppm and will fabricate 2 TESLA cavities from this material after initial
qualification with single cells (the material is sufficient for ~ 16 nine-cell cavities). DESY and
Cornell have started production of single cell cavities from various large grain material
suppliers. Fermilab is currently ordering such material. Wah Chang advertised to offer large
grain/single ingots in the near future.

Pro:

Single crystal or large grain material promises the following potential advantages:

       Reduced costs
       Technical advantages may lead to simplification of fabrication and processing.
       Consistent gradient/Q results
       Possibly lower residual resistances would also lead to cost savings.
       Very smooth surfaces are achieved with BCP, a potential elimination of EP would
        simplify production
       Final cleaning of smoother surfaces might be more effective which may lead to less
        dark current
       Good or better mechanical performance than fine grain material (e.g. predictable
        spring back)
       Less material QA (eddy current/squid scanning) if proven by scanning a large number
        of sheets.

Con

       As a relatively new but exciting development, little experience exists at present. 9-cell
        cavities with stiffeners and couplers need to be made and tested.
       Technology to provide large single crystals needs to be developed (it is the preferred
        option to use single grain material)
       Large grain material with a few crystals might be acceptable.




Required R&D

       Initial R&D is underway at Jlab, DESY, Cornell and Fermilab using material from
        various suppliers

Rev. July 13, 2006
Main Linac                                                                                      12

         Fast, inexpensive cutting techniques need to be identified and tested. The presently
          preferred wire EDM method is too slow.
         The dependence of mechanical, etching and oxidation properties with crystal
          orientation need to be better understood.
         Important topics are: acceptable yield strength of material cut directly from ingot,
          uniformity during forming of half cells, slippage of grains during forming, vacuum
          leaks through grain boundaries, grain boundary problems during EBW.
         Appropriate acid agitation during BCP needs to be developed to achieve smooth
          surfaces and uniform material removal.
         Specifications need to be developed and the first material received from the different
          vendors needs to be qualified.
         The theoretical and experimental investigation of the effects of grain boundaries need
          to be pursued further [University of Wisconsin].

NIST has applied for a grant to the NRC to investigate the forming process on large grain and
single crystal material.

Supporting Documentation

Original Snowmass document by P. Kneisel

Laminated Nb/Cu

Description

The laminated Nb/Cu approach takes advantages of the bulk Nb performance (Nb layer ~ 0.5
mm thick) combined with the increased thermal conductivity and stiffness of the copper
backing resulting in possible significant cost savings. DESY has demonstrated the technology
for multi-cell (3-cell) cavities at the laboratory level.

Pro

         The Nb/Cu laminate approach promises cost reduction because of reduced amount of
          Nb per cavity.
         Gradients comparable to the best bulk niobium cavities have been achieved with
          prototypes.

Better thermal stabilization as a result of the copper backing.

         Stiffening against Lorentz-forces can be obtained without significant performance and
          cost penalty by increasing the thickness of Cu layer. In particular, the stiffening can be
          varied cell-to-cell.
         Seamless fabrication technique (hydro-forming) allows the elimination of equator
          welds in the high magnetic field region of the cavities.
         Most processes used for treatment of bulk niobium cavities (except for post-
          purification with titanium) are applicable.

Cons

Rev. July 13, 2006
Main Linac                                                                                 13

       A first pass production technique was developed at DESY and subsequently at KEK.
        Further refinement of the technique might be needed, especially in the bonding
        process between Nb and Copper (explosive vs hot rolling vs HIP bonding).
       The e-beam welding requires cutting away the copper to make a pure Nb weld. There
        is a risk of contaminated welds, which might leak because of cracks in the weld. Also,
        local RRR reduction typically ensues following the welding.
       Cu/Nb laminate cavities still quench, despite additional thermal stabilization.
       Thermo-currents introduced during the quenches (or during processing in a barrier)
        lead to frozen-in flux, which lowers the Q-value.
       Cool-down has to be very uniform over the cavity volume, because thermocurrents/
        frozen-in flux will destroy the Q-value.
       The presently used methods of cooling down cryo-modules will most likely not work
        with the Nb/Cu composite material.
       This technology is probably not applicable with single crystal unless one would apply
        standard fabrication techniques (half cell forming from composite sheets and welding).
       Cracks appear in iris area during fabrication, when heat treating below the
        recrystallization temperature of the Nb. Heat treatment at higher temperatures causes
        softening of the copper. Doping of Cu with Zr was tested. Intermediate temperature
        heat treatments were not sufficiently explored. More effort is required to get the tube-
        material to state where it can be transferred to industry.
       Industrialization process has not been started yet.

Required R&D

A number of choices need to be made:

       Choice of bonding method: explosion bonding (DESY), hot rolling (KEK-Nippon
        Steel Co.), HIP, back extrusion (DESY);
       Fabrication techniques: e beam, hydro-form, Lorentz force stiffeners;
       How the end groups are handled: composite material (lots of EBW)?, solid niobium?
        Sputtered Nb on copper with end groups flanged onto cell structure? Superconducting
        joint?
       Can such composite cavities be ‘pipe-cooled’, which would change the whole cryo-
        concept?
       Complete cavities with end groups need to be fabricated and tested;
       Cavities must be placed in modules and tested;
       How the cavity is made rigid against Lorentz force must be developed in detail
        (varying the Cu thickness, for example);
       A demonstration of this technology on complete 9-cell cavities with end groups is
        necessary!

Supporting Documentation

Original Snowmass document by P. Kneisel

Other Superconductors

Films and other SC considered not feasible at this time for LC project.

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Fundamental Materials R&D

Overview

Over the last decade a set of procedures has been developed for the fabrication, surface
treatment and assembly of superconducting niobium cavities, which lead to high performance
cavities, if applied properly. These procedures include improved material QA by scanning for
defects, extensive QA during cavity fabrication (e.g. cleanliness of weld joints), appropriate
amount of material removal prior to heat treatments at 600 -800°C for hydrogen removal (to
avoid ‘Q –disease’), BCP and EP, high pressure ultra-pure water rinsing (HPR) for extended
periods of time, clean room assembly, ‘in situ’ baking and finally acceptance testing. The
application of these procedures has in many cases now led to performance levels which
approach the ultimate limits of the material.

Nevertheless, the underlying physics is in some areas not well understood and a fundamental
material R&D program should be aimed at clarifying the physical phenomena and aid to
optimize the processes. In the context of a project of the scale of the ILC improved
understanding of the processes is likely to benefit performance and cost.

Fundamental materials R&D distinguished from R&D in direct support of the project- as
discussed here - has to do primarily with the basics of rf superconductivity, such as the
theoretical rf critical magnetic field, the loss mechanisms at high field, the nature of the RF
surface ,including the Nb-oxide interface and surface contamination.

The two key performance criteria are Gradient and quality factor Q. The main performance
limitations in today’s bulk Nb cavities are:

       Limitation I: Ultimate cavity gradients will be limited by the theoretical RF critical
        magnetic field. The prevailing theory claims this is the superheating field, but its value
        for Nb is still under debate. Experimental data suggests the magnetic field limit to be
        around 185 mT.
       Limitation II: Field emission at high surface electric fields. For the new shapes
        (discussed in the Cavity Shapes section) the surface electric fields exceed 100 MV/m
        for Eacc > 45 MV/m. Therefore, major efforts have to go into control of contamination,
        the development of clean processing and assembly procedures- especially in complex
        assemblies such as cavity strings and cryomodules- and the prevention of re-
        contamination.
       Limitation III: defects in the material, which limit the achievable fields to values
        HRF < Hcrit. Scanning methods need to be enhanced to eliminate defects as small as
        10 um as suggested by thermal model calculations.
       Limitation IV: At gradients > 20 - 25 MV/m a strong degradation of the Q-value (‘Q-
        drop’), which significantly increases the cryogenic losses and limits the achievable
        gradients due to heating. This Q-drop can be eliminated/reduced by ‘in-situ’ baking at
        a temperature of ~ 120°C for a duration >12 hrs. Smoother surfaces (e.g. electro-
        polished ) give more significant improvements.
       Limitation V: Residual resistances of a few nOhm have been achieved, but not on a
        regular basis. Low residual resistances (<3 nOhm) would allow to take advantage of
        the decrease of the surface resistance with decreasing temperature and an accelerator


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        such as ILC could be operated at e.g. 1.8 K, reducing the cryogenic load and the
        operation costs.

R&D investigations (ongoing or under consideration)

The overarching goals of fundamental R&D should be to understand and thereby remove
performance limits, ultimately leading to cost reduction.

       Theoretical studies on the RF critical magnetic field
       Measurements of the RF critical field
       Preparation of field emission free surfaces
       Studies to reduce field emission including emitter processing.
       Improvements in scanning methods to pre-screen defects
       Theoretical and experimental studies on the high field Q-slope and its reduction by
        baking.
       Theoretical and experimental studies on the medium field Q-slope.
       Surface analytical studies of niobium using state-of-the-art instrumentation such as
        XPS, Auger, SIMS, 3DAP and others.
       Studies to delineate the role of impurities (e.g. O, N, C, H, Ta…) on the
        superconducting RF performance of Nb
       Studies to delineate the role of grain boundaries and other mechanical imperfections
        on the superconducting properties of niobium.
       Studies to improve understanding of chemical treatment processes such as
        electropolishing.
       Basic studies (similar to those describe above) on large grain and single grain niobium.

Cavity Fabrication

Overview

The accelerating structures for ILC will need to be fabricated from well-controlled materials
according to established and well-controlled methods. The scope of fabrication starts from the
receipt of starting materials and ends with the completion of tuned structures meeting
specified mechanical configuration criteria. It is critical that the applied methods yield a
consistent ‘defect-free’ interior rf surface. The fabrication steps must not add defects to the rf
surface. The options available vary somewhat with the principal starting material.

The materials options appear to be bulk Nb fine-grained, bulk Nb large-grain, bulk Nb/Cu, or,
perhaps longer term, even thin film Nb on Cu. It may be possible to have combinations in the
future.

Since structure design is treated separately, one may note that fabrication R&D bears only
weakly on couplings to other design elements. Therefore, competitive pressure to develop
lower cost methods of providing the chosen structure may proceed until construction start.

A serious industrialization study has only been made for the current ’standard’ fabrication
method (1.a.). More similar studies may be necessary.

Principal opportunities for cost savings:
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       Reduce material costs by reducing the amount of required Nb, substituting cheaper
        material where possible.
       Reduce the number of components and steps.
       Reduce the ‘hands-on’ time for each cavity.
       Develop methods with low-effort based QA

Options under consideration

    1. Bulk fine-grained niobium from sheet, bar, and plate stock
          1. Machining, deep draw forming, mechanical polishing, pre-cleaning chemistry,
              EBW, inelastic deformation tuning with appropriate intermediate cleaning
              steps
          2. Similar to 1.I., but substituting spinning for the deep draw forming and the
              EBW of cells.
          3. Similar to 1.I., but substituting hydroforming for the deep draw forming and
              the EBW of cells.
          4. Similar to 1.I. for the cavity cells, but treating the fabricating the endgroups
              differently and using Nb film on copper for these low-field parts.
    2. Bulk large or single-grain niobium direct from ingot
          1. Similar to 1.I., but using wire EDM to form sheets for cell blanks Evidence to
              date suggests that the balance of fabrication is not significantly changed with
              respect to ‘standard’ fine-grained Nb material.
    3. Bulk Nb/Cu clad material for cells (and beamtubes?)
          1. Similar to 1.III. - hydroforming

BCD choice

1. Present ‘standard’ fabrication methods, applied with serious attention to QA. i.e.

    1. Bulk fine-grained niobium from sheet, bar, and plate stock
    2. Machining, deep draw forming, mechanical polishing, pre-cleaning, EBW, inelastic
       deformation tuning with appropriate intermediate cleaning steps

(2. is not significantly different in fabrication methods but several complete cavities have to
be made with all features and tested. Yield strength of ingot material may be significantly
lower than 1.I, affecting final rigidity of finished structures)

Description

Pros

       There exists a strong experience basis for describing and implementing appropriate
        fabrication methods for bulk sheet Nb cavities. ~1000 such cavities exist.
       Costs are well understood from low-volume production runs.
       Serious production analyses exist for high-volume scale-up of the ’standard’
        fabrication methods.

Cons

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       ’Touch’ labor is relatively high, with piece part handling, cleaning, and inspection
        required for EBW steps, many nuts and bolts in flanged connections to keep clean.
       The endgroups require as much fabrication attention as the cells. This seems less
        justified for the cost.
       Mass production analysis for TESLA-type cavities showed 77% of residual fabrication
        cost in machining operations.

Required R&D

       Reduce material costs by reducing the amount of required Nb, substituting cheaper
        material where possible.
       Reduce the number of components and steps.
       Reduce the ’hands-on’ time for each cavity.

Supporting Documentation

Original Snowmass document by C. Reece

TESLA TDR Cavity Chapter

P. Schmueser et al.; The Superconducting TESLA Cavities; PRST-AB 3 (9) 092001

Alternatives

Overview

Motivations for considering alternate fabrication choices are almost exclusively related to cost.
No presently considered alternative methods claim to directly improve ultimate performance.

Cost reductions may result e.g. from methods which reduce the cost of required material or
aid the automation of fabrication. Thus the hydroforming, spinning, and film coating might be
considered. Hydroforming and spinning offer the prospect of seamlessly forming the cavity
cells, while eliminating several machining, chemical cleaning, and EBW steps.

Hydroforming of cell structure

Pros

       Technique quite suitable for factory production with automation and reduced total
        fabrication costs.
       The technique can produce equally performing Nb cavities. (One-cell cavities: 42
        MV/m and Q - value ~ 1010 test cavity without EP) (KEK & DESY)
       3-cell structures have been built.
       If applied to Nb/Cu clad tubing, the quantity of required high-purity Nb for cells could
        be reduced by 75%.
       Highly consistent interior cell geometry expected, thus less required tuning.
       Avoidance of machining and welding in the high field regions of the cavity eliminates
        some potential sources of defects which could degrade ultimate cavity performance.

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Cons

       Less experience translates into less awareness of subtle difficulties.
       With Nb/Cu, must manage the end transitions, e.g. Cu removal to avoid Nb weld
        contamination.
       Tube production needs development, this is underway at INFN
       Must assure tubing QA for consistent forming properties.

Potential cost impacts

       Elimination of multiple machining steps and expensive EBW time.
       Expected time required to form 9-cell structure from tube: ~6-8 hours
       If applied to Nb/Cu clad tubing, would reduce quantity of required Nb.
       Needs serious study to determine cost benefit

Risk and Reliability impacts

       Impact on production yield is unknown.
       Delamination of Nb-Cu from stresses due to multiple thermal cycles.

R&D

       Work is proceeding under JRA1 – CARE program.
       Fabrication of seamless bulk Nb tubes of the length sufficient for 9 cell cavity from
        one piece (ca. 1.8 m long)
       Development of ‘industrial’ production routine and qualification of cavities.
       How to avoid or suppress the trapping of magnetic flux caused by thermo - coupling
        effect in Nb/Cu cavities?
       New methods of bimetallic tube fabrication
       More appropriate material for Nb clad cavities instead Cu (Cu alloys etc.)?
       End group cost reduction for Nb/Cu clad cavities
       Seamless cavity of new shapes (low losses, re-entrant etc.)

Time scales for R&D

       Bulk Nb “seamless” 9 cell TESLA shape cavity (e beam weld three 3-cell units)
        suitable for installation in the cryomodule (2006).
       New machines (capital investment) needed for 9-cell hydroforming
       Multi cell NbCu clad cavities from special copper without Cu layer inside (2006)??

Spinning

Pros

       Technique quite suitable for factory production with automation and reduced total
        fabrication costs.
       The technique can produce equally performing Nb cavities up to 40 MV/m in single-
        cell test cavities (INFN Legnaro)
       9-cell structures have been built, awaiting test.
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Cons

       Nb tube production needs development
       Must assure tubing QA for consistent forming properties.
       Multi-cell performance not yet demonstrated
       Substantial inside surface grinding required to remove fissures. This may be reduced
        by 800 C annealing before spinning tube.
       Less experience translates into less awareness of subtle difficulties.

Technical advantages, increased tech potential

       Avoidance of machining and welding in the high field regions of the cavity eliminates
        some potential sources of defects which could degrade ultimate cavity performance.
       Limited experience makes evaluation difficult.

Potential cost impacts

       Elimination of multiple machining steps and expensive EBW time.
       Expected time required to form 9-cell structure from Nb tube: 4 hours in mass
        production (INFN)
       Estimated potential fabrication cost reduction: (Needs Industrial study.)
       Possibility of spinning thicker segments for stiffening against Lorentz-Force detuning

Risk and Reliability impacts

       Impact on production yield is unknown.
       Substantially more material removal is necessary if fissures are present
       Wall thickness uniformity

R&D

       Work is proceeding under JRA1 – CARE program.
       Production of Nb tubing for multi-cells needs development
       Development of industrial production routine with multiple cavities and RF
        qualification of those cavities.

Nb films on Cu for endgroups

Pros

       Potential for reduced cost by using Nb film coating in low-field regions.
       Improved thermal conduction in portions of accelerating structure outside of the
        helium vessel.
       If successfully made demountable, (as proposed by KEK) could maximize QA of high
        field region of cells by providing opportunities for restructuring inspection, cleaning,
        and assembly sequence.
       Successful SC flanging would create a new type of modularity that could be exploited.

Cons
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       Complexity of endgroup shape makes confident coating difficult.
       Added complexity due to material interfaces at joints etc.
       Still at very early conceptual stage
       Lack of experience base leaves potential problems unrecognized.

Potential cost impacts

       Material cost could be reduced
       Greatly reduced need for EBW steps if Cu endgroups can be formed as a brazed
        assembly, followed by Nb film coating and endgroups are flanged to seamless cell
        structure.
       Studies needed to estimate cost impact

Risk and Reliability impacts

       Impact on production yield is unknown.
       Film adhesion problems in complex geometry

R&D

       Develop Nb coating of HOM coupler and end group assembly.
       Develop low-profile reliable superconducting flange joint for use just outside of
        helium vessel.
       Develop less complex HOM damping scheme for easier fabrication and coating.
       Industrial cost study: Is the undeveloped Nb coating cheaper in production than
        endgroups of BCD? Is the yield sufficiently high?

Cavity Preparation

Overview

The preparation of the cavities should finally result in fully assembled cavities (incl. power
coupler) which are ready for string assembly. After delivery from the welder, there are several
important steps:

    1. Leak check, mechanical checks, inspection
    2. Frequency tuning, field flatness
    3. Cleaning
    4. Damage layer removal
    5. Furnace treatments
    6. Final frequency tuning, field flatness
    7. Final surface preparation
    8. Final cleaning
    9. Bake-out at 120-130°C
    10. Low-power acceptance test
    11. Tank-welding
    12. Assembly for high power operation
    13. High-power test


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All these steps need improvements in QA/QC for mass production. The most challenging one
is to define final (electro-)chemical surface preparation to deliver reliable and reproducible
performance. Today, highest cavity performance has been achieved with electropolishing as
main damage layer removal and final surface preparation process followed by baking.

Baseline

The BCD recommendation for this process is:

       Electropolishing for damage layer removal (~120-150 um)
       800°C furnace treatment
       Electropolishing for final preparation (~20-50 um)
       High pressure rinsing for final cleaning

There is no data on multi-cell cavities which suggests that this procedure can be avoided.
Initial results on single-cell cavities with other material (see section Cavity Material) need to
be confirmed by multi-cell experiments.

Required R&D

The major risk in the cavity preparation is the contamination of the inner cavity surface which
then leads to enhanced field emission and performance degradation. In most series
productions of cavities field emission is the reason for performance limitation. Therefore, it is
highly desirable to further improve quality control of the processes applied to the cavity.
Moreover, any development on cavity shapes with an increased ratio of electric surface peak
field to accelerating gradient (Epeak/Eacc) requires an even larger, continuing effort to reduce
field emission.

Substantial experience has been accumulated on cavity preparation systems. Still, most of the
facilities in operation are small scale systems compared to ILC needs, and there are
differences in capabilities of existing systems. The R&D on the cavity preparation should aim
for improved preparation facilities of the next generation designed for improved quality
control.

There are three main areas in the cavity preparation process where a strong R&D plan is
needed:

       Electropolishing (EP) system: A generic parameter set for niobium cavity EP was
        developed at KEK. So far the experience on multi-cell cavities is not as reproducible
        as desired. Several improvements are suggested.
            o Improved parameter control:
                     Development of efficient heat exchangers to improve temperature
                       stability
                     Compensation for losses of hydrofluouric acid (HF) from evaporation
                       and chemical reaction with the niobium by actively adding HF during
                       the process.
                     Optimization of the current distribution for homogenous material
                       removal

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            o   Contamination control:
                    Avoidance of sulphur by either improved post-EP rinsing methods (e.g.
                       alcohol rinse) or changed EP parameters (e.g. bath composition)
                    Definition of measurement techniques for contamination and quality
                       control (e.g. Nb and HF content of the electrolyte)
            o   Quality control
                    Development of a roughness measurement of the inner cavity surface
                       that goes beyond the information supplied by witness samples

       High pressure rinsing (HPR): Currently, the rinse of cavities with high pressure
        water is the only effective means of removing particles from the cavity surface.
        Further R&D is required to reduce performance spread and increase the onset level of
        field emission.
            o Improved parameter control
                     Optimization of the cleaning force by proper adjustment of
                             Nozzle geometry, material and size.
                             Impact angle and flow rates
            o Contamination control
                     Online particulate monitoring on both the high pressure input side and
                        on the drain water from the cavity
                     Reliable online TOC (total organic carbon) monitoring of input and
                        output water.
                     Understanding of specifications needed on other contaminants (e.g.
                        dissolved solids).

       Assembly procedures: Currently, the assembly and cleaning of components is not
        streamlined to fit into a mass production environment. To facilitate this some
        development is needed to simplify procedures and reduce the amount of parts used
        during assembly. An investigation on improved tooling and (semi-) automation is
        necessary.
            o Assembly of components
                    For the quality control of assembly procedures it is desirable to develop
                       a technology to assess particle contaminations of the inner cavity
                       surface.
                    Methods for mass production need to be developed (e.g. cleaning of
                       screws, gaskets etc.)
                    The assembly of the cavity string needs a further improved quality
                       control procedure as it is currently not possible to clean the full string
                       after assembly.
                    The overall workflow during inspection, preparation, assembly and
                       testing needs optimization to avoid contamination and increase cost
                       effectiveness.
            o Drying
                    An evaluation on drying procedures after HPR is needed to avoid re-
                       contamination of the surface
                    An integration of the drying with the ‘in-situ’ bake-out seems desirable
                       and should be explored.



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These are critical R&D issues for a cost efficient production with a high yield. Mass
production issues need to be addressed for tuning and cleaning, as well as a serious effort to
reduce the number of parts to be assembled. Any new R&D infrastructure for the ILC must
take into account the enhanced need for quality control. Some of the topics are:

       Integration of mechanical measurements (eccentricity, etc.), optical inspection (EB
        welds etc.) into the cleanroom area
       Frequency tuning: Integration into clean room seems desirable (see above).

Vertical cavity tests for all cavities are mandatory (preferably with many cavities inserted into
one vertical test dewar) and will potentially allow sorting cavities during production, with
some benefit to overall linac performance.

High-power tests on individual cavities are necessary in an initial production phase where the
critical assembly steps (e.g. high-power coupler mounting) are set up and where sub-
components are qualified (e.g. tuners). With increasing confidence, the number of these tests
may be reduced.

Full tests on all modules are mandatory in the first phase of the production. When the
production process is established, a statistical approach on testing might be feasible especially
if a cool-down of linac sections in the ILC is envisaged, so that major problems (e.g. vacuum
leaks) are detected before machine operation starts. A risk analysis of different scenarios is
desirable, and should include cavity preparation experts.

Supporting Documentation

Original Snowmass document by L. Lilje

TESLA Report 2004-04 Improved Surface Treatment of the Superconducting TESLA
Cavities - L. Lilje, A. Matheisen, D. Proch - DESY; C. Antoine, J.-P. Charrier, H. Safa, B.
Visentin - CEA Saclay, DAPNIA; C. Benvenuti, D. Bloess, E. Chiaveri, L. Ferreira, R. Losito,
H. Preis, H. Wenninger - CERN; P. Schmueser - Universitaet Hamburg

TESLA Report 2004-05 Achievement of 35 MV/m in the Superconducting Nine-Cell Cavities
for TESLA - L. Lilje, D. Kostin, A. Matheisen, W.-D. Moeller, D. Proch, D. Reschke, S.
Simrock, K. Twarowski - DESY; E. Kako, K. Saito - KEK; P. Schmueser - Universitaet
Hamburg; T. Suzuku - Nomura Plating Co., Ltd.

Alternatives

A brief discussion summary on the alternatives to the BCD is given below. For details refer to
the original Snowmass document by L. Lilje.

       Postpurification of material at 1400°C with titanium getter
       Eliminate outside etching
       Hot water rinsing
       Tumbling/barrel polishing to reduce amount of EP necessary
       Dry-ice cleaning instead of high pressure water rinsing

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       Air bake out instead of in-situ vacuum bake at 120 – 130 C.

Required R&D

For all the alternatives

       Single-cell R&D is necessary.
       Multi-cell issues need work.

Supporting Documentation

Original Snowmass document by L. Lilje

High Power Input Coupler

BCD Choice

Our BCD choice is based on the twin cylindrical window architecture of the TTF-III coupler.

Description

Pro:

The principal “pros” of this choice are the following:

       Lengthy experience (~ 100,000 coupler-hours) of this device on the TESLA Test
        Facility.
       Some degree of manufacturing experience – 52 couplers built, mostly in industry.
       Demonstration of use with a cavity at 35 MV/m on CHECIA.
       Tested at a power of 1 MW, 1.3 ms pulse in TW mode.
       This coupler has already been accepted for use with the European X-FEL (~ 1,000
        units needed) and this implies that there will be considerable experience gained with
        this coupler before the ILC is launched.
       In principle, then this power coupler would be sufficient even if the cavities were to be
        run at 35 MV/m and would meet, at least in TW mode, the needs of a 2 x 9-cell
        superstructure at 35 MV/m.

Cons:

The “cons” of this choice, at the time of writing are:

       The present unit cost is prohibitive. However, the couplers have only been built in
        small numbers to date.
           o Note that the cost issue will be dealt with through an “Industrialisation” study
               to be carried out by LAL-Orsay (co-financed by DESY and the IN2P3) in the
               context of the European X-FEL project. We aim for a major reduction in the
               unit cost of the couplers through this study.
       The experience with conditioning indicates that the conditioning time is rather long.

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             o   However, note that the scatter in conditioning time is rather large and the fact
                 that some examples are conditioned rather quickly (< 50 hours) is encouraging.
             o   The issue of conditioning is currently under study at Orsay in the context of a
                 DESY-LAL collaboration and partly financed through the European Union
                 initiative – CARE (Co-ordinated Accelerator Research in Europe). It is hoped
                 that this activity will lead to procedures offering reduced conditioning times.

A period of ~ two years will be necessary to complete the conditioning and indutrialisation
studies (i.e. completion around summer 2007).

Potential Modification

A modification of potential interest is an increase in the diameter of the cold assembly (from
40 mm to 62 mm). This would have the technical benefit of pushing multipactor levels to
higher powers and therefore may be of interest in case of a choice of higher gradient ( ~ 45
MV/m).

Four proto-types of such a coupler has been ordered by Orsay and should be delivered in the
spring of 2006 and tested soon afterwards. The R&D necessary for such a modification could
be complete by the end of 2006.

Required R&D

See above.

Cost Estimation

Supporting Documentation

Original Snowmass document by T. Garvey.

Alternatives

There was a general consensus that different coupler designs incorporating two « disk » type
windows could be potential alternatives to the TTF cylindrical window. Four such couplers
are currently under study and we list them here with no order of priority:

       The “capacitive” disk window coupler.
       The ‘TRISTAN’ like window coupler.
       The TW60 coupler.
       The AMAC window coupler. (this last coupler was added by T. Garvey after the
        meeting. It is currently being developed by AMAC, in collaboration with DESY and
        CPI with funding from a DoE SBIR grant).

Each of these couplers were presented in more or less detail at Snowmass and a description
can be found in the presentation on the Workshop web site.

Description

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Pro:

Some ‘pros’ are as follows:

       Disk windows are (or can be expected to be) relatively free from multipactor.
       Disk windows are mechanically easy to fabricate and therefore may be cheaper (but
        the ceramic is not a cost driver for the coupler).
       Thin disk windows can be positioned at low values of the SW electric field.
       Disk windows should be relatively easy to braze into the coupler.
       One should note that windows based on the TRISTAN design have a history of
        success.

Con:

Some ‘cons’ are as follows:

       The current version of the capacitive coupler cannot be DC biased.
       The present capacitive and TRISTAN like couplers have no possibility to have their
        external Q variable.
       Disk ceramics are in the “line of sight” of the cavity beam pipe. This was seen to be a
        problem on the early CEBAF linac design, which was a waveguide coupler. However,
        the co-axial version may be less problematic as the window surface is smaller and is
        further from the beam pipe, thus reducing the solid angle presented to x-rays or
        electrons which might impinge on the ceramic.
       For all of these alternatives it is too early to estimate the cost impact.

Required R&D

Time for R&D

As for time scales for R&D, the AMAC coupler is currently under test at DESY. A proto-type
of the TW60 coupler will be tested at Orsay in the summer of 2006 at the earliest. The
capacitive coupler will be tested at high power at KEK early in 2006.

Supporting Documentation

Original Snowmass document by T. Garvey.

HOM Couplers and Beam Line Absorbers

Overview

The accelerated ILC beam, if similar to the proposed in the TDR beam of the TESLA 500
GeV collider, will consist of 3.2 nC bunches which rms length will be 300µm. The 14100
bunches accelerated per second will be grouped within five 950 µs long RF pulses. The time
separation between sequential bunches in a RF-pulse will be 337 ns. This beam will generate
spectrum up to 0.4 THz. The beam deposited power in a cryo-module housing 12 TTF-like 9-
cell structures will be ~24 W if no synchronous excitation of parasitic modes will take place.
Big fraction of this power (17.4 W) is carried out by propagating modes above 5 GHz.
Rev. July 13, 2006
Main Linac                                                                               27

The beam deposited power must be removed from cryomodules to avoid an additional heat
load in 2K environment and to maintain the high quality of the accelerated beam (preserving
the low emittance). This will be achieved by means of two kinds of devices:

       HOM couplers
       Beam line absorbers

Each accelerating structure will be equipped with two HOM couplers suppressing mainly the
non propagating part of the spectrum (below 5 GHz). Beam dynamics simulations showed
that preservation of the low emittance demands suppression of high impedance dipoles to
Qext of the order of 10^5. This suppression will also ensure stable operation if resonant
excitation of some high impedance modes takes place.

One beam line absorber will be installed in each interconnection between cryomodules. It is at
present proposed and being under development version should absorb 80% of the energy
propagating out of neighboring cryomodules. The first prototype will be ready for beam test
by the end of October 2005.

Baseline

The suppression scheme as proposed in the TESLA TDR

Description

Required R&D

The careful studies of HOM suppression at TTF linac showed that almost all dipole modes,
but two, are well damped and satisfy the specification. To improve damping of these modes
which Qext was above the spec one of two HOM couplers for all recently produced cavities
has been positioned in a different way. The expected improvement can be first verified when
a cryomodule with new cavities will be installed in the TTF linac. This should happen in 2006.

Further R&D is needed on getting reproducible properties of beam line absorber material. We
also expect that the prototype of the beam line absorber will be installed and tested at TTF
linac also in 2006.

The fraction of HOM power deposited at 2 K should be investigated.

Supporting Documentation

Original Snowmass document by J. Sekutowicz

TESLA TDR Cavity Chapter

P. Schmueser et al.; The Superconducting TESLA Cavities; PRST-AB 3 (9) 092001

TESLA Note 2002-05 Higher Order Mode Absorption in TTF Modules in the Frequency
Range of the 3rd Dipole Band M. Dohlus, S.G. Wipf - DESY, V. Kaljuzhny - MEPI

Rev. July 13, 2006
Main Linac                                                                                 28

Alternatives

    1. TDR scheme with minor improvements
    2. Coaxial coupling fitting in the beam line longitudinally and radial.
    3. Other solutions

TDR scheme with minor improvements

The proposed TDR HOM suppression scheme with minor improvements should work
properly. Some additional improvements like higher heat conductivity feedthroughs for the
output lines can be implemented at any time of cavity production with minimal risk.

Options under consideration

Further improvements reducing costs of HOM couplers and beam line absorbers are also
considered for XFEL cavities and can be later implemented for the ILC collider. Three of
them are listed below:

       Radial positioning of the HOM coupler output in the plane of so called F-part.
       Version of HOM coupler with hidden output capacitor.
       Version of HOM coupler without output capacitor.

Coaxial coupling fitting in the beam line longitudinally and radial.

A more advanced and revolutionary change should lead to a device that replaces the two
HOM couplers rigidly attached to a cavity and that is mountable between cavities. This could
simplify cavity production if remaining body will be cylindrically symmetric. For this also FP
coupler should be removed. The Cu model of beam line FP coupler (very similar to the
Darmstadt linac solution) was built and tested on Cu model of 9-cell cavity. At the moment
there is no conceptual design for the HOM coupler.

Supporting Documentation

Original Snowmass document by J. Sekutowicz

TESLA Note 2002-05 Higher Order Mode Absorption in TTF Modules in the Frequency
Range of the 3rd Dipole Band M. Dohlus, S.G. Wipf - DESY, V. Kaljuzhny - MEPI

Frequency tuner

Overview

Main objectives for the frequency tuner are to provide means to tune the cavity on resonance,
detune a cavity to by-pass operation if needed, and to compensate Lorentz-force detuning.
The tuner further needs to allow for a high linac fill factor (compact design), should be
hysteresis free, and should not cause cross-tuning of neighboring cavities. Long life time of
the tuner is essential; see discussion below. All this needs to be achieved with lowest
cryogenic losses, and at low cost.

Rev. July 13, 2006
Main Linac                                                                                 29

ILC requirements:

       Coarse tuning range: 500 kHz (1.6 mm at 315 Hz/ï•m)
       Coarse resolution: <5 Hz

       Fast tuning range (static at 2K):
           o delta_f=2*K*E^2 (factor 2 for dynamic operation overhead)
           o 2.5 kHz (for K=1 Hz/(MV/m)^2 cavity at 35 MV/m)
           o 3.2 kHz (for K=1 Hz/(MV/m)^2 cavity at 40 MV/m)
           o 4.0 kHz (for K=1 Hz/(MV/m)^2 cavity at 45 MV/m)

The requirement on fast running range is not well known at this point. Significant spread in
the dynamic Lorentz-force detuning constant has been seen (factor 2 at TTF). Unless K can be
controlled well in the ILC cavity/LHe vessel production, more fast tuning range is required.
Also, significant difference can exist between the static range and the dynamic range
(maximal frequency shift within the RF pulse length). A factor of 2 is included in the above
numbers for the ratio static / dynamic range. Dedicated experiments are needed to define the
actually required fast tuning range.

Options under consideration:

The following existing tuners could provide a basis for the ILC frequency tuner (see original
document for pictures):

       Original Saclay / TTF tuner:
           o This type of tuner is in use at TTF/VUVFEL since several years and thus is
               well tested. It does not have a fast tuning element, though a piezo actuator has
               been added for proof-of-principle tests of Lorentz-force detuning. The range
               however of the fast tuner is below 500 Hz, and the tuner was not initially
               developed to implement a piezo actuator.

       Modified Saclay tuner:
          o Similar the original Saclay tuner, this tuner is located at one end of the cavity.
               The redesign however is more compact, and incorporates piezo actuators
               (about 1 kHz tuning range). First tests of this tuner are expected by end of
               2005.

       INFN / DESY blade tuner:
           o The first version of this tuner did not include fast actuators, and was tested at
               TTF with the superstructure. This tuner is located around the LHe vessel, thus
               does not require any clearance at the cavity ends. The recent version of this
               tuner includes piezo actuators (about 1 kHz tuning range), and will be tested
               late 2005.

       TJNAF Renascence tuner:
           o This tuner was developed for the TJNAF upgrade cryomodule. It does
             incorporate piezo actuators (about 1 kHz tuning range), and eight tuners of this
             type will be operated and tested in a cryomodule test late summer 2005.

Rev. July 13, 2006
Main Linac                                                                                  30

                                                 INFN                                KEK
                      Saclay      Saclay                     TJLAF     KEK Slide
                                                 Blade                            coaxial ball
                     original    modified                    upgrade   Jack Tuner
                                                 tuner                               screw
Coarse Range
             440                500         500          500           1100        >4000
   [kHz]
 Coarse Res.
             <1                 <1          <1           <1            <100        <120
    [Hz]
                                                         Piezo/
Fast actuator (Piezo)           Piezo       Piezo                      Piezo       Piezo
                                                         Magnetostr.
  Number of
                 (1 - 2)        2           2            2             1           1
fast actuators
  Fast range
                 <500           1000        1200         1000/ 30000 1900          2500
      [Hz]
  Position of    5     K, 5    K, 5      K,                                        80      K,
                                            5 K, vacuum 5 K, vacuum
 fast actuator   vacuum vacuum    vacuum                                           vacuum
  Position of    5     K, 5    K, 5      K,             Warm,                      80     K?,
                                            5 K, vacuum
     motor       vacuum vacuum    vacuum                outside                    vacuum


       KEK slide jack tuner:
          o KEK design for cavity operation at the baseline gradient of 35 MV/m. The
              uniqueness of this design is that the motor is placed at room temperature
              outside of the vacuum vessel.

       KEK coaxial ball screw tuner:
          o This tuner was designed for the ICHIRO 9-cell cavity at 45MV/m. Both, the
              motor and the piezo are placed at intermediate temperature inside of the
              vacuum vessel. A first prototype test is planed for late summer 2005.

Risk and Reliability impacts:

All but one of the above mentioned designs have a cold drive motor inside the vacuum vessel.
In none of the designs can the fast actuator be replaced without cryostat warm-up. Highest
reliability / lifetime are therefore essential. The main risk is a failure of the motor, the fast
actuators or the gearing. All designs with cold drive can use the same type of motor, gearing
and fast actuator, so that there is no principle difference in risk and reliability between these
tuner designs. A well tested version of the motor and gearing exists. Tests on piezo
performance and reliability are underway.

However, the reliability of a single cold drive might not be sufficient, as was pointed out by
the U.S. Linear Collider Technology Options Study: ‘The cavity tuners and cavity piezo
tuners designs both require opening the cryostat to effect repairs and had over 50 failures per
year. This is an unreasonable amount of work even for the 3 month shutdown. The tuners will
either have to be made very reliable (probably via redundancy) or their failure prone
components made replaceable without warm-up.’

To improve reliability, the following options exist:
Rev. July 13, 2006
Main Linac                                                                                   31

       Redundant motor and piezo, if inside of vacuum vessel
       Improved design with highest reliability for motor and/or piezo, if inside of vacuum
        vessel
       Warm motor

It should be pointed out, that the operation of the fast actuator is essential at high fields. A
failure of this element will result in lower operating fields. The motor on the other side is only
operated during cool-down and warm-up, and to correct for slow frequency drifts. A failure of
the motor will not immediately impact the cavity operation. It can be expected that the total
required step-count of the motor is moderate. The failure mechanism and the MTBF in this
operating mode need to be studied in detail, to verify if a cold motor is acceptable.

Fast actuator:

Two options are under consideration:

    1. Piezo actuator (used in all designs as baseline).
           o Detailed studies have been done to verify pulsed cryogenic operation in
              radiation environment.
    2. Magnetrostrictive actuator (similar size to piezo, so can be used instead of piezo in all
       discussed tuner designs):
           o Has a significant larger stroke than a piezo at 5 K, produces less heat and
              might have a higher lifetime and higher tolerance for preload change than
              piezo. First tests at cryogenic temperatures have been done. A detailed
              characterization is need. This actuator needs a high drive current, and its
              residual magnetic needs to be studied. Also, the cost of this actuator type might
              be higher.

Baseline

Not available. No existing tuner design fulfills the specification on fast tuning range above 30
MV/m. The above mentioned designs give a good starting point for an ILC tuner and for a
cost estimate. The tuner needs to provide 500 kHz slow tuning range and more than 3 kHz
fast tuning range.

The fast actuator should be located inside the vacuum vessel for best performance during
Lorentz-force compensation. A redundant design for the fast actuator is important for
reliability.

Required R&D

       Tuner design for 40+MV/m operation and prototype tests including demonstration of
        Lorentz-force detuning at highest fields with BCD cavity.
       Reliability (MTBF) studies of motor / gearing / piezo / magnetostrictive actuator,
        including failure mechanisms and improved estimate of requirements.
       Performance of magnetostrictive actuator.
       Cavity design with smaller Lorentz-Force detuning.
       Cost estimation for external motor.

Rev. July 13, 2006
Main Linac                                                                         32

Parameter Tables

Supporting Documentation

Original Snowmass document by M. Liepe and S. Noguchi

Cost Estimation

Close to BCD:

    1. Modified Saclay tuner:
          o Pros:
                  Relative simple and compact design
                  Redundant design for piezo element
                  Original Saclay tuner was tested in detail
          o Cons:
                  Maybe difficult to increase fast tuning range
                  Redesign needed with increased fast tuning range
                  Poor maintainability of stepping motor and fast actuator
                  Requires some length between cavities (located at cavity end)
          o R&D necessary:
                  Design with increased fast tuning range
                  Fast actuator R&D
                  Prototype tests with Lorentz-force compensation at 35 MV/m
                  Verification of sufficient MTBF for cold motor
    2. INFN blade tuner:
          o Pros:
                  Compact design (not at cavity end)
                  High stiffness
                  Tested (without fast actuator)
                  Relative easy to increase fast tuning range
          o Cons:
                  Redesign needed with increased fast tuning range
                  Poor maintainability of stepping motor and fast actuator
          o R&D necessary:
                  Design with increased fast tuning range
                  Fast actuator R&D
                  Prototype tests with Lorentz-force compensation at 35 MV/m
                  Verification of sufficient MTBF for cold motor

Supporting Documentation

Original Snowmass document by M. Liepe and S. Noguchi

Alternatives

    1. TJNAF Renascence tuner:
          o Pros:


Rev. July 13, 2006
Main Linac                                                                          33

                        Compact and simple design (not at cavity end)
                        Low cost?
            o   Cons:
                     
                     Not originally designed for ILC cryomodule. May need some redesign
                     to fit.
                  Redesign needed with increased fast tuning range
                  Poor maintainability of stepping motor and fast actuator
         o R&D necessary:
                  Design for ILC cryostat
                  Design with increased fast tuning range
                  Fast actuator R&D
                  Prototype tests with Lorentz-force compensation at 35 MV/m
                  Verification of sufficient MTBF for cold motor
    2. KEK slide jack tuner:
         o Pros:
                  Motor outside of vacuum vessel (inexpensive motor)
                  Piezo can be replaced (cryostat warm-up required)
                  High stiffness
         o Cons:
                  Feed-through to outside needed (penetration of shields and vacuum
                     vessel)
                  Some static losses (0.05 W?)
                  Redesign needed with increased fast tuning range
                  Poor maintainability of fast actuator; no redundancy
         o R&D necessary:
                  Design with increased fast tuning range
                  Fast actuator R&D
                  Prototype tests with Lorentz-force compensation at 35 MV/m
    3. KEK coaxial ball screw tuner:
         o Pros:
                  Wide tuning range
                  Compact design with common technology
                  Cost effective
                  High stiffness
                  Maybe access to piezo (warm-up required; need to pass through all
                     thermal shields)
         o Cons:
                  Poor maintainability of stepping motor
                  Poor maintainability of fast actuator; no redundancy
                  Heavy weight
                  Some static losses
                  Redesign needed with increased fast tuning range
         o R&D necessary:
                  Choice of coating material for balls
                  Weight reduction
                  Design with increased fast tuning range
                  Fast actuator R&D
                  Prototype tests with Lorentz-force compensation at 35 MV/m
                  Verification of sufficient MTBF for cold motor

Rev. July 13, 2006
Main Linac                                              34

Supporting Documentation

Original Snowmass document by M. Liepe and S. Noguchi




Rev. July 13, 2006
Main Linac                                                                                35

Gradient

Baseline

The WG5 recommendations call for TESLA-like cavities to be used. They would be qualified
to operate at a gradient of at least 35 MV/m with a Q > 0.8×1010 in CW tests (cavities not
meeting these requirements would be rejected or reprocessed). Only a small fraction of the
cavities and cryomodules would be pulsed-power tested. With such screening, they expect
that a 31.5 MV/m gradient and Q of 1×1010 would be achieved on average in a linac made
with eight-cavity cryomodules. This assumes that (1) the rf system would be capable of
supporting 33.5 MV/m operation throughout the linac (2) some of the poorer performing
cavities would be de-Q’ed so the associated cryomodule can run at a higher gradient and (3)
the cryomodule power feeds would include attenuators so the average gradient in each unit
can be maximized. For a future upgrade, they recommend that cavities of the low-loss or
reentrant type be used and that they be qualified to at least 40 MV/m with Q > 0.8×1010 in
order to achieve 36 MV/m and Q = 1×1010 on average in the linac.

Alternatives

Since improvements in cavity performance will likely continue, one design strategy would be
to choose a gradient significantly higher than that currently achievable. However, the linac
cost is a weak function of gradient in the 30-50 MV/m range, and operating close the ultimate
45-50 MV/m gradient limit would prevent extending the machine energy by lowing the beam
current (and depending on the cooling overhead, lowering the machine repetition rate). Thus a
better strategy would be to design for a gradient around 30 MV/m, and if the cavities that are
eventually installed perform better than the initial requirement, use this capability to extend
the machine energy reach (e.g., up to 750 GeV if 45 MV/m operation is eventually achieved).
The luminosity would decrease with higher energy, but still may allow for discovery-level
measurements. The WG5 ACD gradient recommendation for 500 GeV operation is the same
as given for the BCD upgrade.




Rev. July 13, 2006
Main Linac                                                                                  36

Cryomodule and Lattice

Baseline

Adopting the WG5 gradient and cavity recommendations and assuming TDR-like rf
distribution losses and overheads, a reasonable baseline rf unit is a 10 MW klystron driving
26 TESLA cavities. This configuration allows 33.5 MV/m operation with 5% rf distribution
losses and a 10% power overhead (below klystron saturation). The overhead is an estimate of
that required to stabilize of the cavity voltages and to operate in a reasonably linear regime: an
overhead based on operation experience with ILC-like cryomodules should eventually be used.

The cavities would be divided into three cryomodules instead of two since (1) this is the
configuration that has been used and will continue to be used for several years (2) there is no
significant cost savings with longer cryomodules, which would be more difficult to build,
transport and make vibrationally stable and (3) the cavity gradient variation can be more
efficiently dealt with if there are less cavities per cryomodule, assuming the power to each
cryomodule can be controlled using attenuators.

Every third cryomodule in the linac would include 8 cavities and a superferric or cos(2*phi)-
type quadrupole (this corresponds to a constant beta lattice with one quadrupole every 26
cavities). The other two cryomodules would each contain 9 cavities. The quad He vessel
would be supported from above by the 300 mm diameter gas return pipe, which itself would
be supported by three posts extending down from the top of the cryomodule. The quad would
be located below the center (fixed) post, and attached to its upstream face would be a BPM
with 10 micron or better bunch-to-bunch position resolution for the nominal bunch charge of
2×1010. On the downstream face of the quad, a combined horizontal and vertical dipole
corrector magnet would be attached. The length of the cryomodule with the quad is 12.543 m,
close to the 12.590 m length of the 9 cavity cryomodule (the rf unit length is thus 37.723 m).

A laser wire scanner would be located in each of the three warm sections between the 2.2 km
long cryogenic units within each linac. The 1.2 km undulator region in the electron linac, and
the corresponding section in the positron linac, would each contain emittance diagnostics and
energy and energy spread measurement capability.

The TDR cavity spacing of 283 mm was optimized based on the flange connection and bellow
scheme used at TTF. Shorter spacing is possible (see ACD below), but the TDR spacing is
assumed until the engineering implications are better understood. With this choice, the linac
packing fraction (ratio of active to actual length) would be about 70%.

For 500 GeV operation, 283 (279) such rf units would be installed in 15-250 GeV electron
(positron) linac, where the additional units in the electron linac would restore the energy lost
in the undulator. These unit counts provide a no energy overhead with 5 degree off-crest beam
operation, however there is an effective 6% overhead if all cavities could operate at the rf
limited gradient of 33.5 MV/m. Overall, 80% of the peak klystron rf capability would be
transformed into beam power with 31.5 MV/m average gradient operation.




Rev. July 13, 2006
Main Linac                                                                                  37

Alternatives

Several variations on the TDR-like layout were discussed including:

Having up to 12 cavities per cryomodule instead of 8 to reduce the number of inter-
connections.

Shortening the distance between cavities to 250 mm based on the connection scheme used at
JLab, or as close as possible to the 180 mm limit from cavity cross talk and heat losses at the
transitions.

Instrumenting the HOM readouts to provide a measure of the average beam position in the
cryomodules. To better center the beam, the cryomodules would either be moved manually
during down periods or equipped with remote-controllable movers to allow corrections during
machine operation.

Putting the quad and BPM in a separate cryo-section to better stabilize them vibrationally and
allow them to be moved independently from the cavities. This would eliminate the need for
corrector magnets.

Putting movers on the middle support post of the gas return pipe to allow adjustment of the
quad and BPM positions (the nearby cavities would move as well). To allow independent
control of the quad and cavity (average of eight) positions, the cryomodules would also have
to be supported on movers.

Reducing the quad aperture by half (to about 35 mm) to allow the use of superferric quads,
which will likely have more stable magnetic centers with respect to quad shunting. Likewise,
reducing the BPM aperture by half to yield higher resolution and smaller common-mode
errors. These changes would increase the short-range wakefield by about 10%.

Improving the BPM resolution to about one micron to allow measurements of beam jitter at a
level smaller than the vertical beam size.

RF Distribution

Baseline

The baseline choice is a TDR-like distribution system that includes a circulator in each cavity
feed followed by a tuner (three-stub or E-H type) to allow control of the cavity phase and
Qext. Currently DESY uses off-the-shelf components for the distribution system: a
customized, integrated design would likely yield significant cost savings. WR770 waveguide
would be used between the klystron and the cryomodules and WR650 would be used along
the cryomodules. WR770 has about 33% lower rf losses than WR650.

Alternatives

The circulator is the largest cost component at about 25% of the rf distribution cost. There are
several alternative distribution schemes that eliminate the circulators but require more precise

Rev. July 13, 2006
Main Linac                                                                                  38

cavity-to-cavity phasing, and make it harder to deal with the variation in the maximum cavity
gradients. Also, in the event of rf breakdown in a coupler or cavity, these schemes would
allow some fraction of the reflected power to propagate to the other cavities. How this
additional power would effect performance has yet to be determined. KEK plans to test such a
distribution scheme at the STF in the next year.

Cryogenic System

Baseline

The basic layout choice is that outlined in the TDR except the refrigerator spacing would
depend on the choice of operating gradient, expected cavity Q and the desired cooling
overhead (the spacing is about 5 km in the TDR – cryo-engineers at FNAL will recommend a
spacing for the BCD). For static heat losses, no uncertainty factor is included in the load
estimate as the actual static load would be know before construction begins. However, a 40%
over-capacity factor on the total load (static and dynamic) is included, of which ~ 20% is for
degradation of plant performance, which may be due to helium contamination, equipment
wear and other factors that are often seen in cryogenic plants, ~ 10% for operational control,
for example the need to recover temperature while already at full load, and finally ~ 8% for
mainly detrimental effects due to warm summer temperatures and warmer cooling water in
summer The maintenance length (i.e., the length that would need to be warmed up to repair a
cryomodule) is half of the refrigerator spacing.

Alternatives

Thermal cycling long strings of cryomodules will be slow and may cause vacuum leaks. To
reduce the maintenance length, U-tubes or turnarounds can be included at periodic locations
to allow one section be thermally isolated (the refrigerators on either side of this region would
cool the other cryomodules). If such sections (each 1.5 m long) were installed every 500 m,
the number of cryomodules thermally cycled would be reduced by a factor of five, the warm-
up time would be reduced by a factor of two and the cool down time would be reduced by a
factor of 10.

Supporting Documentation

bcd:main_linac:ilc_bcd_cryogenic_chapter_v3.doc

Tunnel

Baseline

Relative to a linac with bends, a laser-straight linac would make dispersive emittance control
easier and reduce the likelihood of off-energy beams intercepting the accelerators. However,
this choice would limit the possible sites, especially those near the surface, and would require
that the two-phase He be distributed along the cryomodules in more costly manner.

Until on-going beam dynamics simulations show otherwise, the linac will follow the
curvature of the earth, unless a site-specific reason (cost driven) dictates otherwise.

Rev. July 13, 2006
Main Linac                                                                                   39

Alternatives

Laser-straight linac or one constructed from straight-line segments. Final choice depends on
cost considerations, primarily for the cryogenics and any site constraints, and the results of
further beam dynamics studies.

Required R&D

Aggressive beam dynamics simulations of emittance preservation (beam-based alignment) for
the continuously curved linac, including tolerance studies.

Baseline

The baseline choice is for the rf sources to be located outside of the beam tunnel so they
would not be subject to radiation and could be accessed for repairs while the machine is
running. To minimize rf power losses and cable runs, the sources are to be distributed along a
second tunnel (or surface gallery) that runs parallel and nearby to the beamline tunnel. The rf
power is transported into the beamline tunnel through three WR650 waveguide runs from
each rf unit (one waveguide per cryomodule).

Alternatives

For cost savings, the TDR design is a reasonable choice. The beamline is in a near-surface
tunnel (< 30 m deep) and the modulators, sans transformers, are clustered in surface buildings
located every 5 km (the beamline tunnels contain the modulator transformers and klystrons).
With the transformer in the tunnel, only relatively low voltage pulses (~ 10 kV) need to be
transported, and the required cable impedance is typical of that used commercially (although
four cables would be used per modulator). The disadvantage of separating the modulator from
the klystron is that the cables are expensive ($70 million in the TDR), they pose a fire risk and
they are not easily repairable once the spares are depleted.

For either the baseline or alternative choice, locating the beamline near the surface would
allow easier access and shorten the power and cooling distribution lines that connect to the
surface. The main disadvantages in that case would be larger ground motion and limited site
availability. The final choice in this regard will likely be depend on the surface terrain and the
population density at the proposed sites.




Rev. July 13, 2006

				
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