European Superconducting Accelerator Magnet aimed at the by alicejenny


									                                                              15 March 2006

                           A Strategy
      for European Superconducting Accelerator Magnet R&D
                aimed at LHC Luminosity Upgrade

Dr. Arnaud Devred
CEA/DSM/DAPNIA/SACM (France) & CERN/AT/MAS (International),
CARE/NED JRA Coordinator
Dr. D. Elwyn Baynham
Head of Engineering Department
Pr. Maciej Chorowski
Wroclaw University of Technology (Poland)
Dean of the Faculty of Mechanical and Power Engineering
Dr. Pasquale Fabbricatore
INFN-Genova (Italy)
Dr. Eng. Luis Garcia-Tabares
Head of Applied Superconductivity Group
Dr. Stephen Gourlay
Leader of US-LARP/Superconducting Magnets
Ir. Andries den Ouden
Twente University (The Netherlands)
Faculty of Science and Technology
Dr. Stephen Peggs
Leader of US-LARP
Pr. Lucio Rossi
CERN/AT (International)
Head of Magnets And Superconductors Group
Mr. Laurent Tavian
CERN/AT (International)
Head of Cryogenics for Accelerators Group
Dr. Giovanni Volpini
INFN-Milano/LASA (Italy)
Mr. Louis Walckiers
CERN/AT (International)
Head of Magnet Test and Measurements Group

                                                                                   15 March 2006


After the LHC will have operated for some years at nominal parameters, it will be necessary to
upgrade it for significantly higher luminosity. The most direct way of increasing luminosity is
to focus the beam more tightly at the collision point (reduce the so-called * parameter) which
calls for a redesign of the machine optics in the Interaction Regions (IR) and a replacement of
the final-focusing quadrupole magnets. Furthermore, depending on the limitations revealed by
machine operation (such as turnaround time or dynamic field effects occurring in the
superconducting magnets at the beginning of acceleration), it might also be required to upgrade
the injector chain and raise the injection energy by adding a new superconducting ring. The
time scale for replacing IR magnets is in part determined by the lifetime of the present magnets
under high radiation doses. It can be estimated as being around 2015. The need for restructuring
the injector chain will be assessed at the time of commissioning and can be envisioned as on the
2020 horizon.

LHC Luminosity Upgrade Scenarios and Magnet Requirements

The first task in preparation of an LHC luminosity upgrade is to develop and compare various
scenarios for IR and injector chain upgrades and to derive meaningful sets of magnet
requirements. The terms of comparison should be: peak luminosity, potential for efficient
operation (integrated luminosity), prospects for effective machine protection against beam
losses, high threshold versus luminosity-induced energy deposition, radiation resistance,
technical risks, costs and potentials for the future. The groundwork for this task should be
carried out within the next two years (2006-2007) so as to guide and focus superconducting
magnet R&D. In addition to a phasing in time, it is already clear that the magnet requirements
for IR upgrade and for a new injector ring are different: the former calls for a small series of
magnets, with an emphasis on high-field performance rather than on cost, while the latter calls
for a larger series of medium-field magnets, with an emphasis on cycling capabilities and cost.

Technological Development of High-Field Nb3Sn Magnets for an LHC IR Upgrade

The clearly identified need for lower * has been investigated for some time. The most
promising optics solutions are based on large-aperture, high-field and/or high-field-gradient
magnets moved closer to the interaction point. All potential high-luminosity solutions call for
operation under high-energy depositions, radiations and heat loads. All of the superconducting
magnets presently implemented in the LHC rely on NbTi superconductor technology. The
magnet programs carried out around the world within the framework of LHC were successful in
pushing this technology to the required level of maturity, but they also clearly demonstrated that
NbTi technology has achieved its performance limit (around 9 T). In order to go beyond (and
cross the10-T threshold while maintaining proper operational temperature margin), it is
necessary to change the superconductor material.

Nb3Sn is the only other superconducting material that can be seriously considered within the 10-
year time frame left between now and the 2015 target for the replacement of IR magnets. Nb3Sn
has the potential to operate in the 10-to-15-T range, but it has one disadvantage: once formed it
becomes brittle and its properties are strain sensitive. The brittleness and strain sensitivity of

                                                                                   15 March 2006

Nb3Sn have so far limited its use to specific niche applications and require a complete rethinking
of all manufacturing processes currently used for NbTi conductors and magnets. However, over
the last decade, significant progress has been made thanks to the ITER Engineering Design
Activities and to vigorous accelerator R&D programs carried out mainly in the USA which have
led to a factor 3 to 4 increase in critical current density at 4.2 K and 12 T and to the successful
test of a short and small-aperture magnet model at fields up to 16 T. Although the Nb3Sn
technology has not yet been brought to full maturity, its proven potential to open a new
parameter space for IR optics design is particularly attractive and fully justifies thorough and
well focus R&D programs. This all the more true because we only need a limited number of
these magnets.

Bringing a new technology to maturity is always a challenge. It requires clearly defined goals
and milestones, supported by adequate resources. Working backwards from 2015, this means
that the viability of Nb3Sn technology for high-field and/or high-field-gradient dipole and
quadrupole magnets should be demonstrated by 2010. Hence, the top priorities in terms of
superconducting accelerator magnet R&D are
                      • to invest in the development of homogeneous, reliable and high
                         performance Nb3Sn wires and cables in suitable unit lengths (with at
                         least two manufacturers around the world),
                      • to design, build and cold test several short, large aperture (~90 mm),
                         high-field and high-field-gradient dipole and quadrupole magnet models
                         (~15 T conductor peak field),
                      • to demonstrate the scalability of Nb3Sn technology to multi-meter long
                         accelerator magnets.
In addition, detailed studies must be carried out to characterize temperature margins and
optimize cooling schemes of IR magnets.

In the USA, the Department of Energy has promoted efforts on Nb3Sn R&D for several years.
These activities are now funded and coordinated through the US-LHC Accelerator Research
Program (LARP). LARP is a consortium of four national laboratories –BNL, LBNL, Fermilab,
and SLAC. The primary magnet R&D goal of LARP is to design build and cold test by 2009 at
least one 4-m-long, 90-mm-aperture, 200 T/m (~12 T conductor peak field) quadrupole magnet
prototype and short, 90-mm-aperture, 250 T/m (~15 T conductor peak field) quadrupole magnet
models. In pursuit of this goal, LARP supports the production of Nb3Sn wires at one US vendor.
The LARP effort to demonstrate the feasibility of long Nb3Sn quadrupole magnets is vigorously
encouraged by CERN.

In parallel, a consortium of 7 European institutes (CCLRC/RAL in the UK, CEA in France,
CERN, CIEMAT in Spain, INFN in Italy, Twente University in the Netherlands and Wroclaw
University of Technology in Poland) is working on the so-called Next European Dipole (NED).
This Joint Research Activity is embedded in the Coordinated Accelerator Research Project in
Europe (CARE) project partly funded by the European Commission. The initial goal of NED
was to design, build and cold test by 2008 a 1.5-m-long, 88-mm-aperture, 15-T-conductor peak
field dipole magnet model. Thus, NED and LARP goals are fully compatible and
complementary. Rather than competing, NED and LARP goals are synergistic –each supports
the other. NED development of Nb3Sn technology is also vigorously encouraged by CERN.

                                                                                  15 March 2006

Unfortunately, the NED funding was capped at only 25% of the requested budget. This forced a
dramatic re-scoping. At present, NED is limited to
                    • Nb3Sn conductor development and characterization in collaboration with
                        European industry (aiming at a non-copper critical current density of
                        1500 A/mm2 at 4.2 K and 15 T and an effective filament diameter of
                        50 μm),
                    • development and characterization of Nb3Sn conductor insulation
                        (including heat transfer measurements in superfluid helium environment),
                    • conceptual magnet design studies.
The initial phase of the NED Activity will come to an end in early 2007. It is of strategic
interest that the missing resources be made available through to the end of 2009, so that the
NED collaborators can complete the detailed design, manufacturing and test of at least one
dipole magnet model. This would maintain and develop European capabilities. It would also
complement and crosscheck LARP activities.

The timely and successful completion of the LARP and the NED programs will be instrumental
(and be mandatory) to get prepared for the very challenging requirement of an LHC IR upgrade.
Their successful conclusion (by 2010) will enable a wise strategic decision to be made about
whether Nb3Sn magnet technology should be used. Once that decision is made, an aggressive
program should be put in place to finalize the design, and to produce, cold test and implement
the required IR magnets. A strategy will be devised in due time on how to share the magnet
production between European and non-European partners.

Exploring the Limits of Cycled NbTi Magnets for a potential LHC Injector Chain

In addition to present CARE/NED and LARP activities, a number of other options can be
considered to raise the LHC luminosity, such as increasing the number of bunches, reducing the
bunch size, increasing the number of protons per bunch and so on. However, there are
limitations on how far these parameters can be pushed, such as the beam-beam limit and the
long-range beam-beam interactions, the electron cloud effects, the implication on collimations
and machine protection, pile up of events in the experiments and so on. These limitations are not
well understood at present and can only be explored during machine operation. Machine
commissioning experience will also provide a clear assessment of the level of criticality of the
so-called dynamic field effects generated in the main superconducting magnets at the beginning
of the acceleration ramp. The sum of these considerations may reveal a need for an upgrade of
the LHC injector chain, possibly including the addition or the replacement of one of the
injectors by a superconducting ring.

At the present time, it is too early to clearly identify details of the machine parameters and
magnet performances that will be needed, except that the field requirements should be within the
reach of NbTi technology and that a new injector will be fast-cycling. Hence, the associated
magnet R&D program should not be too narrowly focused, but should leave all options open.
For the time being, the program should concentrate on generic tasks. Also, it should be carried

                                                                                      15 March 2006

out in synergy and in complement to other fast cycling, NbTi accelerator magnet programs, such
as FAIR at GSI (Germany) and NTA_DISCORAP at INFN (Italy).

For the next few years, the generic tasks that are foreseen as useful extensions of ongoing efforts
                  • to promote the development of ultra-low-loss NbTi wires and cables and
                       to set up standardized measurement procedures and facilities to
                       characterize the AC performances of such conductors,
                  • to carry out comparative studies of magnet cooling schemes and detailed
                       investigations of heat transfer to supercritical helium,
                  • to develop suitable quench detection and quench protection systems for
                       fast-cycling strings of magnets,
                  • to adapt at least one horizontal magnet test bench of an existing European
                       facility to supercritical helium operation and to carry out an extended cold
                       test of a FAIR-type prototype (including magnetic measurements at high
                       ramp rate and a large number of powering and thermal cycles) in order to
                       explore the limits of the present technology.
Relevant LHC magnet tooling and equipment that could be used and/or adapted to manufacture
fast-cycled, NbTi magnet models and prototypes should be kept available in order to reduce lead
time and costs and to be ready to react promptly when needed.

When the results of the LHC commissioning starts to come in and exploration of the machine
limits will start, a Task force should be put in place to develop a coherent plan for an LHC
injector chain upgrade and a roadmap for superconducting magnet development and production.

Accelerator Magnet Design and Optimization

It is of the utmost importance to maintain up-to-date and experienced electromagnetic,
mechanical and thermal design capabilities and to pursue the development of existing magnet
design tools. The ongoing effort within the NED Activity that is aimed at comparing various
large-aperture, high-field dipole magnet configurations should be extended to large-aperture,
high-field-gradient quadrupole magnet configurations. When this happens, a joint working
group should be set up between the European collaboration and LARP to enforce cooperation. A
similar comparative study should be launched for fast-cycled accelerator magnet designs.

It is also extremely critical to develop a methodology, to identify and/or set up facilities and to
carry out representative and systematic radiation studies on critical magnet components.


The authors wishes to thank the many people who contributed actively to the elaboration of this
strategy, in particular: A. Dael, O. Napoly, B. Mansoulié (CEA), J.P. Koutchouk, P. Lebrun and
D. Leroy (CERN). The authors also thank B. Baudouy, L. Bottura, P.J. Limon, L. Oberli,
J.M. Rifflet and E. Todesco for helpful discussions.


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