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									                     Crystal Collimation
                        for the LHC

                                      R. Assmann (CERN)
                                   V. Previtali (CERN, EPFL)
                                for the LHC Collimation Project
                                         and CERN/AB
                              Crystal Collimation Workshop, Fermilab
RWA&VP, FNAL 12/07
              LHC Collimation Staged Concept
• The LHC collimation system has been proposed and is being
  implemented as a staged system:
     – Phase 1: Collimation and machine protection functionality for beam
       intensities up to 40% of nominal.
     – Accept predicted limitations in cleaning efficiency and impedance
       during the first few years of LHC operation.
     – Phase 2: Upgrade of the system for requirements of nominal &
       ultimate parameters and beyond.
     – Goals:
          • Gain factor 10 in efficiency.
          • Reduce impedance by factor 4 or more.
     – Several routes will be considered in parallel to achieve these goals.
     – Most likely: Several smaller improvements will bring required gain.
RWA&VP, FNAL 12/07                                                             2
                            The Staged LHC Path
                                      Energy density          Stored energy           Number of
                                       at collimators           in beams              collimators
                                       (nominal 7 TeV)

 State-of-the-art in SC colliders
                                        1 MJ/mm2                   2 MJ

 Phase 1 LHC Collimation               400 MJ/mm2                150 MJ *                    88

 Nominal LHC                            1 GJ/mm2                  360 MJ                     122

 Ultimate & upgrade scenarios          ~4 GJ/mm2                 ~1.5 GJ                 ≤ 138

 Limit (avoid metal damage or
                                       ~50 kJ/mm2            ~10-30 mJ/cm3

                      * Limited by cleaning efficiency (primary) and impedance (secondary)
RWA&VP, FNAL 12/07                                                                                  3
                     Specified Allowed Proton
                     Losses with Collimation
• Collimators are the LHC defense against unavoidable losses:
     – Irregular fast losses and failures: Passive protection.

     – Slow losses: Cleaning and absorption of losses in super-conducting

     – Radiation: Managed by collimators.

     – Particle physics background: Minimized.

• Specified 7 TeV peak beam losses (maximum allowed loss):

     – Slow:              0.1% of beam per s for 10 s             2.1 MW

     – Transient:         5 × 10-5 of beam in ~10 turns (~1 ms)   83.3 MW
                                                                  20 MW

     – Accidental:        up toMJMJ in 200 ns into 0.2 mm2
                           4.2 1                                  20.8 TW
                                                                  5 TW

                           Upgrade                                          4
RWA&VP, FNAL 12/07
                            CERN White Paper
                           Implementation Plan
• There is no “magic bullet” for gaining a factor 10 in performance without
  major side effects.
• All proposed improvements require careful R&D on principle and
• Two phases in implementation:
     – Up to 2010/11:         R&D on phase 2 secondary collimators and advanced
     – 2010 – 2012:           Production and installation.
     – 2012:                  Operational usage of first collimator upgrades.
• Important milestones:
     – 2010:         Tests of phase 2 secondary collimators in the LHC.
     – 2011:         Tests of advanced collimation in the LHC (this includes crystals).

RWA&VP, FNAL 12/07                                                                        5
               Collimation R&D Scope for LHC
• Phase 2 secondary collimators for warm regions (WP, LARP, FP7):
     – Prototype from SLAC/LARP.
     – Prototypes from CERN.
• Cryogenic collimators for dedicated places or regions (FP7 – collaboration
  with GSI in Germany).
     – Developed and built for the FAIR project (collimation in SC ring).
     – Adapt to LHC parameters if cryogenic collimators are required.
     – Requires shorter and higher field SC magnets.
• Other options pursued (no LHC prototyping or construction agreed yet):
     – Non-linear collimation (FP7).
     – E-lens scraper for the LHC (tbd).
     – Crystal-enhanced collimation (WP, LARP, FP7):
     – Magnetic collimators (tbd).

RWA&VP, FNAL 12/07                                                          6
                       Tevatron Input Crucial
• LHC should start from the present state-of-the-art and extrapolate into
  unknown territory: Tevatron lessons are essential!
• Examples:
     – Maximum loss rates.
     – Collimation set-up and tuning.
     – Electron lens effect on losses.
• Concerning crystal-enhanced collimation:
     – Does crystal-enhanced collimation reliably work with stored beam and
       diffusive losses?
     – What is the gain in efficiency?
     – Lessons of using crystals in the environment of a high energy physics collider.
• CERN Accelerator&Beam department and EPFL participate through
  existing LHC collimation project – LARP connection. Important studies…

RWA&VP, FNAL 12/07                                                                       7
                                 The Concept…

                                                                                                                                           Beam axis
                  Beam propagation                                                     Impact                                              Collimator
  Core                                                                                 parameter

                   Unavoidable losses                                                 1.          Phase 2 materials for system improvement.
                                                                                      2.          Crystals AP under study (surface effects,
 halo (p)                       Secondary                                                         dilution, absorption of extracted halo).
                              p halo
             Crystal                 p                                              Shower
                                                                                                                       Tertiary halo
 Impact                                                                               p
parameter                                                                                                               p

 ≤ 1 mm

                                    Phase 1 Colli- 1 Colli-

                                                              Hybrid Colli-

                              e p
                                                              mator TCSM
                                             mator TCSG

                           Shower                                                          Absorber                                          SC magnets

                                                                                                         Super-                              and particle
                                     mator TCSG

                                                                                                       conducting                            physics exp.

              CFC &                 CFC Phase 2                                        W/Cu                                  W/Cu
             Crystal                    material                               Low electrical resistivity, good absorption, flatness, cooling, radiation, …8
RWA&VP, FNAL 12/07
             “Phase 1”
                         System Design



“Final” system:
Layout is 100%
                                         C. Bracco

RWA&VP, FNAL 12/07                                       9
                         Possible Crystal
                     Implementation into LHC
• Reference (further work ongoing):
        R. Assmann et al, “Optics Study for a Possible Crystal-Based Collimation
        System for the LHC”. EPAC06.
• LHC requires efficient collimation during all stages of operation (injection
  – ramp – physics – dump).
• Crystals can only be placed close to primary collimators in IR3 and IR7.
• In this case, we have two scenarios:
     – Crystal not tuned up: “Conventional” collimators will establish multi-stage
     – Crystal tuned up: Halo will be extracted to dedicated downstream dump (up to
       2 MW over 10 s).
• Beam divergence at crystal varies with beam energy (present optics):
     – Adjustment required during energy ramp to maintain crystal enhancement.

RWA&VP, FNAL 12/07                                                                   10
                     Beam Divergence at Crystal

RWA&VP, FNAL 12/07                                11
                                           Downstream Trajectory for
                                              Crystal Channeling
  normalized amplitude (sigma)

                                 20                                                   Amplitude
      Normalized offset [s]


                                                            50 urad
                                                20 urad
                                           0 urad
                                 -10                                                  Phase shift


                                       0     100      200           300   400   500
                                                            s (m)

RWA&VP, FNAL 12/07                                                                                  12
                     Crystal Effect: Increase of
                     Amplitude and Phase Shift

                                                   7 TeV
                                                    crit ~ 2-2.5 urad
                                                    v.r. ~ 3-4 urad
                                                    chan ~ 50 urad

RWA&VP, FNAL 12/07                                                       13
              Phase Advance Versus Distance

       Required dumps:   ~ 70 m downstream of crystal for channeling
                         ~ 230 m downstream of crystal for reflection

RWA&VP, FNAL 12/07                                                      14
                     Considered Parameters
• Reflection:
     – Reflection angle:     - 4 mrad (~1.5 times critical angle)
     – Amplitude increase:   6  6.3 s
     – Opt. phase advance:   160 degrees
     – Distance to dump:     ~ 230 m
• Channeling:
     – Channeling angle:     50 mrad
     – Amplitude increase:   6  27 s
     – Opt. phase advance:   77 degrees
     – Distance to dump:     ~ 70 m

RWA&VP, FNAL 12/07                                                  15
                     Energy Deposition (FLUKA)


                                                                                        FLUKA team

           Primary   Dump channeled   Dump reflected     Dump channeled   Primary
       Collimators      particles       particles           particles     Collimators

RWA&VP, FNAL 12/07                                                                                   16
             Consequences of Diffusive Impact

RWA&VP, FNAL 12/07                              17
            Always Channeling and Reflection?
• Diffusion generates exponential distribution of impact parameter from
  zero to typically 100-200 nm.
• A small fraction of halo will always see the channeling, even if crystal is
  oriented for reflection. Same is true vice versa.
• Need to install dumps (absorbers) for both processes.
• Further understanding required to quantify effects.

RWA&VP, FNAL 12/07                                                              18
                     Multi-Reflection Issues

                                               Assume ideal crystal:
                                                   2.5 mm length,
                                                  15 mrad@450 GeV
                                                  refl angle.
                                               Multi-crystal assembly:
                                                   8 crystals with
                                                  1 mm interdistance.

RWA&VP, FNAL 12/07                                                   19
                Issues for LHC Implementation
• Several issues have been identified for crystal implementation into the
  LHC (some of them illustrated in previous slides):
     – Halo beam dynamics with crystals (effect of small impact parameter).
     – Dump of extracted energy (up to 2 MW over 10 s).
     – Surface effects (interested in 1-100 nm regime).
     – Implementation into high radiation zone (radiation damage).
     – Operational stability and efficiency (change in divergence).
     – Multi-turn turn behavior and associated issues (e.g. heating of crystal).
• Simulations ongoing for multi-turn collimation with crystals (crystal routine
  implemented into CERN tracking code). However, basic uncertainties.
• Tevatron is the best place to experimentally assess basic feasibility of
  crystal-enhanced collimation.
• LHC efforts on crystal will depend on outcome at Tevatron.

RWA&VP, FNAL 12/07                                                                 20
                           Tevatron Experiments
•   Results that we are interested in:
     – Accurate geometry of bent crystal measured.
     – Measured change in multi-turn beam halo distribution with crystal channeling and
       reflection (resolution 10-4 level).
     – Measured beam loss maps around the ring in different regimes.
     – Measured temperature in the crystal and/or its holder.
•   Tests proposed:
     – Change diffusion speed and therefore impact parameter on crystal (allows determining
       impact of amorphous layer, ratio channeling/reflection).
     – Change multi-turn dynamics by changing collimator settings.
     – Calibrate absolute loss rates with known intensity loss.
     – Check on “spiky” loss behavior when crystal close to beam?
     – Evaluate crystals with different bent angle, length, material, ...

RWA&VP, FNAL 12/07                                                                            21
            Contributions from LHC Collimation
• Multi-turn simulations of crystal-enhanced collimation with different
  diffusion models and parameters (will be done for LHC – numerical
  models can also be used for Tevatron).
• Comparison with simulations results for Tevatron from other codes.
• Participation in experiments and in analysis of experimental data,
  including comparison with simulation results.
• Support for measurements of beam loss around the Tevatron (might
  include participation from CERN beam instrumentation group, including
  LHC hardware – if needed and if agreed).
• Information exchange concerning precision movement of material inside
  the vacuum (as done for LHC phase 1 collimators) – if needed.
• Evaluation of results for possible LHC implementation of crystals.
• People: R. Assmann, V. Previtali, S. Redaelli
  (maybe + BI if requested and agreed)
RWA&VP, FNAL 12/07                                                        22
•   Crystals could possibly enhance the efficiency of the LHC collimation system.
•   Basic feasibility in a storage ring with diffusive beam loss must still be proven.
•   There are a number of issues to be addressed in order to arrive at a system
    design for crystal-enhanced collimation in LHC.
•   We are working on these issues but many input parameters are not fully defined
    and require experimental input. Several questions mentioned in this talk.
•   Tevatron can perform the needed experiments and clarify key issues and input
•   Possible contributions from the LHC collimation team have been defined and
•   Looking forward towards productive years on studying crystal collimation. Results
    will determine LHC effort on this direction (goal are beam tests in 2011).
•   Thanks to N. Mokhov and Fermilab for providing this unique opportunity.

RWA&VP, FNAL 12/07                                                                       23
                     Reserve Slides

RWA&VP, FNAL 12/07                    24
                  Diffusion Process & Impact Parameter

 Slow loss:            Beam lifetime: 0.2 h     Loss rate:       4.1e11           p/s
                                                Loss in 10 s:    4.1e12           p     (1.4 %)
 Uniform “emittance”
                                                                 (~ 40 bunches)
                       Assume drift:      0.3   sig/s
                                          5.3   nm/turn          (sigma = 200 micron)

                                                                Transverse impact

                                                                Almost all particles
                                                                impact with

                                                                y ≤ 0.2 mm


RWA&VP, FNAL 12/07                                                                                25
                      pp, ep, and ppbar collider history                Higgs +
                                                                        SUSY + ???
                                                                        ~ 80 kg TNT
                                                                        Machine Pro-
                                                  1992                  tection

                                                     SC magnets
                    1971                             1987


The “new Livingston plot“ of proton colliders: Advancing in unknown territory!
A lot of beam comes with a lot of garbage (up to 1 MW halo loss, tails, backgrd, ...)
 Collimation. Machine Protection.
             Comparison: Phase 1 Collimators

RWA&VP, FNAL 12/07                             27
                     LHC Need for Collimation
• Ideally, stored proton beams would have infinite lifetime and no protons
  would be lost.
• However, a multitude of physical processes will limit the lifetime of the
  beams and unavoidable proton losses must be taken into account.
• Conditions for quenching a SC magnet:

     – Transient loss of 10-9 fraction of beam (within 10 turns)

     – Slow loss of 3×10-8 fraction of beam per s and per m (< 10000 h lifetime)

• Proton losses must be intercepted and absorbed by specifically
  designed devices, namely collimators. These constrain the aperture.
• Multi-turn process: protons diffuse to limiting aperture bottleneck. Process
  also called beam cleaning.
• 2 out of 8 straight sections in the LHC are dedicated to collimation!

RWA&VP, FNAL 12/07                                                                 28
                     LHC Collimator Gaps

                                           Collimator settings:

                                           5 - 6 s (primary)
                                           6 - 9 s (secondary)

                                           s ~ 1 mm (injection)
                                           s ~ 0.2 mm (top)

                                           Small gaps lead to:

                                           1. Surface flatness
                                              tolerance (40 mm).

                                           2. Impedance

                                           3. Mechanical
                                              precision demands
                                              (10 mm).

RWA&VP, FNAL 12/07                                                 29
                     Required Efficiency

  Allowed               Quench threshold
  intensity             (7.6 ×106 p/m/s @ 7 TeV)
                                                            Illustration of LHC dipole in tunnel

  N    max
       p          Rq  Ldil /c                   Cleaning inefficiency
                                                   Number of escaping p (>10s)
                                                    Number of impacting p (6s)
     Beam lifetime              Dilution
     (e.g. 0.2 h minimum)       length
                                (~10 m)

  Collimation performance can limit the intensity and therefore
  LHC luminosity.

RWA&VP, FNAL 12/07                                                                            30
           Intensity Versus Cleaning Efficiency

                                              For a 0.2 h
                                              minimum beam
                                              lifetime during
                                              the cycle.

                            99.998 % per m efficiency
RWA&VP, FNAL 12/07                                              31
                  The LHC Phase 1 Collimation
•   Low Z materials closest to the beam:
     – Survival of materials with direct beam impact
     – Improved cleaning efficiency
     – High transparency: 95% of energy leaves jaw
•   Distributing losses over ~250 m long dedicated cleaning insertions:
     – Average load ≤ 2.5 kW per m for a 500 kW loss.
     – No risk of quenches in normal-conducting magnets.
     – Hot spots protected by passive absorbers outside of vacuum.
•   Capturing residual energy flux by high Z absorbers:
     – Preventing losses into super-conducting region after collimator insertions.
     – Protecting expensive magnets against damage.
•   No shielding of collimators:
     – As a result radiation spread more equally in tunnel.
     – Lower peak doses.
     – Fast and remote handling possible for low weight collimators.

RWA&VP, FNAL 12/07                                                                   32
                                       Efficiency in Capturing Losses

                                                                                             Beam1, 7 TeV
                                                                                             Betatron cleaning
                                                                                             Ideal performance
Local inefficiency [1/m]

                                              99.998 % per m                                 Quench limit
                                                                                             (nominal I, =0.2h)

                                                                                             Beam2, 7 TeV
                                                                                             Betatron cleaning
                                                                    TCDQ                     Ideal performance
                                            99.998 % per m
                                                                                             Quench limit
                                                                                             (nominal I, =0.2h)

                                                                                             99.998 % needed

                      Local inefficiency: #p lost in 1 m over total #p lost = leakage rate      99.995 %
RWA&VP, FNAL 12/07                                                                                                 33
                        5) Beyond Phase 1
• The LHC phase 1 system is the best system we could get within the
  available 4-5 years.
• Phase 1 is quite advanced and powerful already and should allow to go a
  factor 100 beyond HERA and TEVATRON.
• Phase 2 R&D for advanced secondary collimators starts early to address
  expected collimation limitations of phase 1.
• Phase 2 collimation project was approved and funded (CERN white
  paper). Starts Jan 2008. Should aim at complementary design compared
  to SLAC.
• Collaborations within Europe through FP7 and with US through LARP are
  crucial components in our plans and address several possible problems.
• We also revisit other collimation solutions, like cryogenic collimators,
  crystals, magnetic collimators, non-linear schemes.

RWA&VP, FNAL 12/07                                                           34
                          Draft Work Packages
                     White Paper (WP), Europe (FP7), US (LARP)

WP1 (FP7)                  –   Management and communication

WP2 (WP, FP7, LARP)        –   Collimation modeling and studies

WP3 (WP, FP7, LARP)        –   Material & high power target modeling and tests

WP4 (WP, FP7, LARP)        –   Collimator prototyping & testing for warm regions

                 Task 1    –   Scrapers/primary collimators with crystal feature

                 Task 2    –   Phase 2 secondary collimators

WP5 (FP7)                  –   Collimator prototyping & testing for cryogenic regions

WP6 (FP7, LARP)            –   Crystal implementation & engineering

RWA&VP, FNAL 12/07                                                                      35

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