LHC Collimation Requirements

1
LHC Collimation Requirements

• Ralph W. Aßmann
• CC-2005
•  CARE-HHH-APD mini-Workshop onCrystal
  Collimation in Hadron Storage Rings
• CERN March 7th-8th, 2005

2
Outline

• Why do we need beam cleaning for the LHC?
• Phase 1 cleaning and collimation system
• Crystals for phase 2 of LHC collimation?
• Conclusions

3
Why do we need beam cleaning for the LHC?

• The LHC machine
• Physics ? High luminosity at high
  energy Great discovery potential!
• Accelerator design ? Handling of ultra-intense
  beams in a super-conducting
  environment Great risk of quenching damage!

Control losses 1000 times better than present
state-of-the-art!
4
Running at the Quench Limit
Quench threshold (7.6 106 p/m/s _at_ 7 TeV)
Allowed intensity
Illustration of LHC dipole in tunnel
Cleaning inefficiency Leakage rate Number of
escaping p (gt10s) Number of impacting p (6s)
Beam lifetime (e.g. 0.2 h minimum)
Dilution length (50 m)
Collimation performance can limit the intensity
and therefore LHC luminosity. Efficiency should
be better than 99.9.
5
Two-Stage Cleaning
Beam propagation
Core
Diffusion processes 1 nm/turn
Primary halo (p)
Secondary halo
p
p
p
Tertiary halo
Impact parameter 1 mm
p
e
Primary collimator
p
Secondary collimator
Shower
e
Sensitive equipment
Shower
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Beam Loss Specification

• The collimation system was designed based on the
  following assumptions on loss rates
• Loss rates based on experience. Not too
  conservative Peak loss at 7 TeV is 1 of beam in
  10s!
• Supported by external review, taking into account
  Tevatron, HERA and RHIC experience!

7
Allowable Intensity in the LHC
For peak loss rates 0.1 h lifetime at
injection 200 kW 5 loss in first s of ramp 1
MW 0.2 h lifetime in collision 500 kW
8
Outline

• Why do we need beam cleaning for the LHC?
• Phase 1 cleaning and collimation system
• Crystals for phase 2 of LHC collimation?
• Conclusions

9
Phase 1 cleaning and collimation system

• LHC collimation system has been redesigned in the
  last two years!
• In view of tight LHC boundary conditions the
  following decisions were taken
• Rely on proven multi-stage cleaning process.
• Rely on conventional collimators with advanced
  features for the start-up of the LHC (phase 1).
• Integrate space reservations into the LHC layout
  for later test/installation (phase 2) of advanced
  concepts for cleaning (advanced collimators,
  crystals, )
• Layout optimized for phase 1 of collimation.

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The LHC Cleaning Insertions
Two warm LHC insertions dedicated to
cleaning IR3 ? Momentum cleaning IR7 ?
Betatron cleaning
11
Machine Layout IR7 (V6.5)
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Phase 1 From halo tracking to losses
M. Brugger et al
Where does the energy go ? Fluka
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Energy flux and dose in IR7
K. Tsoulou et al
IP
RR73
UJ76
RR77
Flux (cm-2/y)
4 orders of magnitude
NoAbsorbers
A6vC6Eh6v Absorbers
beam1
beam2
A6v
E6v
E6v
C6h
A6v
C6h
0.1 MGy/y
Dose (Gy/y)
Mean values 2m horizontally and 1m vertically.
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Power flow IR3, t 1h , Ptot 90kW
J.B. Jeanneret, I. Baishev

• Need active and passive absorbers to limit load
  on auxiliary systems
• Consequences for vacuum ...

15
The LHC phase 1 collimator
Beam passage for small collimator gap with RF
contacts for guiding image currents
Designed for maximum robustness Advanced CC jaws
with water cooling!
Vacuum tank with two jaws installed
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Robustness Test with Beam
Take and hit each jaw 5 times!
450 GeV 3 1013 p 2 MJ 0.7 x 1.2
mm2 equivalent Full Tevatron beam ½ kg TNT
C-C (left) and C (right) jaws after impact
No sign of any damage! Required robustness was
demonstrated!
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Damage Limits in Hardware Design

• Danger to regular machine equipment and metallic
  absorbers
• Above 1e12 p at injection 4e-3 of beam
• Above 5e9 p at 7 TeV 2e-5 of beam
• Danger to C-C collimators/absorbers
• Above 3e13 p at injection 10 of beam
• Above 8e11 p at 7 TeV 3e-3 of beam
• Maximum allowed loss rates at collimators (goal)
• 100 kW continuously.
• 500 kW for 10 s (1 of beam lost in 10s).
• 1 MW for 1 s.

Crystals?
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Impedance Limit for Movable Devices

• Collimators and absorbers are close to beam A
  resistive wall impedance is induced (gap size
  depends on b)!
• C-C material has reduced electrical conductivity
  (price to pay for a robust system). Fix with
  phase 2 advanced collimators.
• Increase from collimators (nominal settings) for
  the imaginary part of the effective vertical
  impedance
• 8 kHz factor 3 for injection factor 69 for
  7 TeV
• 20 kHz factor 3 for injection factor 145
  for 7 TeV
• Large increase in impedance must be actively
  counteracted by transverse feedback and
  octupoles!

19
Outline

• Why do we need beam cleaning for the LHC?
• Phase 1 cleaning and collimation system
• Crystals for phase 2 of LHC collimation?
• Conclusions

20
Crystals for phase 2 of LHC collimation?

• Requirements for phase 2 of LHC collimation
• Improve cleaning efficiency!
• Reduce collimator-induced impedance!
• Maintain robustness and operational
  reliability!
• Any solution that helps in these goals is very
  welcome! We can also imagine a combination of
  different technologies!
• Can crystals help to achieve the phase 2 goals?

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Two-Stage Conventional Cleaning
Beam propagation
Core
Diffusion processes 1 nm/turn
Primary halo (p)
Secondary halo
p
p
p
Tertiary halo
Impact parameter 1 mm
p
e
Primary collimator
p
Secondary collimator
Shower
e
Sensitive equipment
Shower
22
A possible crystal collimation scheme?
Beam propagation
Core
Diffusion processes 1 nm/turn
Primary halo (p)
Crystal
Impact parameter 1 mm
Shower
p
p
Collimator-like object
Absorber
Sensitive equipment
e
Primary halo directly extracted! No secondary and
tertiary halos!?
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Scattering properties collimator
Primary collimators (0.2m) give typical
scattering angle of 2 mrad! Wide tails in
strength of kick (deflect particles onto
secondary collimators). If small deflection Come
back after some turns onto primary! What is the
kick probability spectrum from the crystal?
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Large deflections can be bad!
Much stronger kick at injection!
25
Possible vertical collimator set-tings during
in-jection, ramp and top energy
Afterb squeeze
Smaller leakage rate with E increase!
26

• Larger deflections at injection result in higher
  leakage rates (worse performance)!
• Why?
• System was optimized for top energy (7 TeV)
  scattering Secondary collimators at optimal
  phase locations for 7 TeV kicks.
• Similar system must be designed for crystals
  Need to know
• How many absorbers?
• Where to place them and for what energy?
• How to handle collimation for different
  energies?

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Conclusions

• Crystals are an interesting advanced technology
  for phase 2 of LHC collimation.
• To evaluate its benefit in detail the following
  information is required
• Damage limit of crystal for instantaneous shock
  beam impact (expect 3MJ, 0.21.0 mm, 200 ns).
• Damage limit of crystal for integrated dose
  (expect 51016 p/year at 7 TeV).
• Handling of crystal during normal operation 500
  kW power impact. Heating problems and need for
  cooling?
• Probability spectrum for proton deflections
  (channeling and others) for energies from 450 GeV
  to 7 TeV. Include all effects down to 10-5
  probability!
• Number, opening (impedance) and locations of
  absorbers for extracted and scattered beam. How
  do the absorbers look like?
• How to handle different LHC energies with
  crystals to ensure efficient cleaning from 450
  GeV to 7 TeV?
• Sensitivity to beam angle and angular spread?
• Requirements for alignment and operational set-up
  (tolerances, time, )?

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Conclusions continued

• Can we collect this information by the end of
  this workshop?
• Detailed simulations must show the benefit of the
  crystal approach (include proton simulations,
  showers from crystals and absorbers). Done?
• Experimental test is important. In the SPS
• What aspects can be tested in the SPS?
• Channeling efficiency is no good measure of
  cleaning efficiency. E.g. if 95 is channeled
  where do the 5 other particles go? We care at
  least on the 0.1 level!
• Can we measure cleaning efficiency with crystals
  in the SPS?
• How much time is required to establish crystal
  collimation?
• When should such a test be done? Mandatory for
  2006?
• When could we arrive at a detailed evaluation for
  a crystal collimation system?
• Space has been integrated into the LHC for a
  phase 2 upgrade!

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Collimator Specification
Driving criteria for material was robustness ?
Carbon-carbon Resistivity (7-25 mOm)Short lead
times
0.5
0.5