1. LHC Collimation Requirements
Ralph W. Aßmann
CC-2005
 CARE-HHH-APD mini-Workshop on Crystal Collimation in Hadron Storage Rings
CERN March 7th-8th, 2005
R. Assmann

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
R. Assmann

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!
Factor ~ 200
Control losses ~ 1000 times better than present state-of-the-art!
R. Assmann

4. Running at the Quench Limit
Allowed
intensity
Quench threshold
(7.6 ×106 7 TeV)
Illustration of LHC dipole in tunnel
Cleaning inefficiency
Leakage rate =
Number of escaping p (>10s)
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%.
R. Assmann

5. Two-Stage Cleaning Core Primary halo (p) Secondary halo p p p
Beam propagation
Core
Diffusion
processes
1 nm/turn
Primary
halo (p)
Secondary halo p p p
Tertiary halo
Impact parameter
≤ 1 mm p e
collimator
Primary p
Secondary
collimator
Shower e
Sensitive
equipment
Shower
R. Assmann

6. 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!
R. Assmann

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
R. Assmann

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
R. Assmann

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.
R. Assmann

10. The LHC Cleaning Insertions
Two warm LHC insertions dedicated to cleaning:
IR3  Momentum cleaning
IR7  Betatron cleaning
R. Assmann

11. Machine Layout IR7 (V6.5)
R. Assmann

12. Phase 1: From halo tracking to losses
M. Brugger et al
Where does the energy go  Fluka
R. Assmann

13. Energy flux and dose in IR7
K. Tsoulou et al
RR73 IP
UJ76
RR77
Flux (cm-2/y)
NoAbsorbers
4 orders
of magnitude
A6vC6Eh6v Absorbers
beam1
beam2
E6v
A6v
C6h
A6v
C6h
E6v
0.1 MGy/y
Dose (Gy/y)
Mean values ± 2m horizontally and ± 1m vertically.
R. Assmann

14. Power flow IR3, t = 1h , Ptot = 90kW
Q7L
Q7R
PRIM
SEC
ABS
VAC 8%, 7kW
Warm Magnets 60%, 54 KW
3% , 2.6 kW
7% , 7 kW
Side leakage 20%, 19 kW
F’wd leakage
1%, 1 kW
J.B. Jeanneret, I. Baishev
Need active and passive absorbers to limit load on auxiliary systems
Consequences for vacuum ...
R. Assmann

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
R. Assmann

16. Robustness Test with Beam
Take:
… and hit each jaw 5 times!
450 GeV 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
C-C jaw
C jaw
TED Dump
No sign of any damage!
Required robustness was demonstrated!
R. Assmann

17. 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?
R. Assmann

18. 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!
R. Assmann

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
R. Assmann

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?
R. Assmann

21. 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
collimator
Primary p
Secondary
collimator
Shower e
Sensitive
equipment
Shower
R. Assmann

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!?
R. Assmann

23. 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?
R. Assmann

24. Large deflections can be bad!
Much stronger kick at injection!
R. Assmann

25. Smaller leakage rate with E increase!
Possible vertical collimator set-tings during in-jection, ramp and top energy:
After b squeeze
Smaller leakage rate with E increase!
R. Assmann

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?
R. Assmann

27. 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, 0.2×1.0 mm, 200 ns).
Damage limit of crystal for integrated dose (expect ~5×1016 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, …)?
R. Assmann

28. 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!
R. Assmann

29. Collimator Specification
Driving criteria for material was robustness:
 Carbon-carbon
Resistivity (7-25 mΩm) Short lead times
0.5
0.5
R. Assmann