1. CERN, Geneva, Switzerland PAC 2003 – Portland, Oregon, USA
DESIGNING AND BUILDING A COLLIMATION SYSTEM FOR THE HIGH-INTENSITY LHC BEAM
R.W. Aßmann for the Collimation Team
O. Aberle, R. Aßmann, M. Brugger, L. Bruno, H. Burkhardt, E. Chiaveri, B. Dehning, A. Ferrari, J.B. Jeanneret, M. Jimenez, V. Kain, M. Lamont, F. Ruggiero, R. Schmidt, P. Sievers, V. Vlachoudis, L. Vos, J. Wenninger,
CERN, Geneva, Switzerland
I. Baishev, IHEP, Protvino, Russia
D. Kaltchev, TRIUMF, Canada
PAC 2003 – Portland, Oregon, USA
RWA PAC03

2. The LHC Challenge for Collimation
High stored beam energy ~ 350 MJ/beam
(melt 500 kg Cu, required for 1034 cm-2 s-1 luminosity)
Small spot sizes at high energy 200 mm (at coll.)
(small 7 TeV emittance, no large beta in restricted space)
Large transverse energy density 1 GJ/mm2
(beam is destructive, 3 orders beyond Tevatron/HERA)
High required cleaning efficiency % (~ 10-5)
(clean lost protons to avoid SC magnet quenches)
Collimation close to beam s
(available mechanical aperture is at ~10 s)
Small collimator gap ~ 3 mm (at 7 TeV)
(impedance problem, tight tolerances: ~ 10 mm)
Activation of collimation insertions ~ 1-15 mSv/h
(good reliability required, very restricted access)
Big system coll / 2 insertions
RWA PAC03

3. At less than 1% of nominal intensity LHC enters new territory.
There is no easy start-up for collimation!
RWA PAC03

4. Allowable Intensity in the LHC
Allowed
intensity
Quench threshold
(7.6 ×106 7 TeV)
Cleaning inefficiency =
Number of escaping p (>10s)
Number of impacting p (6s)
Beam lifetime
(e.g. 0.2 h
minimum)
Dilution
Length
(50 m)
RWA PAC03

5. The V6.4 LHC Collimation System
Two warm LHC insertions
dedicated to cleaning:
IR3 Momentum cleaning
1 primary
6 secondary
IR7 Betatron cleaning
4 primary
16 secondary
Two-stage collimation system
54 movable collimators for high efficiency cleaning, two jaws each + other absorbers for high amplitude protection
Significant system: ~ 200 degrees of freedom!
RWA PAC03

6. Beam Halo and Collimation Philosophy
Secondary collimators
Primary collimators
Protection devices
Strategy:
Primary collimators
are closest.
Secondary collima-
tors are next.
Absorbers for protec-
tion just outside se-
condary halo before
cold aperture.
Relies on good know-
ledge and control of
orbit around the ring!
Cold aperture
RWA PAC03

7. Achievements… …and open questions
System layout has been worked out and provides required cleaning efficiency.
Collimation has been integrated into machine optics and layout.
Foreseen collimator materials do not withstand the expected beam impact (~8 bunches out of 2808). Require factor better resistance!
Impedance from collimators is critical (similar to the rest of the machine or larger).
High activation imposes severe restrictions for access. How to service the cleaning insertions?
Mechanical and operational tolerances are tight. How to maintain, commission, optimize, operate such a system?
…and open questions
RWA PAC03

8. Material Studies
Irregular dump → Proton impact on jaw → Proton-matter interaction
Two possibilities:
Find a solution where collimator jaws resist expected failures (preferred).
Accept that jaws can be damaged and foresee in-situ repair/exchange.
Pursuing possibility 1 for the moment but also getting informed about 2 (LC’s).
RWA PAC03

9. Temperature Increase in Materials
Typical length of secondary jaw: 0.5 m (Cu) to 1.0 m (C)
RWA PAC03

10. Stress and Fatigue Analysis
Calculated stress in simple Graphite is almost OK
Divide numbers by 2.5 for recent improvement in dump re-triggering time.
Just 30% too high… Would survive most erratic dumps.
Other forms of Carbon are expected to be more robust (Carbon-Carbon). Under study.
Beryllium would not sur-vive due to large stress.
About factor of 3 beyond limit.
FLUKA energy deposition as input to ANSYS calculations
RWA PAC03

11. Impedance Constraint
Typical collimator half gap at 7 TeV
LHC impedance without colli-mators
A doubling can be accepted at
7 TeV with full powering of octupoles
Half gap b [m]
Graphite requires different collimation strategy (larger openings).
Not evident – possibilities under study (local triplet collimation, …).
RWA PAC03

12. Other Ongoing Studies Length of collimator jaws for low Z:
0.2 m/1.0 m for prim./sec. jaws gives sufficient efficiency
Re-design of cleaning insertions for longer collimators
Vacuum and outgassing studies for C jaws
Outbaking, heat treatment, jaw temperature <50ºC, local e-cloud
Checks of downstream equipment, activation – damage
Mechanical design
Moving mechanisms, vacuum tank, cooling design, jaw surface properties: flatness, stability, …
Possibilities for experimental tests
RWA PAC03

13. Summary and Outlook
Collimation of the high intensity LHC beams is a challenging task with many difficulties. New territory is entered.
Requirements are now clearly defined and a strong project team is pursuing a solution at full speed.
Design choices will be done in the next few months. The solution will be a compromise between different constraints.
April 2004: First prototype.
: Production of 56 collimators plus spares.
2006: Installation into the LHC. Ready for beam in 2007.
In parallel: Studies on procedures for commissioning, optimization, and operation of such a system.
LHC will rely on collimation as no other collider before:
Opportunity for all of us to learn more about the handling and control of very high intensity beams!
RWA PAC03