1. Plan for Collimator Beam Commissioning & Phase 2 Plans
R. Assmann, CERN/AB
13/06/2008
for the Collimation Project
LHC MAC
Slides, data and input provided by O. Aberle, A. Bertarelli, C. Bracco, F. Caspers, J. Coupard, A. Dallocchio, W. Hoefle, Y. Kadi, L. Lari, R. Losito, A. Masi, E. Metral, R. Perret, S. Perrolaz, V. Previtali, S. Redaelli, T. Weiler, AB/BDI (R. Jones et al), …
RWA, LHC MAC 6/08

2. 1) Production and Installation
All collimator locations under vacuum. Good end of a long race…
RWA, LHC MAC 6/08

3. Jaw Flatness (Ring & TL)
360 MJ proton beam
1.2 m
Total: 148 jaws
Flatness better than many feared. Out of tolerance collimators were placed in locations with more relaxed tolerances, meaning larger beta (limited sorting). Enough collimators for tightest places (40 mm).
RWA, LHC MAC 6/08

4. Minimum Collimation Gap (Ring)
Total: 32 TCSG, 30 TCT
TCSG (fiber-reinforced graphite)
TCT
(tungsten)
High precision collimators produced adequate for LHC conditions!
Note: No time to discuss here production problems with a few CERN collimators.
Important: Readiness for 2008 run and parameters is ensured.
RWA, LHC MAC 6/08

5. 2) Hardware Commissioning and Residual Problems
Collimator beam commissioning is only efficient if a thorough and successful hardware commissioning is done before (sort out as many issues without beam as possible). Therefore summarized here…
Hardware commissioning procedure published and approved.
MTF system is set up.
Collimator HWC comes after vacuum commissioning (bakeout). Request from vacuum group to perform bakeout without water in collimator.
80% of collimators have completed hardware commissioning. Now working on getting all data into MTF.
About 2 weeks still needed for completion of tunnel work.
Fully integrated into LHC schedule. Frequent shifts and adjustments to access conditions (cooldown has priority)…
Present scheduled end of collimator HWC: July 7th.
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6. Last Collimator Commissioning Schedule
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7. New Issue: Lifetime of Rail System
velocity: v
fixed
Cage
velocity: v/2
Rollers
Connected to movable jaw
Two rails per axis.
Pressed together to support jaw weight (pre-load).
Without grease.
Outside of vacuum.
RWA, LHC MAC 6/08

8. New Issue: Lifetime of Rail System
Cage “creeps” too fast in collimators from early series production. Random effect.
Not seen in tests for prototype collimator (32,000 cycles).
Stop not in all collimators  cage can get damaged.
Seen in lab during controls tests.
Cage
velocity: v/2
Connected to movable jaw
Rail
velocity: v
Rail
fixed
creeping
Rollers
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9. Collimator Creeping Collimator Cage type Orientation
Creeping expected [mm/km]
Maximum creeping observed [mm/km]
Creeping observed [mm/cycle]
TCS
Inox
Vertical
~ 5
278
16.7
Horizontal 20
1.2
Aluminum 53
3.2
TCT 10
0.6
1 cycle = 60 mm
Statistical behavior and reason for increased creeping not understood.
Studies ongoing in CERN mechanical engineering group (TS/MME).
Creeping not an issue for collimator lifetime if cage is adequately stopped (demonstrated 22,000 cycles, all but 28 first collimators equipped following production issues).
RWA, LHC MAC 6/08

10. Roller Cage Summary
No problems in 2008 run to be expected based on lifetime data available presently. Several collimators now used for cycling tests (originally restricted to 50 cycles to keep lifetime) and reached > 20,000 cycles.
10 collimators (early production) with lifetime of about 1-2 nominal years (first problems towards end of 2009 run?).
15 collimators (early production) with lifetime of about 10 nominal years.
All other collimators conform with specified lifetime of > 20 years (> 22,000 cycles).
We are very lucky that we found this problem in the lab and not in the tunnel with beam. If not fixed, we will adjust operational procedures to known weakness: minimum number of cycles, avoid using collimators with inox cage, …
Preparing to retrofit stops for 28 collimators before end of July. We have good hope that this will succeed, solving this new issue. Now (June 12) validated for 2,500 cycles (factor 8 improvement: 1 y  8 y). Cycling ongoing…
RWA, LHC MAC 6/08

11. 3) Controls Readiness and Remote Commissioning
Collimators are fully operational after HWC (single collimator). Lifetime problem not an issue for the first year and will anyway be fixed at latest in the shutdown.
Low, middle and top level software is operational with most required features.
Allows to generate movement functions versus time for any collimator:
Specify setting in number of sigma.
From database information (later including beam-based calibration) generate settings in mm (take into account local beta, emittance, local orbit, calibration data).
As the first collimators have become available, functionality has been remotely commissioned for ensembles of collimators. Many single collimators must work as a coherent system!
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12. For Fun…
Example:
Remote, function- driven movement of collimator jaw (10 min).
Remote reading of jaw position in tank reference system with LVDT’s.
What does this read???
Initially in 2008: Control 88 collimators for the two beams (~ 400 DOF)!
RWA, LHC MAC 6/08

13. First 7 Collimators in IR3: Remotely Executing 30 min Ramp Function
Position
Gap
Realistic LHC ramp functions generated for 7 collimators in IR3. Executed synchronously (software trigger used: timing signal not available).
Different absolute settings due to local beta function and different types of collimators (families). Automatically generated at top level application.
Jaw and gap positions surveyed independently of motor drivers with 10 sensors per collimator.
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14. Jaw Position Error: Requested - Measured
7 IR3 collimators: 14 jaws with 28 sensors.
Remotely controlled from CERN control room.
Example interlock level
width of human hair
Demonstrated: (1) Accurate jaw positioning. (2) Synchronous movement. (3) Precise position readout and survey (collimator and machine protection).
RWA, LHC MAC 6/08

15. Problem: Magnetic Interference on LVDTs of TI2 Collimators (TCDIV
Problem: Magnetic Interference on LVDTs of TI2 Collimators (TCDIV and TCDIH.29050)
See last MAC!
The resistive magnets (5KA peak current) current cables in TI2 generate a magnetic field that perturbs the LVDTs collimator nominal behavior
The position drift follows the magnet’s current cycle. Drift of up to 150 um have been experienced.
Reproduced in lab. Shields tested…
RWA, LHC MAC 6/08

16. Countermeasures Applied for TCDIV.29012 and TCDIH.29050
60% reduction of the LVDTs excitation voltage as best trade-off between magnetic interference reduction and reading accuracy reduction (the standard deviation of most LVDT’s is still below 1 um)
Installation of a magnetic shielding to reduce the external magnetic field on the LVDTs
The LVDT deviation is now below 50 um (apart from one LVDT. A better shielding strategy is under study).
Magnetic Interferences summary for collimator TCDIH.29050
Magnetic Interferences summary for collimator TCDIV.29012
RWA, LHC MAC 6/08

17. Problem: Overshoot on the Position Reading after Large Displacement
As consequence of the long cables capacitance between LVDT’s and conditioning electronic a phenomenon of reading overshoot has been remarked. The overshoot can reach more than 100 um after a displacement of 40 mm. The recovery time can reach several minutes.
Problem: 0.3 % overshoot in reading with decay over minutes!
Impact on operation: Run collimators slowly.
Anyway good for protection. No reason for cm scale fast movements with beam!
No major problem with foreseen ramp and squeeze functions.
Nevertheless, ATB works on improving this.
The phenomenon has been reproduced (even if not yet understood..) in the lab and different scenarios have been tested. The figure refers to a 70 mm displacement
Countermeasure:
Again at half excitation voltage the overshoot is reduced by a factor 4.
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18. Collimator Interlocks
Two checks agreed (10 position sensors/collimator):
Measured position and gap at a given time checked versus allowed position versus time (already implemented into low level controls).
Measured gap at a given energy (beta) versus allowed gap for this energy (beta). Keep this as simple as possible.
Energy and beta function from timing signal.
Work ongoing for implementation by end of June.
Working on MP check procedure for commissioning this (run special automatic cycle to trigger interlocks).
Note temperature, state-driven and BLM interlocks in and close to collimators have also been defined (not reported here).
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19. Position Interlock Test TCDI2905
Example showing that interlock logic works (low level controls).
Once interlock (“dump limit”) is reached jaw movement is stopped.
At the same time interlock generated  any beam would be dumped.
Can decide later to relax (e.g. “just stop movement but do not dump”).
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20. Position Interlock Check Procedure (automatic verification of position interlocks)
Interlocks generated
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21. 4) Plan for Beam Commissioning
By start of beam operation (much is already there – see previous slides):
Collimators fully operational.
Controls system ready for driving collimators following any function.
Controls system ready for setting any warning and interlock levels safely (MCS).
MP functionality fully working by commissioning jaw position interlocks already without beam.
Collimator safety (closest devices to beam) ensured by temperature interlocks and interlocks on BLM rates next to collimators.
Other systems operational (BIC, MCS, BLM, Timing incl. E signal), especially relative reading of BLM’s close to collimators. BLM’s already working.
Beam commissioning plan then means:
Decide what collimators to use when and at what settings.
Beam-based calibration of collimators (input to functions).
RWA, LHC MAC 6/08

22. Reminder: Beam-Based Calibration of Collimators
Not reported in detail since the status is the same as in December:
Method has been worked out (based on Tevatron/RHIC/HERA experience).
Method has been tested and realistic accuracy established with the LHC prototype collimator in the SPS (see past reports).
Machine conditions for calibration defined (up to few nominal bunches at top energy). See report in December.
Full set-up is lengthy: 6 shifts of about 8 hours. Faster if only updating, checking calibration.
Agreed goal is to have automatic beam-based calibration (as TEVATRON) available for 2009 run:
Hardware connections prepared (collimators – BLM’s).
Wok package in AB/CO group. Further SPS tests this year.
No more details here this time to avoid repetition… Note: big expected impact from collimation upgrade in phase 2 (much better)!
RWA, LHC MAC 6/08

23. Collimator Operational Modes
For simplification, several modes defined:
Mode 1: Primary collimators and protection collimators only.
Mode 2: Primary collimators, absorbers and protection collimators.
Mode 3: Full 2008 system (5 instead of 11 secondary collimators in IR7 per beam).
Mode 4: Full phase 1 system, as installed for 2009.
Settings, performance reach and tolerances defined for each mode.
Realistic assumptions for efficiency:
Factor 2-3 margin for BLM thresholds and uncertainties in FLUKA.
Factor ~10 reduction in cleaning efficiency with realistic imperfections (shown in PhD thesis of Chiara Bracco).
Master table for collimator commissioning, defining settings for various operational stages. A few examples in next slides…
RWA, LHC MAC 6/08

24. Commissioning Table: Settings
Here listed for nominal settings (b*=0.55m): tightest gaps and tolerances.
Settings in normalized sigmas for every collimator family.
Converted into mm with the tools described before.
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25. Commissioning Table: Performance
Here listed for nominal settings: tightest gaps and tolerances
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26. Comm. Table: Optimized Ramp Settings
Just one out of 3 fully calculated ramp cases shown here. Used also for b*.
For each case settings, performance reach and tolerances calculated (not fully shown).
Our basis for placing LHC collimators.
Too many numbers  some figures for visualization…
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27. Collimators Closed with Relaxed Tolerances During Energy Ramp
with imperfections
Transient b beat: 10% 40%
Orbit: 0.3 s 1.2 s
Collimator: 0.4 s 1.6 s
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28. Collimators Closed with Tight Tolerances During Ramp (Nominal)
with imperfections
Transient b beat: 10% 10%
Orbit: 0.3 s 0.3 s
Collimator: 0.4 s 0.4 s
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29. Performance Reach 7 TeV (Nominal Settings)
Work on imperfections to approach ideal performance reach (40% of nominal intensity)! Here assume peak loss rate of 0.1% per second. If lower, higher intensity can be reached!
RWA, LHC MAC 6/08

30. The Collimation Plan on One Slide
Pilot bunch:
Start with primary collimator only and protection elements.
Keep open during first ramps and close as we learn about the ramp.
Bring in additional collimators and test ramp functions & stability.
End of ramp compatible with b* = 11 m.
43 bunches and/or squeeze to b* = 2 m:
Use full installed system, according to collimator hierarchy.
Close during ramp with optimized ramp settings (maximum tolerances).
Squeeze without closing collimators (end of ramp compatible with b* = 2 m).
156 bunches and/or squeeze to b* = 0.55 m:
Close collimators to nominal settings and tight tolerances.
Squeeze with tight collimator settings. Compatible with b* = 0.55 m.
75 ns running (push intensity and luminosity):
Reduce imperfections (machine & collimators) and improve machine stability.
RWA, LHC MAC 6/08

31. Summary Beam Commissioning
All collimation locations under vacuum. HWC well advanced: scheduled to be completed July 7th. Residual problems being addressed (“cage creeping”).
A strong team has been trained for collimator commissioning. See results from various team members presented. Try to learn as much as possible from Tevatron, HERA and RHIC and adapt to LHC (no simple copy possible).
Installed collimators are precision devices, as specified. Remote commissioning of collimator ensembles has started (7 collimators remote controlled and surveyed over 30 min to 30 mm, the width of a human hair).
An extensive collimator table has been worked out for the beam commissioning: takes into account performance reach, realistic imperfections, defines used collimators, their settings and tolerances for different energies and intensities.
Plan has been established, minimizing number of complications at given time (first ramp, first squeeze without collimator movements, …).
Time to get the beam…
RWA, LHC MAC 6/08

32. CERN Plan for Phase 2 Collimation
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33. Reminder: Constraints Phase 1
Strict constraints in 2003 for phase 1 system:
Availability of working collimation system for beam start-up (2007 originally)
Robustness against LHC beam (avoid catastrophic problems)
Radiation handling (access for later improvements)
No modifications to SC areas (due to short time and problems with QRL)
Compromises accepted:
Limited advanced features (e.g. no pick-ups in jaws).
Risk due to radiation damage for fiber-reinforced graphite (electical + thermal conductivity changes, dust, swelling, …). Kurchatov data shows factor 4-5 changes with irradiation in various important parameters.
Steep increase in machine impedance due to collimators.
Excellent cleaning efficiency, however, insufficient for nominal intensity.
RWA, LHC MAC 6/08

34. The Phase 2 Path
Due to LHC extrapolation in stored energy and predicted limitations in phase 1 system: The LHC collimation system was conceived and approved during its redesign in 2003 always as a staged system.
Phase 1 collimators will stay in the machine and will be complemented by additional phase 2 collimators.
Significant resources were invested to prepare the phase 2 system upgrade to the maximum extent.
Phase 2 does not need to respect the same constraints as the phase 1 system.
The challenge we put to ourselves: Improve at least by factor 10 beyond phase 1!
RWA, LHC MAC 6/08

35. Constraints: Phase 2 Strict constraints in 2003 for phase 1 system:
Availability of working collimation system for beam start-up (2007 originally)
Robustness against LHC beam (avoid catastrophic problems)
Radiation handling (access for later improvements)
No modifications to SC areas (due to short time and problems with QRL)
Phase 2 constraints:
Gain factor ≥10 in cleaning efficiency.
Gain factor ≥10 in impedance.
Gain factor ≥10 in set-up time (and accuracy?).
Radiation handling.
Sufficient robustness, also against radiation damage.
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36. Phase 2 Collimation Project
Phase 2 collimation project on R&D has been included into the white paper, thanks to strong support by CERN top management :
We set up project structure in January. Key persons in place.
Budget requested and allocated.
Manpower request for white paper post sent.
We are gaining momentum. Emphasis now on technical progress…
FP7 request EUCARD with collimation work package:
Overall marks very high (14.5/15.0).
Expect that this will fly and make available significant additional resources (enhancing white paper money).
Remember: Advanced collimation resources through FP7 (cryogenic collimators, crystal collimation, …).
US effort (LARP, SLAC) is ongoing and we are well connected (not reported here). Expect first basic prototype results before EPAC.
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37. General Work Plan
So far 6 meetings for phase 2 specification (R. Assmann). In parallel collimator design meetings going on in TS/MME (A. Bertarelli)
Overall work plan:
Define general directions until July 08.
Prepare conceptual design until October 08.
Discuss conceptual design and organize project details in November 08.
Testing of hardware in 2009/10 (lab and beam tests).
Time plan will be affected by start of LHC beam operation (highest priority to make phase 1 collimation system work).
However, once LHC intensity is limited (see previous slides) time will be short (prepare now!).
Note: Phase 2 locations in IR3 might initially be used for installing temporary betatron cleaning to live with electronics problems in IR7 (study for combined betatron/momentum cleaning for LHC ongoing  not reported here).
RWA, LHC MAC 6/08

38. Concept to Realize Improvement on Phase 2 Timescale
Factor 10 efficiency for protons and ions (work Thomas Weiler/Ralph Assmann):
Place metallic, advanced phase 2 collimators (efficiency study by Chiara Bracco). 2-3 complementary development paths in CERN and US (SLAC rotatable design).
Place cryogenic collimators into SC dispersion suppressor (use missing dipole space).
Different material for primary collimators (to be evaluated).
Factor 10 in set-up time (and accuracy?):
Integration of pick-ups into collimator jaws for deterministic centering of jaws around circulating beam. Support from BI group (R. Jones et al).
Gain accuracy due to possibility to redo for every fill (avoid reproducibility errors fill to fill).
Factor 10 in impedance:
No magic material yet (factor 2 seems possible). Pursue further the various advanced ideas! Work by Elias Metral and Fritz Caspers. Tests ongoing.
Rely to some extent on beam-based feedback. Work by Wolfgang Hoefle.
Open collimators or use less collimators with improved efficiency (see above) and increased triplet aperture (phase 1 triplet upgrade), if feedback cannot stabilize beam.
RWA, LHC MAC 6/08

39. 1) Concept for Improving Efficiency
Fundamental problem:
Particle-matter interactions produce off-momentum particles in straight cleaning insertions (both p and ions). These are produced by different basic physical processes that we cannot avoid (single-diffractive scattering, dissociation, fragmentation).
No dispersive chicane after collimation insertion: Off-momentum particles get lost in SC magnets after first bend magnets downstream of straight insertion.
Conceptual solution (no decisions taken – under study):
Reduce number of off-momentum particles produced (phase 2 primary and secondary collimators).
Install collimators into SC area, just before loss locations to catch off-momentum particles before they get lost in SC magnets.
Might be beneficial to install around all IR’s, for sure in IR3 and IR7.
Elegant use for space left by missing dipoles!
RWA, LHC MAC 6/08

40. Schematic Solution Efficiency
Collimator
Warm cleaning insertion
(straight line)
SC bend dipole
(acts as spectrometer)
SC quad
(acts as collimator)
Off-momentum particles generated by particle-matter interaction in collimators
Ideal orbit (on momentum)
RWA, LHC MAC 6/08

41. Schematic Solution Efficiency
Collimator
Warm cleaning insertion
(straight line)
SC bend dipole
(acts as spectrometer)
SC quad
Off-momentum particles generated by particle-matter interaction in collimators
Ideal orbit (on momentum)
Add cryogenic collimator, using space left by missing dipole (moving magnets)
RWA, LHC MAC 6/08

42. No longitudinal displacement.
Change in Layout of DS
3 m to left
3 m to right
No longitudinal displacement.
Moves inwards by 3 cm.
Layout and optics checked with MADX. No problem for the optics and survey seen. Optics change (move of Q7) small even without optics rematch. More careful work is required. Note, that impact on infrastructure was not checked yet!
RWA, LHC MAC 6/08

43. Proton Collimation Efficiency with Phase 2 Cu Collimators and Cryogenic Collimators
%/m  %/m
Inefficiency reduces by factor 30 (good for nominal intensity). Lower losses in the experimental collimators (background). Should also work for ions.
Caution: Further studies must show real feasibility of this proposal (energy deposition, heat load, integration, cryogenics, beam2, … ). Just a concept at this point.
Cryogenic collimators will be studied as part of FP7 with GSI in Germany.
RWA, LHC MAC 6/08

44. Zoom into DS downstream of IR7
quench level
Impact pattern on cryogenic collimator 1
Impact pattern on cryogenic collimator 2
RWA, LHC MAC 6/08

45. 2) Concept for Improving Set-Up
Standard method relies on centering collimator jaws by creating beam loss (touching primary beam halo with all jaws).
Procedure is lengthy (48h per ring?) and can only be performed with special low intensity fills for the LHC.
Big worries about risks, reproducibility, systematic effects and time lost for physics (integrated luminosity).
Tevatron and RHIC must rely on collimator calibration and optimization performed at the start of each physics run.
LHC can only do better if non-invasive methods are used (no touching of primary beam halo and no losses generated):
integration of pick-ups and loss measurements into jaws.
RWA, LHC MAC 6/08

46. Schematic 1
Jaw 1
Jaw 2
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47. Schematic 2
Jaw 1
Jaw 2
1) Center jaw ends around beam by zeroing difference signal from pair of pickups (not touching beam halo  no or very low losses.
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48. Schematic 3
Jaw 1
Jaw 2
2) Put the same gap at both ends as measured from jaw position (phase 1 feature).
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49. Collimator - BPM Study
No time for detailed studies and simulations this year. Will start next year.
In the meanwhile implement “best guess” electrodes into mechanical design.
Crucial help from BI group (R. Jones et al). Engineering design driven by TS in phase 2 collimation project.
Ansatz: Implement some reasonable buttons, build a prototype and test with beam how well it works (improve then with second generation design).
Needed for high intensity: Should not be too difficult to reach much better accuracy than with collimator beam-based alignment method.
Will still require knowledge of local beta function. Can in principle be evaluated with movable BPM buttons. However, chance to measure with global methods regularly (1000 turn small kicks).
RWA, LHC MAC 6/08

50. Engineering Design for Prototype
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51. Electrode Design
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52. Improvements Beyond Phase 2
We should not forget these advanced directions because we might need to have them at some point to advance LHC intensity.
Time scale is beyond phase 2 collimation (2011/2).
Several advanced directions have been proposed but are too early for starting engineering design now. They are pursued as longer term improvements:
Crystal collimation, waiting for successful results from Tevatron and SPS.
Non-linear collimation.
Hollow electron beam lens.
Laser collimation.
Partly funded through FP7 proposal.
RWA, LHC MAC 6/08

53. Conclusion Phase 2
Within the last months we have gained quite a bit in knowledge: thanks to many colleagues for their support in very busy times.
Based on this work we can hopefully propose a big step forward for LHC collimation, evolving the existing system with relatively modest modifications (no new magnets). Concept being evaluated for:
Factor 30 in efficiency (AP OK, check with energy deposition studies).
Factor 50 in setup time, some factor in accuracy.
Factor 2 in impedance, hope to stabilize with feedback, use increased aperture after phase 1 triplet upgrade, trade-off with efficiency.
Higher radiation robustness.
Feasibility will now be addressed in more detail. The LHC tunnel is very constrained and we might encounter showstoppers.
Important milestone: Review of conceptual design with parallel development paths in late autumn 2008.
RWA, LHC MAC 6/08