Summary notes of the LCU meeting on 17/12/2007

Present: IA, MA, GB, CB, HB, SF, MG, ,PL, WH, JJ, MM, VP, TR, FR, TW, SW, SY

Report from meetings

• ICC -> MG
  Sector 4-5 is at 2K which is just above super fluidity and powering tests compatible with this temperature are in progress. These tests already included Q6 and the RF and no particular problems were found.
  Instead, there appear to be short circuits at  Q11.L4 and Q7.R2.
  The vertical corrector at Q5.L4 seems also to be not working. This is a serious issue due to the long drift space without correctors, the tight aperture in the D4 and the fact that no warm correct can be used to fix this problem (as it was done for the Q5 in IR8).
  In sector 8-1 a vacuum leak in the beam vacuum (first time) is observed in at the level of Q21.L1 in aperture V2. It is planned to remove from the circuit the upper capillary (it is the source of the leak) in both aperture to avoid unbalancing the overall circuit. Only the lower capillary will remain operational. 
  It is expected that all sectors will have been pressure tested before the end of the year.
  Communication of problems with magnets and power convertors to our group should be improved.
• SLM -> MG
  Muriel will leave by the the end of January. Lauriane Bueno will help and be present in mornings, do not hesitate to contact here directly.
  The room 354-1-001 will not be available on Monday mornings in 2008. The proposal is to move the LCU meetings to Tuesday morning in room 6-2-004.

Recent results on optics flexibility at top energy -> MA (pdf)

Masamitsu reported about his optics flexibility studies. The idea is to allow compensating the loss in tune in the case of high beta optics in IR1 and/or IR5 using other insertions or the arcs. This could potentially allow to use the standard injection and ramp also for the high beta configurations requested by TOTEM/CMS and ATLAS.
Using IR2, IR8 or also IR4, tune changes of at least +/- 0.2 in H and +/- 0.5 V should be feasible. Masamitsu also found optical solution in, e.g., IR2, compatible with the nominal triplet gradients of 205 T/m.
Recovering the missing integer (or half integer) by trimming the main arc quadrupoles requires a change in gradient by 1 - 2 per cent. however, this results in about 20% peak beta beating, which corresponds to the total beta-beating budget and it does not include any other source of beating. Of course, for the time being no re-matching of the insertions was performed, which should improve the situation.
The study should be completed by providing optics solutions for both Beam 1 and Beam 2.

Influence of missing closed orbit correctors on LHC orbit -> WH (pdf)
This study was triggered by discussions at the LTC on spares power converters for the arc closed orbit correctors and it should provide a basis to define with which frequency repairs in the tunnel of the power converters should be carried out.
WH recalled, that there are dedicated correctors in the straight sections which are absolutely necessary for crossing angles and separation bumps. Instead, some missing correctors in the arcs will only gradually downgrade the correction performance, see his transparencies. For this study, WH used his own MAD-X version with extra options to simulate missing correctors, including the number of missing correctors, the number of successive missing correctors and their location. MG comments that these features should be ported to the official version of MAD-X.
It is important that the missing correctors are known to the correction package. Some local bumps will remain, but the orbit correction package should still be able to correct globally the orbit. The impact of missing correctors appears to be less important than for missing monitors (BPMs): results on the impact of missing monitors will be presented at a forthcoming LCU meeting. SF comments that the assumed quadrupole misalignment (0.1 mm rms) is substantially smaller than the actual value. WH replies that this will have an impact on the actual rms and peak closed orbit, but not on the conclusions concerning the dependence of the corrected orbit on the number of missing correctors.
RA reminded about the tight orbit tolerances at collimators. MG replies that the study focuses on the arcs, essentially: non orbit leakage in the IR3 and IR7 insertions is generated by the missing arc correctors.

Issue with TCTVB in IR2 for ions? -> RA & JJ (pdf)
JJ stressed the importance of the ZDC for ALICE heavy ion physics. As for the other IPs, IR2 will be equipped with tertiary collimators, which have mainly been designed to protect the triplet aperture for operation with protons.
It turned out that the standard TCTVB settings partially shadow the cone of  neutrons generated at the IP in ion operation at beta*=0.5 m, thus preventing a correct reconstruction of the events.
Physicists consider this a serious issue and discussions co-ordinated by D. Macina between ABP (JJ and RA) and physicists from Alice are on-going. Several ideas how to improve this were mentioned and will be further followed up by JJ together with the collimation team: to make clear to which extent the tertiary collimators are actually needed for ion operation, what can be done at the level of the optics to open up the collimator (SF proposed to go to 0.55 m beta* as in IR1 and 5) and if tolerances can be relaxed so to allow for an increased opening of the collimators while maintaining the triplet protection.

AOB
None

Last update: 17-December-07

MG & HB

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