Summary notes of the LCU meeting on 16/06/2009

Present: MA, CB, HB, SF, MG, WH, BH, EL, JM, JN, FR, AR, FS, YS, RT, GV, SW, DW, FZ

Report from meetings

• SLM -> MG and general information (slides)
  Last week we had the ABP forum with presentations from ABP/SU. This was very interesting and several points require further follow up with input from us. Realignment requirements that were well adapted to LEP were the vertical plane was very critical, will have to be adapted for the LHC which is designed for round beams with equal requirements in the vertical and horizontal plane. WH will help to review the smoothing algorithm used by SU and check the relevant length over which smoothing is applied. Concerning the intermediate CAS this year, the training budget may be insufficient and require a discussion of priorities and additional resources from the group travel budget.
  The aperture modeling used in LHC was improved using details from drawings for experimental beam pipes (see also later).
  There has been a clarification and change of courses for the "habilitation electrique", which will now not require additional practical tests.
  MG illustrated (see the slides) changes in names and element type assignment to improve both the nominal sequence, but also to provide guidelines for the as built LHC MAD-X sequence. The transverse damper for example was previously declared as RF which is misleading. A new element type PLACEHOLDER for generic do nothing to optics (but with an aperture assigned) elements has been introduced and used for example for the DFBs. There was a discussion on pros and cons for a single built in sequence. The conclusion was that we need a stable LHC sequence for the machine as planned and then deal with exceptions, faults and temporary changes in a separate file.

Latest results of DA studies for LHC Phase 1 upgrade -> BH (slides)

BH reported about studies for the LHC phase 1 upgrade and results from tracking on the dynamic aperture.
Lowering beta* from 0.55 m to 0.30 m will increase the beta max in the triplet to over 11 km. A minimization and correction of higher order multipoles in the triplet will be important.
Field quality for the new triplets and the cold D1 magnet were provided by magnet experts. Worst case estimates (magnetic errors in new triplets and cold D1 without ay correction) show that the dynamic aperture could drop below 6 sigma at beta* of 0.30 m.
The contribution of DA reduction form the field quality of the cold D1 is about 2 sigmas. Then, most of the study is performed without the field quality of the D1 magnet and focusing on the impact on the triplets including also strategies for correcting the magnetic multipoles.
The correction of b3 and b6 seems essential, while the contribution to the DA of the matching section quadrupoles is excluded.
The beta-beating compensation has an impact on DA at the level of about 1 sigma (at least for small angles).
It was also tested that by defining a target field quality table by re-scaling the multipolar errors of the current triplet quadrupoles including the dependence on the reference radius and the new optical parameters, the DA is restored.
This is a rather clear sign that the field quality should be improved. BH reports that a recent meeting with magnet experts it turned out that proposed field quality is obtained by extrapolation form the cable position of MQs, rather than MQXs (a factor of three better).
For the future, the tracking studies should aim at defining a target error table for both the new triplets and the cold D1.

LHC Phase 1 upgrade: beam-beam contribution to DA -> EL (slides)

EL reported about recent beam-beam studies for the LHC phase 1 upgrade. Nominal tunes for the upgrade are Qx = 63.31 and Qy = 60.32. Even if intensities would be kept at the original level, the parasitic effect would increase due to extra (19 instead of 15) parasitic collisions due to the increased D1 distance from the IP.
He observes roughly that the dynamic aperture with beam-beam decreases from 9 sigma for the nominal LHC to 7 sigma for the upgraded LHC, even if the beam-beam tune footprint size is not increasing. At present the reason for this is not clear. SF reminded that the upgraded LHC has about two times stronger sextupoles and HB asked if there could be more resonance excitation from the increased crossing angle in the upgrade. Further studies should also include ultimate intensities and beam-beam at injection.

Experimental beam pipes aperture studies for LHC Phase 1 upgrade -> JM (slides)

JM reported on ongoing studies to see if the size of the experimental beam pipes could be reduced. ATLAS would like to modify their pixel detector and allow for a bit of extra space to be able to slide in and out the detector. The requirements for the present beam pipes were defined in 1997. To the beam stay clear of 14 mm, 2.6 mm survey precision and 2.6 mm mechanical construction, a nearly 10 mm uncertainty was added to allow for tunnel displacements by the excavation of the experimental halls. This turned out to be generous and should allow tighter tolerances. JM looked at the impact on various optics. Low beta* optics are optimized for small beams at the interaction point and should not suffer. The margin for the high beta optics instead which have larger beams at the interaction points will be decreased.

New aperture files -> MG (slides)

MG made more detailed aperture files for MAD-X for the CMS beam-pipe. The model is based on what reported in the LHC Project Note 418 by R. Wanzenberg. Assuming the standard tolerance of 11 mm (2.6 mm - alignment - 2.6 mm - mechanical tolerance - 9.8 mm - other effects and in particular cavern movement) the pipe section called CT2 features a reduced aperture. MG, following a discussion with R. Veness,  determined that the mechanical tolerance should be reduced to 11 mm to ensure a n1 of about 11-12 sigma in the CT2 section.

TOTEM optics and aperture studies -> HB  (slides)

HB shortly reminded about the intermediate 90 m TOTEM optics which is planned to be commissioned in the first LHC run and the high beta (~ 1500 m) options for the subsequent years. He matched and studied parallel separation schemes for these optics. For the 90 m optics, he finds that aperture and corrector strengths are sufficient both at 5 and 7 TeV for  the nominal 3.75 micrometer emittance. For the very high beta (~ 1500 m) optics, the aperture is most critical close to the IP. It may become difficult to accommodate the separation bump. HB and WH argue that high-beta operation will be done with few bunches without parasitic collisions and crossing angle and that if necessary, the final un-squeeze to very high beta* could also be done without separation as was the case in the SppbarS, i.e., starting from colliding beams. SF comments that the fact of having beams of same charge might alter the picture from previous CERN colliders. From discussion in and after the meeting it became clear, that the n1 aperture calculations for the high-beta were done with too pessimistic tolerances. The calculations and slides attached to these minutes have meanwhile been updated and now show that the n1 aperture remains above 7 for the high-beta optics at 1μm emittance also for separated beams. The issue of aperture for high-beta optics will be followed up in parallel with a revision of the tolerance for the experimental beam pipes.

AOB

None

Last update: 19-June-09

MG & HB

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