Summary notes of the LCU meeting on 05/04/2011

Present: CA, RB, HB, RC, RdM, MF, MG, BH, TM, RM, FS, MS, GV, FZ, EHM

Report from meetings

• SLM and general information -> MG
  The fellows and students situation and budget was reviewed. Few more students could be possible. The students committee is in April and for fellows in May. Fellow request will be reviewed in the light of the modified schedule with operation in 2012.
  The submission deadline for IPAC abstracts to MG is today.
  Work is in progress to make tracking based on MAD-X files easier. The idea is to allow for tracking with thick quadrupoles. Slicing with makethin would then only be necessary for dipoles, and higher multipoles and could be automatic without need to rematch the optics. Thys Risselada helps in this efforts and already generated a special sequence with thin dipoles, but thick sliced quadrupoles, mimicking the future behaviour of makethin. FS checked that the sequence file can be used with SixTrack: indeed the basic optical parameters are in good agreement (SixTrack and MAD-X). The next step will be the computation of DA and comparison for the standard thin lattice and the new one, in both cases only with magnetic field errors in the main dipoles.
  There is further follow up on smaller beam pipes with most stringent limits coming from the high beta optics.

• LHC -> BH
  The 1.38 TeV run and the short technical stop are over and the next step is the scrubbing for electron cloud. See the latest-news for further information.
  MS was asked to comment on the chromaticity correction. He confirmed that the on-line magnet modeling was improved for the MS and spool pieces, in particular including hysteresis effects. The tune decay at injection is well corrected and the correction of the chromaticity change by the snap-back improved. However, during the tests performed, the decay of the chromaticity was over-corrected and new tests are required.
  MG commented that at the last FiDeL meeting it was showed that MCOs have a rather low powering at injection (thanks to the fact that b4 in main dipoles is small). On the other hand this implies that they are fully within the width of the hysteresis region. Hence, strictly speaking, their magnetic status is undefined. In this respect it might be worth testing what happens by switching them off.

• LBS -> HB
  There was an LBS meeting just after the last LCU meeting including a presentation on the vacuum conditions. Details can be found on indico.

• LMC -> FZ
  There was an LMC with a discussion of the upgrade needs for collimation in the next shutdown and planning for the electron cloud scrubbing. For more information see the LMC page. 

Is it possible to link DA and beam losses variation over time ?  -> MG  (slides)
MG reported about a recent investigation on how beam losses and dynamic aperture could be linked. This is a rather general question which has been discussed also in the LHCCWG a while ago in conjunction with the definition of tolerance intervals on key beam parameters for LHC beam commissioning.
The starting point is the possibility of fitting the evolution of DA with time using a rather simple expression that depends on three parameters, only. This was found already some time ago in the case of single-particle effects. The parametrisation is capable of describing situations with finite-size DA as well as zero-DA.
The proposed approach is based on such a parametrisation and the assumption that all particles beyond the DA at time N are lost. From this scaling laws can be derived.
Two data sets were found compatible with the assumptions used for the derivation of the proposed approach. One refers to the intensity variation at injection in the Tevatron for a single proton beam. The original computations modeled the intensity evolution using a simple diffusion model with constant diffusion coefficient. The proposed approach is capable of fitting the data with at the least the same degree of accuracy as the diffusion model.
Another set of data is from the SPS at 55 GeV during a coast aimed at measuring beam conditions in view of future crab cavity tests. Strong intensity reduction and emittance growth were observed. Interestingly enough the second derivative of intensity vs. time is negative, unlike the Tevatron case. This rules out immediately the diffusion models. Once more the proposed model agrees rather well with the experimental data.
FS comments that the situation might be completely different for the case with in which beam-beam effects are included. Indeed, he found the existence of chaotic spikes that generate jumps in the variation of DA with time. MG agrees that the situation might be different when beam-beam is included and this is a topic under consideration.

AOB

None

Last update: 06-April-2011

MG & HB

Back to LCU Minutes