Summary notes of the LCU meeting on 20/04/2010

Present: RB, HB, OD, MF, MG, JJ, YL, FS, MS, RT, DW, GM

Report from meetings

• General information -> MG
  The deadline for fellow requests for the next committee is 29 April. One new fellow position is planned for work with ions in the LHC in 2011.
  LHC commissioning progresses well. All IRs were successfully squeezed to beta* = 2 m, so far, however, without separation bumps.
• LBS -> HB
  The last LBS meeting had reports from the vacuum group, from ALICE, a discussion on the more extended luminosity scans for calibration planned for this week and and update on LHC parameters used for simulations.

Optics and Knobs in MadX and LSA -> Gabriel Mueller (slides)

Gabriel Mueller from BE/OP discussed how optics files as produced by our group on the Mad-X level get into the control system on the LSA level.
The control system always needs a full optics description of the whole machine.
Changes on the optics have to be propagated into the optics dependent knobs.
The case of knob generation for crossing scheme and separation was discussed at length: in case of changes in the knob definition the update might be difficult. In fact, the new knobs should be commissioned with beam and the trims/settings found might not be re-usable. This is the case of the recent version of the knobs for the squeeze sequences that feature and increased number of MCBXs in order to comply with the limitations observed during the hardware commissioning. As the injection settings loaded in the machine use a smaller number of MCBXs we should revert the optics database to a previous solution.
It is also suggested to create the knobs using the maximum number of magnets even if some of them are, for the moment, set to zero.
It is also discussed at length strategies to cope with failing magnets. This happened when some MQTs circuits were lost during operation and the standard knob for changing the tune could not be used anymore. It was suggested to compute a sensitivity matrix with all the circuits and the knob should be obtained by inversion of such a matrix taking into account the number of circuits available.

Matching thick vs thin Lens -> OB (slides)

YL recently noticed in his work on background tracking surprisingly large differences between beta functions in the thick and thin lens versions of the optics.
OB had a look at this. The thin lens version is always an approximation and a compromise between optics match and gradients agreement between the thin and thick lens version. Of course, the situation can be improved by increasing the number of slices used in the thin lens sequence. However, this might have side effects, e.g., on the speed of numerical simulations.
For the current V6.503 optics it also turned out that the thins lens matching was not done in an optimal way, resulting in larger discrepancies than, e.g., in the previous V6.500 version. The V6.503 thin optics will be reviewed and improved to avoid large deviations.
Furthermore, a thin optics corresponding to the squeezed configuration in the LHC at 3.5 TeV (i.e., 2 m beta* in all experimental insertions) will be prepared. Users are asked to report about any potential obstacle to the increase in the number of slices.

AOB

None

Last update: 20-April-10

MG & HB

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