Summary notes of the LCU meeting on 29/01/2008

Present: IA, MA, RB, HB, RC, SF, MG, WH, JJ, PL, MM, JN, TR, FR, FS, RT, TW, SW, SY, FZ

Report from meetings

• ICC -> MG
  The LHC access system was tested recently. The test was very useful for the commissioning and debugging of the system. A possible combined LHC hardware and beam commissioning will be very challenging for the access system.
  Power tests of sector 4-5 have started. Successive ramps reached 8.5 kA, 9.5 kA and 10 kA (roughly equivalent to 6 TeV) before the first quench.
• SLM -> MG
  MG reminded about the deadline for summer student requests on the 7th of February.
  A general AB meeting is planned for the 26th of February.

Impact of longitudinal displacement of low-beta triplets -> FS (pdf)

FS showed us slides that he started to prepare for a future LTC meeting to get our comments and feedback. As previously reported, we were asked to study the effects of the significant longitudinal misalignments of the magnetic centre of the triplet magnets. The observed misalignment of up to 17 mm results into beta beating and tune shifts during the squeeze. The beta beating from this source alone is more than 10 % or half of the total tolerable budget. The effects are largest at low beta*. A correction below a beta* of about 4 m is needed.

The effects of misalignment were also checked in collaboration with JJ for IR2 for the nominal beta* = 0.5 m for ion operation and found to be not too bad as the displacement is only just above 10 mm. More details can be found here.

As far as the situation of IR8 is concerned, the measured displacement is between ±7 mm, which is not to far from the specified tolerance (see LHC PR 866). It contribution to the global beta-beating is certainly negligible. Furthermore, the squeeze sequence is still to be optimized.
FS also showed, that it should in principle be possible with optics correction methods to largely correct for the beta beating generated by the misalignment. The following feedback was given:

• Report the tolerances derived for the longitudinal alignment of the low-beta quadrupoles.

• Report beta-beating generate by each single insertion (one at a time).

• Add the plot of the beta-beating evolution during the squeeze of IR1 or 5.

• Review the parameters used for the simulations of the beta-beating correction and verify the beta-values at key locations (IPs).

• Report the impact of the uncorrected longitudinal position on LCU activities (more optics to be maintained etc.)

Bound free pair production in RHIC and LHC -> RB (pdf)
RB reported about studies of bound free pair production (BFPP ) using simulations and tests at RHIC. The measurements at RHIC showed clear evidence for BFPP. Localized losses were observed in the dispersion suppressor upstream from the IP. This is the first time that losses from this process have clearly been measured in an accelerator environment.
The simulations are based on tracking + FLUKA and were found to be in good agreement with the observations (within a factor of two).
Simulations for the LHC predict a peak of losses in the dispersion suppressor at about 400 m from the IP, just below the quench limit. Incidentally, this is almost the location of FP420: FeRo comments that the collaboration should investigate this point.
RB also showed a comparison of the longitudinal energy distribution of proton and ions in matter. Ions deposit much more energy right after the impact but quickly fragment into many hadrons which makes the distributions similar after an interaction length as relevant for the loss monitors placed outside the magnets. This implies that BLM thresholds should not be different for ions. Normally, the losses occur mostly at the first peak in dispersion upstream of the IP. Using orbit bumps, it could be possible to redistribute the losses over at least two dispersion bumps.

AOB
None.

Last update: 31-January-08

MG & HB

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