Summary notes of the LCU meeting on 12/02/2008

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

Report from meetings

• ICC -> MG
  Power tests progressed in sector 4-5 and reached values above 10kA / 6 TeV. The few quenches observed were on different dipoles. It appears that some re-training was needed for some dipoles after they were moved into the tunnel. Main dipole powering stopped and tests are now performed on other magnets including the insertion quads. The powering tests also revealed some inconsistencies in the nominal current value for separation dipoles.
  Sectors 8-1 and 5-6 started cool-down. The latter is well-advanced.
  Another issue discussed was the load on the jacks of the Q2 quadrupoles. It turned out that the central one supports the whole weight of the magnet, while the extreme ones are almost un-loaded. This situation is not acceptable as the supports are working outside their nominal conditions and no remote-controlled alignment is possible. The distribution of the weight on the three supports of the Q2 all around the LHC circumference was measured and found to vary considerably from magnet to magnet. 
• SLM -> MG
  There were first contacts between ABMB and the designated DG.
  An AB general meeting was announced.
  There is a request for volunteers to help for the CERN open day in April.

Bunch filling schemes for early running  -> WH (pdf)

WH recalled that this is not a new subject and that there is in fact a lot of flexibility, in particular for schemes with a small number of bunches. The minimum spacing considered is 10 rf wavelength or one slot, corresponding to 25 ns or 7.5 meter.

There have been recently discussion with the physics co-ordinator aiming at defining the optimum schemes for early operation with few bunches. There is symmetry between IR1 and IR5, which always see the same bunches colliding and request maximum luminosity.
For IR2 and IR8, several alternatives were presented. IP8 is shifted compared to the LEP-DELPHI IP8 by 3/2 slots or about 11m. LHCb requests a luminosity of 2-5×10³² cm^-2 s^-1 , while Alice prefers very low luminosities around 10²⁹ for the proton runs.
Early operation with 43 bunches can be arranged such that only 4 of these would collide in IP2.
The recent discussions also renewed the interest in operation with 50 ns spacing and 1404 bunches. Potential advantages are fewer long range interactions and the possibility to provide over 1000 bunch collisions per turn in IR1,5,8 and only 2 in IR2. An important advantage of these new schemes is the intrinsic reduction of luminosity in IP2 without the need to un-squeeze or separate the bunches at the collision point. It is also clear that symmetric displacements of bunches in both beams should be preferred with respect to shifts in one beam, only.
This subject will also be discussed in the LHCCWG / LTC.

IP Coupling/Dispersion Knob by using Sextupole Bumps on LHC optics -> AM (pdf)
Akio Morita described coupling and dispersion knobs used successfully in KEKB and discussed possible applications for the LHC. Simulations at KEK are based on the SAD program. Using orbit bumps and sextupoles, it is in principle possible to correct both dispersion and coupling.
For the LHC, skew quadrupoles will be used for global coupling corrections.
Bumps in sextupoles are proposed to tune dispersion (no means of correcting vertical dispersion from the crossing schemes are available in the LHC). The main issue is the amplitude of the bumps, which make it difficult to use them due to the related aperture reduction. A similar results was obtained when closed orbit bump were studies to compensate the dispersion mismatch at injection (in that case the plane concerned was the horizontal one). At that it was found that tuning capability was very small, i.e., to generate a sizable dispersion rather large bumps should be introduced.

AOB
None.

Last update: 14-Februar-08

MG & HB

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