Summary notes of LCU meeting on 04/06/2007

Present: IA, MA, RA, RB, HB, UD, SF, MG,WH, JJ, DK, MM, TP, TR, FR, RT, SW, FZ, KP

Report from meetings

• SLM -> MG
  Please give your feedback to MG concerning the possible continuation of the ABP forum, as requested by OB in the group meeting.
  PAC papers should be submitted to OB by June 15. Oral pre-presentations are scheduled for June 19 and 21.
• ICC -> MG
  The Q5 magnet in IP8 left behaved very well in powering tests. The neighboring Q4 magnet quenched during tests and required training.
  The Q5R4 features a short to ground and it will need to be replaced.
  Q11L2 will be replaced in CW32.
  Triplet repair:  the Q1/3 in PI5 right and Q3 in IP2 right require de-cryostating. The request from ABP is to perform full fiducialization after cryostating. The moles to measure the geometry with the beam screen in place are not available. It will be important to monitor the progress of the repairing actions and that of the mole development to avoid missing the fiducialization in case of late arrival of the moles.
• Aperture analysis of proposed optics V6.501 -> TR (pdf file)
  TR showed  slides comparing the minimum aperture at injection in the insertions for Beam 1 and Beam 2 for optics V6.500 and V6.501. IR6 was not changed. In general, TR managed to get rid of several aperture restrictions. The aperture clearly improved for V6.501 in IR7 (both beams). For the other insertions the situation is less clear, i.e. in IR2 Beam 1 is clearly improved, but for Beam 2 the aperture bottleneck is slightly improved and moved to Q6 (focusing aperture). For IR3, there is an overall improvement, even though the situation at the Q6 is slightly worse. For IR8 the situation is essentially unchanged. The next step is to decide which IRs should be modified and to reconsider tunes and phase advance between IP1 and 5.
• Impedance measurements for FP420 -> FeRo (pdf file)
  The FP420 experiment requires the installation of special beam pipes with modified cross section and material in the dispersion suppressors around IP1 and 5 at about 420 m from the IP. The impedance of the proposed modified beam pipe has been investigated by analytical and numerical calculations as well as laboratory measurements using the wire method. A good match between HFSS calculations and measurements was observed for the resonance peaks between 2 and 3 GHz. The total impedance contribution from FP420 is already expected to be relatively small compared to the sum of the LHC collimator impedances. It can probably be further reduced using tapering or RF-fingers, as suggested by Fritz Caspers and Tom Kroyer.
  It is proposed to install permanently FP420 in the LHC with detectors operated at about 15 sigma from the beam axis. Several other issues in addition to impedance will require further study. One item is orbit stability and steering and another to check that particles intercepted in FP420 do not significantly reduce the quench limits of the downstream LHC magnets.
• Results of recent beam-beam simulation studies -> DK (pdf file)
  Dobrin showed results of beam-beam simulations performed last year. He calculated the dynamic aperture for several values of beta*, nominal (1.15e11/bunch) intensities and different values of beam-beam separation as a function of tune, along the diagonal, keeping Qx-Qy = 0.01. In the simulations, no magnetic errors are taken into account.
  As expected, the dynamic aperture generally scales with the separation. As expected, there is more margin for larger beta*. Long range beam-beam effects only become significant below beta*=1 m.
  In the nominal case (intensity and beta*) beam-beam resonances 4/13 and 5/16 are found to limit the DA as a result of the tune scan. These resonances can be corrected, but not at the same time, by appropriate adjustment of the phase advance between IP1 and IP5.
  He also looked at beam-beam effects for collisions at the injection beam energy of 450 GeV for various beta* values. For this he assumed intensities of 4e10/bunch and a limited number of bunches such that only head on beam-beam effects contribute and no crossing angle. In this case various combinations of magnetic field errors are included in the numerical simulations. In spite of the reduced bunch intensity, he finds that beam-beam effects are already important, mainly due to the interplay between beam-beam and machine nonlinearities. It will be interesting to have tunes scan results for the collisions at injection as well as footprint to understand the role of coupling corrections in the simulations results.
  As a next step it is planned to simulate beam-beam for the commissioning scenarios with the machine as-built and linear imperfections: a detailed simulation programme will be established.
• AOB
  None

Last update: 4-June-07

MG & HB

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