Summary notes of LCU meeting on 26/03/2007

Present: CB, RB, HB, RdM, SF, MG, WH, JJ, IP, TP, VP, TR, FS, RT, FZ

Follow up of actions

• Aperture model for the LHC transfer lines -> TR
  During a meeting concerning the TI8 injection tests, it was requested to have the apertures of the TI2 and TI8 transfer lines available as input for Mad-X. It appears that Samy Chemli was not fully aware of the need for transfer line data and that there is currently not enough manpower to implement transfer line apertures quickly in the layout database. This implies that we will meanwhile need an intermediate solution for our studies. TR will meet with VK and see how the aperture information, which is available from BT, can be implemented soon in the optics files.
  TR also mentioned that new LHC ring aperture files were made available with the LHC optics V6.501 files. The computations presented by TW should be repeated using these data.

Report from meetings

• Injection WG -> HB
  HB reported from the injection working group meeting on the 23 March 07. The main subject was on implications of changes from V6.500 to V6.501. The reason for these changes were explained by TR and illustrated by HB, see his slides. The optics parameters remain unchanged between the IPs and Q4 up to the injection points. The phase advances in the insertions IR2, 8 were allowed to vary and the new solutions improve the aperture for the circulating beam and to some extent also for the injected beam in the septum were the vertical aperture is particularly tight. The strength of the Q5 magnets which is also seen by the injected beam increases by 0.65% in IR8 with rather negligible consequences. However, it decreases by 7.3% in IR2. This would result in a 22% increase in betax and a 4.4% decrease of betay at the end of TI2 and requires a rematch of the transfer line optics. From a first look at a rematch of TI2 for V6.501 it appears that this would change the phase advance between the TCDIs by up to about 6 degrees, which can probably not be recovered at this stage by repositioning. This would result in some, albeit tolerable, loss in protected aperture. A final re-matching of the transfer line should be made (Action: HB). It is mentioned that the phase advance of one of the TCLIs is no more conform to the requirements in terms of phase advance. The history of the change of the phase advance should be determined (Action: TR) and then further actions launched.
• SLM -> MG
  MG said that Louis Rinolfi will be our representative for the academic training in stead of Michel Martini.
  MG also mentioned that there was an email exchange concerning the ramp rates of LHC power converters, as well as the limits on the minimum current for the MQM and MQY magnets. For small currents, ramp rates may be as low as 10 A/s rather than 50 A/s. Such ramp rates differ from those used during the magnetic measurements in SM18, possibly requiring additional measurements.
  As far as the minimum current is concerned, there is a limit set to 3% of the nominal current of insertion quadrupoles. Indeed, smaller values corresponding to as low as 2% could be tolerated. These two issues were raised in the framework of the discussions for the definition of the magnetic cycles for the various LHC magnets. These discussions are led by Rob Wolf in the framework of the FQWG.
  We were already informed about the possibility of not being able to power lattice sextupoles and octupoles in one octant in early operation. There was some concern if this would still be the case in 2008. MG will check the details with R. Saban.

  Consequences of phase advance differences between IPs on beam-beam effects -> WH (pdf file)

  The question whether special phase advance difference between points 1 and 5 would be essential for beam-beam effects in the LHC was raised some time ago, and the answer given by WH was no!
  Today WH gave more details on this subject including a pedagogical introduction, see his slides. He also recommended as good introductory material the lecture notes by Chao and the book of E. Forest. It is in principle possible to suppress the effect of selected resonances by a particular choice of the phase advance between IPs. For the current design LHC working points, the most relevant resonances are expected at fractional tunes of 4/13 and 5/16. Already the cancellation of one of these would require unrealistic tight tolerances on the phase advance between 1 and 5. Simulations confirmed that the design optics has a good dynamic aperture and that the exact choice of tunes and phase advance close to the design values is not too critical.
  FZ reported that UD works on beam-beam simulations with broad tune scans and it was proposed to ask him to give a presentation in one of our next meetings.

  Beta-beating from longitudinal misalignment of low-beta quadrupoles -> FS

  FS reported on measurements of the longitudinal alignment of triplet quadrupoles and their impact on the beta-beating, see his slides. This issue was raised during the approval process of the triplet quadrupoles. Due to the uncertainty in the longitudinal position of the magnetic centre with respect to the cryostat and the beam screen the nominal position of the magnets might not be the optimal one. FS determined the resulting beta-beating using the measured values including also the effect on measured errors (corresponding to about 5 mm). In addition, the impact of longitudinal shifts aimed at minimizing the beta-beating was studied. It turned out that with reasonable shifts of about 4 mm, it would be possible to reduce the beta-beating down to about +/- 5%. During the discussion it emerged that it would be important to compute the beta-beating for each insertion separately (to avoid any cancellation) and to use the squeezed optics for IR2, as this will be worst case (Action: FS). It is also recalled that in addition to the implementation of longitudinal shifts, re-matching of the optics should be used to improve the situation.

  AOB

  None

  Last update: 17-Apr-07

  MG & HB

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