Summary notes of the LCU meeting on 18/11/2008

Present: MA, CB, HB, RC, SF, MG, WH, JJ, YL, JM, EM, JN, SP, VP, TR, FR, FS, RT, TW, SW, SY, FZ

Report from meetings

• SLM -> MG
  An email was circulated to remind about PAC'09 abstract submission by Monday noon to Oliver and Massimo.
  There will be a general AB meeting on 4 Dec. with a presentation by Steve Myers and a review of 2008 activities including up to 4-5 slides from each group. MG proposes to have the on-line model, aperture measurements, polarity checks, and beta-beating measurements presented in three slides. the global presentation will be discussed at the SLM.
  There will be a general ABP group meeting on November 28th. The new department and group structures for 2009 were defined (see details on the accelerator sector here).
  LHC sector 3-4 is warm and repair in progress. The MEB will be revived to deal with the magnets affected by the repair (dipoles and SSS). Further analysis and measurements revealed two dipoles with high internal splice resistance, one in 1-2 and the other in 6-7. While the first one will be certainly replaced, no decision is taken for the second one. MG highlights that in tomorrow's LHCPerC meeting the latest results about the new techniques developed to assess the status of the splices will be reviewed.

50 ns LHC beam through the injector chain in 2008  -> EM (pdf)

This study was requested by the LHCCWG on 13/02/2008. 50 ns beams for the LHC were studied in MDs in booster, PS, and SPS this year. For the booster the main difference compared to 25 ns is the reduction to half intensity by transverse shaving. In the PS, the final quadruple bunch splitting is reduced for the 50 ns beam to a double splitting, only. The 50 ns beams were injected and accelerated in the SPS and it was possible to obtain rather small transverse emittances of about 1 - 1.5 micrometer. If requested, it will be possible to increase the emittance in the SPS by controlled blow-up (also studied this year by EM et al. in the SPS).

SF asked if ultimate intensities would be possible for 50 ns. After the meeting EM contacted the PS RF specialists, who confirmed that 50 ns beams are the most challenging in terms of increasing the intensity/bunch, while 75 ns one are the least difficult.
FZ asked about the value of the longitudinal emittance, namely how the same emittance in the PSB can produce the same in SPS with a different number of longitudinal splittings in the PS (for the 25 ns and 50 ns beams). The situation is clarified by EM after the meeting (referring also to a presentation by Roland Garoby at Chamonix 2003): a different longitudinal blow-up is applied at PS after the triple splitting, namely about 115% for 25 ns beams and only 28% for 50 ns beams.

Impact of MD results for the 50 ns LHC beam -> WH (pdf)

50 ns were proposed for the LHC. They are interesting in several respects as intermediate solutions towards the nominal performance. Operation with 50 ns allows for much flexibility to distribute the luminosity according to the different needs of the experiments. With 50 ns it will be possible to adjust the fraction of bunches which collide in LCHb, give maximum luminosity to ATLAS and CMS and allow to keep the luminosity for ALICE at an approximate low constant level as requested.
The smaller emittances possible with 50 ns will be beneficial for aperture and reduce long range beam-beam. On the contrary, the local energy density relevant for collimation and the head on beam-beam tune shift would increase. Still, it is believed that starting off with a smaller emittance would make operation easier and more flexible with the possibility for controlled blow up if needed. As for the 25 ns, there will be bunch by bunch intensity and emittance variations. HB said that a minimisation of bunch differences will be important for beam-beam and background and proposes that bunch equalisation would be looked into for the LHC.
Follow up:  check / re-optimisation of the nominal tune working point for 50 ns with less long range and same or more short range beam-beam effects as well as methods to generate controlled transverse blow-up. In this respect there are synergies to be found as robust techniques to manipulate the transverse beam emittance are important in all accelerators in the LHC injectors' chain.

Optics studies for LHC upgrade -> JM
JM reported about optics studies for the first phase of the LHC upgrade with increase aperture in the triplet. He confirms the findings of SF and already reported in the LIUWG meeting last May.
No optical solution is found with beta*=25 cm and the current layout of the matching section. Hence, in case it is confirmed that the matching section cannot be modified as part of the insertion upgrade, then the only viable approach consists in finding the minimum beta* compatible with new triplets and the current layout of the matching section.
In parallel, JM provided also field quality tables for the closed orbit correctors based on scaling from the current triplet field quality, taking into account the constraint that the field quality of the corrector should be in the shadow (10%) of the one of the quadrupoles, and the beta-function ration (nominal and upgrade - estimated - optics). Similar tables are also produced for the cold D1 magnet in order to compare the proposed field quality with some targets. It is clear the ultimately, the answer on the required field quality will rely on tracking studies once the optics solution will be defined.
Early in December WH will review the situation of the closed orbit correction at the LIUWG. It is already clear that the proposed approach with only one closed orbit corrector between Q3 and D1 is not aceptable.

Synchrobetatron optics: the grazing function g -> Steve Peggs
Postponed to the next meeting.

AOB

SF received data on measurements of the CMS magnet fringe field which is much larger than originally anticipated (see details here). The effect on beam dynamics maybe non negligible and requires modeling and possibly correction. SF already worked out the most general form for solenoidal fringe field assuming radial symmetry. This expression could be used to perform some analytical computations about the effect of the stray field. In any case proper implementation is required before any numerical simulation is performed.

Last update: 2-December-08

MG & HB

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