Minutes of the LIS Section meeting held on 21 May 2012

Present: G. Arduini, H. Bartosik, E. Benedetto, M. Bodendorfer, V. Forte, A. Garonna, A. Lachaize, A. Molodozhentsev, A. Ulsroed, R. Wasef, C. Yu

General Information (Gianluigi)

Gianluigi reminded that he is expecting input for the ABP contribution to the BE Annual report by Friday 25/5.

LHC status:

Progress has been done in the understanding of the losses and of the lower performance of the LHC in collision. The main problems and solutions found:

IPAC12 papers: the were quite some delays in the submission and revision of the papers. The procedure needs to be improved for the next IPAC.

 

PS space charge studies  (Elena) - slides

Elena presented a summary of the measurements performed in the PS to study space charge effects at 2 GeV (this will be the new injection energy after the Injector Upgrade).

Last year measurements have been performed with natural chromaticity. It is planned to repeat these measurements after correction of the chromaticity. Raymond will conduct these measurements.

The tune scan with high intensity beams in the vertical plane has been conducted by varying the vertical tune above the coupling resonance to avoid head-tail instabilities. The correction of the chromaticity and the setting-up of the transverse feedback should allow operating the machine below the coupling resonance without instabilities.

During the measurements the skew quadrupoles were ON at the settings used in the past to stabilize the LHC beams. Following recent investigations it is now known that the present setting do not provide coupling compensation. At present we know that the coupling cannot be corrected. Simone is following up the issue with the magnet team. Measurements performed during the technical stop indicated that some quadruples were not correctly powered although more recent measurements conducted by the power converter teams showed that the measured load corresponded to the expectations.

Gianluigi noted that it might be interesting to evaluate separately the core emittance and the tails in order to disentangle core emittance blow-up from tail generation. These two phenomena might have different weight according to the selected working point. Elena is presently trying to fit the profiles with a Levy-Student distribution (with the help of M.Martini), to characterize the tails.

Gianluigi asked whether the scans in vertical tune have been performed in both directions as the loss pattern might depend on the working point history. He suggested to do scans in both directions for the tune. Static measurements should be also performed.

The space charge tune spread at 2 GeV has been obtained by shortening the bunches. Replying to Sasha Elena explained that two measurements of the emittance were taken one at the beginning and one at the end of the 2 GeV plateau and after bunch shortening. The interval of time between the two measurements was 1 second. Sasha suggested to conduct measurements also within few ten of ms as this likely corresponds to the maximum number of turns that can be realistically simulated.

Gianluigi asked whether simulations have been performed with the beam characteristics used for the experiments conducted by Elena. Raymond replied that the simulations have been done for the LHC beam so far. Gianluigi suggested to run simulations also for beam parameters corresponding to the experiment.

Gianluigi asked whether it would be possible to change the integer tune in one of the two planes to study the effect of the Montague resonance. In the past this was considered to be one of the potential limitations.

 

Round table (all)

Elena has started PTC-ORBIT simulations for the PSBooster with short dipoles.

Vincenzo, Elena and Sasha have studied the impact of the double harmonic RF. There are indications that by changing the ratio of the h=2 RF voltage with respect to the h=1 RF voltage the beam size oscillations and blow-up observed by Vincenzo in the simulations can be suppressed. the observed blow-up presented by Vincenzo at the last meeting could be the result of periodic resonance crossing due to the synchrotron motion and to the non completely matched longitudinal distribution resulting in bunch length oscillations and therefore in modulation of the maximum extension of the tune footprint with corresponding transverse blow-up.

Gianluigi suggested to benchmark these simulation results with measurements in the PS Booster.

 

Raymond is preparing for resuming the working point studies conducted by Elena last year and he is studying convergence for space charge simulations.

 

Hybrid extraction is being set-up by Cedric. Measurements and correction of the tune and chromaticity changes during the extraction bump raise are ongoing. It is not evident to correct non-linear chromaticity.

Antoine has resumed the measurements to understand the origin of the fluctuations of the island position observed last year.

 

Adrian and Hannes have conducted measurements of the dispersion and dispersion mismatch in the injection line TT2-TT10

OTRs are not working and there are difficulties to set-up the first turn measurement to measure the first turn when the energy of the extracted beam is changed in the PS to measure dispersion.

Hannes has compared the measurement of the beam instability observed in the LHC with 48 bunches and a spacing of 25 ns last year with PyCLOUD/HEADTAIL simulations. The agreement is good..

 

Adriano is simulating the slow extraction form LEIR and determine the settings for the extraction sextupoles and satisfy the Hardt condition. Gianluigi asked whether it is conceivable to tune the optics to have low dispersion at the extraction point.

A brainstorming will be organized at CERN with the possible users of the LEIR radiobiological facility to determine what are the requirements for the ion beams.

 

Next meeting

Monday, 4th June 2012 at 14:00 in room 6-R-012

Agenda:

General Information (Gianluigi)

 

Round Table (all)

 

Back to Minutes