Minutes of the LIS Section meeting held on 2nd April 2007.

Present: M. Aiba, G. Arduini, S. Aumon, G. Bellodi, C. Carli, M. Chanel, A. Franchi, F. Gerigk, M. Juchno, E. Métral, M. Martini, S. Maury, G. Rumolo, B. Salvant, G. Smirnov.

General information (G. Arduini) - slides

The study of the expected performance of the PS Booster with LINAC4 H- injection requires tools to simulate reliably the effects of space charge. The meeting was mainly devoted to a review of the benchmark effort on space charge simulation done so far and in addressing the possibility of an experiment to benchmark simulation codes (ACCSIM and/or ORBIT) in the PSBooster.

Space Charge Experiments and Benchmarking in the PS (E. Métral) - slides

The benchmark data showed concerning the crossing of the integer or half-integer resonance in the PS refer to a dynamic scan (i.e. the tune and RF voltage are changed during the cycle after injection). Link to EPAC04 paper (S. Cousineau et al.). Link to PAC03 paper (M. Giovannozzi et al.). G. Arduini asked whether machine errors and chromaticity were included in the simulation. E. Métral replied that very likely the non-linear model of the machine has been included. It is not clear whether dipolar or quadrupolar errors have been included.

Measurements (both in static mode - i.e. the working point was changed in different cycles and kept constant during each cycle - and in dynamic mode) have also been performed to benchmark the simulation codes for the Montague resonance (Link to EPAC04 paper (S. Cousineau et al.)). Measurements in static mode and IMPACT (full 3D code from Berkeley) simulations evidence an asymmetric stop-band. Link to HB04 paper (E. Métral et al.) and a fair agreement between measurements and simulations. Some discrepancies remain close to the resonance line. The agreement between simulations is good (Link to PAC05 paper by I. Hofmann et al.). It must be noted that the benchmarking with the Montague resonance is mainly testing the behaviour of the space charge codes with respect to the modelization of the space charge fields and for "short" time scales (few tens of turns).

The agreement between simulations and measurements is not good on the other hand for the dynamic case (Link to HB04 paper (E. Métral et al.)), where an important mixing of the emittances is observed. According to simulations, when the longitudinal motion is taken into account this should occur only if the crossing of the resonance takes place in a time scale which is much longer than the synchrotron period. (Link to EPAC04 paper by I. Hofmann et al.). Even taking into account this effect and the parameters of the experiment the disagreement persist. C. Carli asked whether losses were observed during the process as it seems that the sum of the emittances after the resonance crossing is smaller than the sum before the resonance crossing. E. Métral replied that he was not sure about that.

E. Métral presented also a list of the available space charge codes available with their basic characteristics (slide). F. Gerigk noted that very likely IMPACT cannot be used for long bunches as those in the PS and PSB.

Space charge tune spread can drive part of the beam onto resonances and depending on how the space charge tune spread overlaps the resonance, core emeittance blow-up or halo (and losses) can be observed. A good agreement exist between measurements and simulations for the emittance blow-up although some discrepancy remains for the measured beam losses. The discrepancy can be reduced (but not completely eliminated) by taking into account the effect of chromaticity (Link to HB2006 paper by G. Franchetti) and longitudinal motion. C. Carli and G. Rumolo noted that the remaining discrepancy could be due to power supply ripple or to a simplified aperture model.

E. Métral concluded presenting some possible experiments for space charge code benchmarking, in particular:

Simulations for emittance blow-up at 160 MeV (M. Martini) - slides

M. Martini presented the results of the ACCSIM simulations for the nominal CNGS and LHC beams at 160 MeV. In both cases an emittance blow-up of up to 30 % is observed in ~15000 turns for the unperturbed tunes QH=4.28 and QV=5.47 and assuming operation with a single harmonic RF system h=1 at 8 kV. G. Arduini asked whether these conditions are realistic. M. Chanel replied that it should be possible to run at 160 MeV with the above RF parameters. He noted that the considered tunes are those at 50 MeV but during the ramp the tune is dynamically changed. M. Martini asked to have the values of the tunes that one can expect at 160 MeV. Action: M. Chanel. M. Chanel also asked whether the case with the same integer tune has been considered. M. Martini replied that this was done in the past and no significant difference was observed.

It was concluded that the following experiment should be performed as a case for benchmarking the codes:

Acceleration of a single LHC and CNGS nominal bunch to 160 MeV with double harmonic RF system to study comparatively the emittance blow-up when keeping the h=2 ON during the 160 MeV plateau and when switching it OFF. Monitoring of the beam transverse and longitudinal coherent motion should be performed to identify the onset of instabilities on the 160 MeV plateau.

According to M. Chanel it should be possible to start these studies at the beginning of June.

The following additional simulations should be performed for both beams ==> M. Martini:

M. Martini noted that he has no more the double-core PC that he used to run the simulations and the queue on LXPLUS is limited to 1 week. (After the meeting B. Salvant found that a a longer queue exists and he informed M. Martini). G. Arduini will ask for a new double-core PC for M. Martini. Action G. Arduini.

After the meeting A. Franchi noted that the cluster OPLAPRO could be used also for running ACCSIM provided that this can run on LINUX.

M. Chanel noted that the tune footprint extends to very low fractional tunes and he asked how the tunes of each particle are calculated. M. Martini replied that probably this is done by counting particle zero crossing but it was not clear over how many turns this process is applied. This should be investigated. Action: M. Martini.

First comparison of ORBIT and ACCSIM (M. Aiba) - slides

M. Aiba presented the first results of the simulations performed with ORBIT for the nominal LHC beam for the same parameters and initial beam distribution as assumed by M. Martini for the ACCSIM runs. The evolution of the rms and 100% horizontal emittances is comparable to that obtained with ACCSIM although in the vertical plane there is a significant discrepancy appearing suddenly. Comparing the particle distribution vs. amplitude for the two cases the differences are mainly localized in the tails and the observed behaviour could be the effect of a statistical fluctuation. During the discussion it was noted that the rms and even more the 100% emittance are very sensitive to tails and that it might be better to quote the percentage of particles inside a given physical emittance or the profiles.

M. Aiba noted that it is not yet clear what is the tracking option (linear or non-linear) used by ORBIT while ACCSIM uses a linear tracking. Contrary to what was said during the meeting ACCSIM takes into account of the machine chromaticity in the tracking (using a momentum dependent betatron frequency as indicated in the ACCSIM user guide - link in Section 16).

A. Franchi noted that there were some problems in the last few weeks with the cluster (OPLAPRO) where ORBIT is installed in order on run ORBIT in parallel on several (14) processors. This problem has been fixed and now a procedure exist and has been transmited by A. Franchi to M. Aiba after the meeting.

 

Next meeting

Monday, 16th April 2007 at 09:00 in room 354-1-001

Agenda

General Information (G. Arduini)

Model of the losses in the PS and modelization issues (J. Barranco)

Status of the machines: PSB (M. Chanel)

A.O.B.

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