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

Present: M. Aiba, G. Arduini, S. Aumon, E. Benedetto, O. Berrig, R. Calaga, C. Carli, M. Chanel, F. Gerigk, A. Franchi, S. Gilardoni, M. Juchno, M. Martini, Y. Papaphilippou, G. Rumolo, B. Salvant, G. Smirnov

General information (G. Arduini)

The White Paper has been approved. C. Carli asked whether the this is available. G. Arduini replied that he sent the paper out few months ago but he will re-send it again. (done after the meeting).

M. Chanel noted that there is still a water leak from the roof in the corridor of building 9 although the TS/FM service has been informed since several weeks. G. Arduini will contact M. Nonis. (done after the meeting - the roof has been repaired only locally. The leak has stopped but we need to monitor the situation).

G. Arduini reminded that the deadline for the submission of proposals for students in the frame of the VIA  programme to Simon is Friday 6th July 2007.

PAC07 conference:

• SNS is on track with the commissioning although the planning was not very aggressive. They are at 90 kW beam power on the target out of 1.4 MW (nominal operation). High radiation levels are observed at injection and a redesign of the injection is ongoing.
• DOE said that ILC construction cannot be started before 2025
• German Government has given the green light for the start of the FAIR project at GSI. Official start ceremony in November 2007. The German government will pay 75% of the overall cost the remaining part should come by other states. For the time being the external contributions amount to about 3 % of the total cost. civil engineering expected to start early 2009.
• Emphasis on medical machines and non-conventional acceleration techniques.

Preliminary results of the E-Cloud Instability MDs at the SPS  (G. Rumolo) - slides.

Preliminary measurements were already performed in 2006 (movie1). Signs of instability were observed only at 26 GeV/c. More detailed studies have started in 2007 before and after the scrubbing by comparing the behaviour of the LHC beam (1 batch consisting of 72 bunches) at injection momentum (26 GeV/c) and at 37 GeV/c. These studies have been performed on the MD cycle in parallel to physics.

The vertical chromaticity required to stabilize the beam at 37 GeV/c is larger than that required at 26 GeV/c.

During the test performed before the scrubbing run losses were observed at the flat-bottom, the losses were mainly affecting the centre of the batch. M. Chanel and G. Arduini noted that this could be due to beam loading. G. Arduini noted that very likely the setting-up of the feedback and feed-forward was not done properly, during the following week a problem with the cable connecting the Wide-band pick-up used for the feed-forward to the Faraday cage has been found. This could explain also the observed bunch intensity signals that increase with time and the larger losses observed at the start of the ramp. Observations at 37 GeV/c could be compatible with both a coupled-bunch (when losses at the beginning of acceleration occur - movie 2) and single-bunch instability at the end of the batch (movie 3). The behaviour at 26 GeV/c is more consistent with a coupled batch instability (movie4). Bunch lengthening and a larger spread in bunch length is observed in at the time of the instability. C. Carli noted that this could be the consequence of the losses and of the poorer quality of the fit to the bunch profile to determine the bunch length. Measurements at 26 GeV/c with lower bunch intensity (~70 % of the nominal) showed also instabilities for the same value of the chromaticity (movie 5). During this first MD the head-tail monitor was not working and no data could be taken.

A second machine study was performed after the scrubbing run. The losses at the beginning of the ramp were smaller (very likely as a result of the better adjustment of the feedback and feedforward). Instabilities affecting a larger fraction of the batch were observed both at 26 and 37 GeV/c (movie6 and movie7). G. Arduini suggested to performed a 2D FFT of the data in order to determine the frequency of the modes that are becoming unstable and to analyze the head-tail data taken during the last MD.

The following points seem to be in contradiction with the past experimental evidence of an electron-cloud instability:

• no clear improvement of the threshold of the instability after the scrubbing run, although the RF setting-up was completed only during the scrubbing run and the intensity was higher in this second experiment
• no clear evidence of a single bunch effect in particular at 26 GeV/c

G. Arduini asked whether the transverse feedback was switched off at the moment of the measurement. G. Rumolo replied in the negative.

E.Benedetto suggested to take one batch in which the last 12 bunches have lower intensity and to observe the effect on the instability. 

Status of the machines

PSB (M. Chanel)

The high intensity beam is transversally unstable in ring 4 (horizontal plane) ~1 ms before extraction when operating at intensities larger than 7.5x10¹² p/ring/cycle. This could be due to the large orbit distortions required to steer the beam in the extraction channel. This induces large offsets at the damper pick-up and could hamper its operation. G. Arduini asked whether it is planned to perform a machine realignment. M. Chanel replied that this will be done during the next technical stop.

The tune measurement is not yet working. M. Gasior is working on it.

G. Arduini asked whether any progress has been done on the 160 MeV/c beam for space charge studies. M. Chanel replied that he had to work on the stability problem above mentioned. G. Arduini stressed the importance of these studies.

PS (S. Gilardoni, Y. Papaphilippou)

It has been a difficult week. Several trips of the figure-of-eight loop. There has been a long stop during the weekend because of a suspected earth fault on one of these circuits.

The measurements of the TT2 initial dispersion conditions performed on AD with lower intensity are now consistent with the results obtained for the LHC beam last year. The initial discrepancy could be the result of losses occurring at extraction for large momentum offsets. It was also observed that the radial position at extraction is approximately -7 mm. This should be adjusted before continuing the matching measurements without QKE58. G. Arduini proposed to adjust the extraction and perform the measurements in the next PS dedicated MD (included in the planning after the meeting).

The tune measurement (BBQ) is not working properly in the PS. When there are losses the values that are provided are shifted by 0.02. This prevents progressing on the measurement of the 5-current matrices for chromaticity correction.

The loss pattern at injection on SFTPRO has changed and needs to be understood.

Status of the space charge simulations for the LINAC4 to PSB injection (M. Aiba) - slides

M. Aiba presented the latest results of the simulations performed with ORBIT for parabolic and gaussian distributions. The sudden vertical blow-up observed for a parabolic distribution is not observed with a gaussian distribution although a continuous but larger blow-up is observed. The latter can be understood because for a bi-variant gaussian distribution there is a larger fraction of particles at smaller amplitudes as compared to a parabolic distribution. The analysis of the evolution of the first and second order momenta of the distribution show that the sudden blow-up is due to the onset of quadrupolar and dipolar oscillations. The latter persist for quite some time. G. Rumolo and G. Arduini were surprised to observe the above behaviour as in the simulations no boundary conditions are imposed. G. Arduini suggested to contact S. Cousineau to have her feedback. Action: M. Aiba.

The following possible causes were mentioned (to be followed-up by M. Aiba, C. Carli):

numerical noise: M. Aiba noted that the sudden blow-up occurs later and later if the number of macro-particles is increased. E. Benedetto suggested to symmetrize the distribution by adding for each macro-particle its symmetric one in phase space. G. Arduini asked whether a "stabilization" of the behaviour has been observed as a function of the number of macro-particles. M. Aiba replied that this needs to be done.

non-linear elements (e.g. fringe fields). C. Carli suggested to repeat the simulations with linear tracking.

M. Aiba presented also the comparison between PTC and ORBIT with non-linear tracking. The two codes provide the same chromatic tune shift. G. Arduini asked whether the evolution of the r.m.s. emittances from PTC and ORBIT (without space charge) has been compared. M. Aiba replied in the negative.

G. Arduini asked whether it is possible to freeze the space charge potential: in a paper of Ohmi on beam-beam the "re-calculation" of the beam potential was found to be a source of numerical noise. M. Aiba replied that this is not foreseen in ORBIT.

G. Arduini concluded that the needs in terms of computing should be identified and support required to the LINAC4 project leader. Need for support from FERMILAB should be also identified (use of their cluster, or possible modifications of the code to freeze the space charge potential).

Next meeting

Monday, 23^rd July 2007 at 09:00 in room 354-1-001

Agenda

General information (G. Arduini)

Status of the PS magnet model (M. Juchno - to be confirmed)

Preliminary results of the non-linear chromaticity measurements in the SPS (R. Tomas or R. Calaga)

Status of the machines (supervisors)

A.O.B.

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