Minutes of the LIS Section meeting held on 10^th August 2009

Present: S. Aumon, G. Arduini, J. Barranco, E. Benedetto, O. Berrig, H. Damerau, M. Chanel, C. Carli, S. Gilardoni, P. Jackson, D. Quatraro, C. Zannini.

General Information (Gianluigi)

LMC has recommended to run the LHC at 3.5 TeV and later (once we have a better understanding of the machine) to energies up to 5 TeV. A formal decision will be taken today although the experiments (represented at the meeting) have expressed their approval for that proposal.

IEFC

The second main 400-66 KV transformer is back in service Simone has presented the status of MTE and the results of the study of the impact of a failure of all the PFW in one or few PS magnets. 30 to 50 spare PFW will be produced. The risk of damage on the main coils is considered to be extremely low.

SPS:

In 2 occasions after the replacement of dipole magnets with spares large orbit errors have been observed. After investigation it has been found that these magnets have been measured but no trim of the shims has been applied to obtain the required integrated dipole field. Gianluigi has discussed the issue with D. Tommasini. Corrections will be applied to the new magnets based on measurements of the effect of shimming on a reference magnet.

SPS studies for low SEY coatings. Although the liners installed in the LSS5 test bench still confirm the lower of the SEY for carbon coated liners, the measurements performed in MBB dipoles with coated vacuum chambers are not conclusive. The transmission RF measurements performed by F. Caspers aimed at evidencing a phase modulation due to the electron cloud feature a high noise. Local pressure measurements are not conclusive either. Improvements on the measurement set-up are being studied.

LHC:

Sector 12: the arc is now at less than 5 K and ELQA tests have started

Orbit distortion at transition in the PS with new configuration of pick-ups for the radial loop (S. Aumon) - slides

The machine experiments performed in 2008 have shown that part (1-2 mm) of the excursion of the radial position at transition can be explained by the fact the the beam is not going through the centre of the gamma jump quadrupoles. The rest (1-2 mm) could not be explained and losses can be reduced by adjusting the radial offset at transition. 

The behaviour of the radial loop has been studied. It has been observed that the dispersion is decreasing during the transition jump in some of the pick-ups used for the radial loop. In particular this is the case for the pick-up 55.

A new pick-up has been connected for the calculation of the radial position.

The configuration including pick-ups 22-76-36-96 allows to reduce the mean radial position (MRP) jump to 0.5 mm for the AD and SFTPRO beams.

Gianluigi and Giovanni noted that It would be interesting to plot the evolution of the average dispersion among the pick-ups used for the radial loop for the various combinations studied in order to confirm the reason of the better performance which is very likely due to the larger value of the average dispersion during transition for the above set-up.

For the TOF beam the visible jump in radial position is likely due to the fact that the average orbit is not centred on the gamma jump quadrupoles. The gamma jump doublets have the main effect on the orbit. The strength of these quadruples is varying very rapidly (within few hundreds of microseconds) and their effect on the orbit when the beam is not passing through their magnetic centre cannot be compensated by the radial loop which has a response time of approximately 2 ms.

Gianluigi congratulated Sandra for the good results and suggested to implement the new configuration in operation. This would require a dedicated MD to set-up all the operational beams with the new configuration and would require good support from OP to take references before moving to the new set-up and to set-up the beams. Gianluigi suggested to present these results to the MSWG so that the required support can be organized. Elias will plan that for a future dedicated MD.

Study of the longitudinal beam behaviour at transition with experiments and HEADTAIL simulations (S. Aumon) - slides

The study aims at getting information about the low frequency component of the PS longitudinal broad-band impedance. The low frequency part of the broad-band impedance, together with space charge, affect the longitudinal beam size evolution around transition. The comparison of this measured evolution of the bunch length (without gamma jump) with HEADTAIL simulations can therefore provide useful information about the low-frequency broad band impedance.

Some discrepancies between the analytical model and the HEADTAIL simulations are likely due to a some initial mismatch and can be improved.

When the known longitudinal broad band impedance model of the PS is introduced the simulations show that a microwave instability is occurring at bunch populations larger than 3×10¹² p due to the resistive component of the impedance. This is qualitatively consistent with the observation that a gamma jump is needed in the PS to cross transition.

Heiko pointed out that it is very important to adjust the RF phase jump properly (i.e. at the moment the transition energy is crossed). Although this is easy when the gamma jump is active, this is less evident when this is not the case. Phase jump timing errors affect the bunch length evolution around transition and therefore must be taken into account (if known) when comparing to simulations.

Round-table (all)

Olav mentioned an issue with the maximum strength of the correctors used for the crossing scheme for the LHC. This is not an issue for the 3.5 or 5 TeV run.

LEIR

With 16 microamperes from LINAC3 10¹⁰ charges can be delivered to the PS (the design early beam intensity is 1.2×10¹⁰ charges)In order to have the design intensity 20 microamperes are needed. The injection efficiency has been significantly improved (from 50% to 80% - although the absolute values should be taken with care the relative improvement is evident)

An MD early cycle has been made available. This feature an injection plateau with more injections in case of need to compensate for low Linac3 intensity

The RF low level control is now fully digital There are still some problems with the availability on OASIS of some signals. The control of the voltage on the cavity requires also further improvement.

Signs of self-bunching are visible when the cavity short circuit is opened and could be responsible for the losses observed at low energy. This requires further investigations in view of minimizing the losses for the nominal beam.

 The intensities of the beam in the injection and extraction lines are now available on the VISTAR but there are still issues for the ring intensities.

Dispersion measurement in the line have been made in the TT2 line and are not consistent with the values in 2007. Polarities are being verified in TT2.

LINAC4

Simulations will be launched to study a working point with higher fractional vertical tune to minimize the blow-up. Masamitsu has transferred most of the tools for space charge simulations to Matthias. Work is ongoing in the collimation studies (Penny): some bugs have been identified and corrected.

PSB

Diego is analyzing the data for the instability measurements conducted in the PSB. Gianluigi reminded that it is very important to summarize them in a detailed paper.

Some instabilities are observed in Ring 3 even when the transverse feedback is on. This requires further analysis.

The observed discrepancy between the wire scanner and SEM grid profiles has been understood. The discrepancy was due to saturation of the photomultiplier signal. Gianluigi will mention that to Simone to see whether the same effect is responsible for some of the discrepancies observed in the PS.

Impedance

Carlo is conducting some simulations with Particle Studio (with the direct integration method) for the kickers and he is trying to reproduce Tsutsui's results. In the longitudinal plane there is a good agreement. The simulations for the transverse plane are ongoing.

MTE

The dispersion measurement in TT2 have been analyzed even taking into account the non-linear behaviour. Elena is matching a ne optics based on these measurements. She will also take some additional measurements (including one of the islands) on Wednesday at the latest.

Neutrino Factory and Beta Beams

Elena has presented the results of her stability studies for the accumulator for a proton driver at NuFact09. She is now working on the production ring for the beta beams based on the proposal of C. Rubbia. At FNAL she has discussed with D. Neuffer who has also made some studies on Rubbia's idea.

A lattice for the production ring has been designed by M. Schaumann and Elena with the help of B. Holzer.

Next meeting

Monday, 7^th September 2009 at 14:00 in room 354-1-001

Agenda to be defined.

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