Minutes of the LIS Section meeting held on 23^rd July 2007.

Present: M. Aiba, G. Arduini, S. Aumon, J. Barranco, O. Berrig, R. Calaga, A. Franchi, S. Gilardoni, M. Juchno, Y. Papaphilippou, G. Rumolo, R. Tomas.

General information (G. Arduini)

A new SLS scheme is being put in place as circulated by e-mail

At the LTC it was mentioned that LHC should not take ions in 2008 and that the SPS commissioning with ions should go ahead as much as possible this year and should concentrate on the Early beam. The question might be raised about the not having ions in 2008 at all in the Injectors.

S. Gilardoni noted that the restart of the ion injectors for LHC in 2009 after one year stop might imply some risks. G. Arduini replied that one could imagine to restart the ion injectors in 2009 well in advance to sort out any potential problem in due time.

Status of the PS main magnet model (M. Juchno) - slides

Both a 2D and 3D model of the PS magnet have been produced. The 2D model results were compared with the results of magnetic measurements performed by D. Cornuet are in good agreement with the measured gradient and sextupole components measured at the centre of one magnet block and along the radial direction. Given the 2 approximation the same cannot be said for the dipole magnetic field along the beam direction (slide 3 - the x coordinate is there the coordinate along the beam direction). The measured variation of the dipole field is due to the structure of the PS magnet consisting of 10 blocks (5 for the defocussing part and 5 for the focussing part). The two half-units are separated by a gap of 20 mm (called junction). This explain the difference between the measured and calculated dipole field levels (the latter is represented by the dashed red lines).

Attempts have been made to compare the measured beam parameters with the machine parameters obtained with MAD8 assuming the gradients and multipolar components obtained from the ANSYS simulations and adding the edge field effects as calculated from MAD8. For the moment no conclusion could be drawn due to the poor reproducibility and ripple performance of of the New PFW power supplies. The comparison is important to understand whether the 2D magnetic model and the representation of the edge field effects in MAD8 (or PTC) would be sufficient for a good representation of the PS magnets in terms of machine settings prediction from the powering currents and the calculation of the matrices for tune and chromaticity correction.

The 3D model requires a lot of computing power and it is approximate because the exact geometry of the PFW and (in particular) figure of eight loops at the edges of the magnets and at the junction is not know exactly.

The 3D model is certainly required to understand the magnets at injection and extraction but this might not require the knowledge of teh exact geometry of the PFW and figure of eight loop geometries.

Non-linear chromaticity measurements in the SPS  (R. Tomas) - slides.

These studies are ongoing since a few years with the aim of providing a non-linear model of the SPS for the LHC beam and possibly to reduce the spread in tune resulting from the non-linear chromaticity and momentum spread of the LHC beam which is significant.

R. Tomas presented the results of the non-linear chromaticity measurements performed on the LHC cycle with a pilot beam in order to minimize intensity effects (instabilities and error on the non-linear chromaticity measurement as discussed at the APC on 11/05/2007. There is still some discrepancy between the values measured in the 2004-2006 and 2007. This can be partly explained by:

• different intensities were used (larger in 2004 and 2006)
• smaller momentum span due to instabilities in 2006

The low intensity used in 2007 (the lowest achievable and measurable) should provide the most reliable information. Furthermore the measurement has been performed all along the injection plateau and the ramp by measuring the tune every 20 ms with chirp excitation in "blocks" 4 second longs. The following features have been observed:

• the vertical tune is changing along the flat-bottom. G. Arduini noted that this might be due to some eddy currents persisting on the vacuum chamber after the ramp down of the main magnets from 45 GeV to injection. An attempt has been performed to minimize this effect by adding a pre-injection plateau at 14 GeV/c. This has been only partly successful. The observed drift is smaller than in the past but it is not completely cancelled.
• the horizontal tune varies during the injection plateau along each 4 second block. This is very likely due to detuning with amplitude in the horizontal plane and to blow-up due to the chirp excitation amplitude.
• a tune jump was observed at 10 s after injection and it was not understood as no trim was found at that timing.
• a small bump in chromaticity was observed at the beginning of the ramp

A possible scheme for reducing the tune spread due to non linear chromaticity was also calculated based on the measured data.

Next steps:

• Understand the origin of the tune jump observed at the injection plateau at 10 s (done: it was found that the HW compensation trims were not done correctly and contained a trim at that timing. This has been fixed during the long MD in week 30).
• correct for the chromaticity bump (done during the long MD in week 30).
• Correct for the detuning with amplitude at injection in the horizontal plane and use a reduced chirp amplitude
• Possibly use the PLL to track the tune to minimize the excitation level (the PLL was not available during the previous MD).

Status of the machines

PSB and LEIR (from M. Chanel after the meeting)

Studies of the beam behaviour at 160 MeV as compared to 50 MeV have started and some preliminary measurements are available.

The orbit correction dipole power supplies have been put into service and the orbit has been corrected. No alignment work is required for this run.

The tune measurement is not yet working.

LEIR cold check-out is progressing well so far.

PS (S. Gilardoni, Y. Papaphilippou)

There are still problems with the 5 current mode of operation: the new power supplies have a large ripple. This explains the observed tune fluctuations in the various beams and the ripple observed by DIRAC on the extracted beam. As a result of that we are back to the 3 curent mode of operation until PO has fixed the problem (due to the DSP control of the power supplies).

As a result of that the non-linear chrmaticity and matrix measurements could not be performed. There is no way to control tunes and chromaticity for the time being. For that reason no progress can be done with the MTE studies (the 2004-6 performance could be obtained with 3 currents already).

For the AD extraction without QKE 58 we are also stuck because the extraction conditions (radial position at extraction) are not clean. The radial position at extraction is 15 mm inwards and could not be corrected so far. The reason why this was not possible are being investigated by the RF colleagues.

Tune measurement: there are still problems with the BBQ. Measurements are possible only with single bunch and no losses. The BBQ measurement would not require any excitation as it is sufficiently sensitive but the front-end does not return any data if the kicker is not ON. This feature will be modified and should solve the problem of the perturbation to the measurement due to losses. G. Arduini proposed that we address the issue of the tune measurement at the APC with the BI colleagues.

In the mean time S. Gilardoni has asked M.G. Angoletta to make the x-x' measurement available as a back-up solution.

The observed orbit deformation at transition is due to a change in radial position during transition crossing. The RF people are looking into this problem. Part of it (and of the displacement of the beam at extraction for the AD beam could be due to the limited number of pick-ups available for the calculation of the radial position combined with the orbit distortion occurring at transition or extraction due to the pulsing of the transition and extraction quadrupoles.

Injection losses: there are some doubts on the offsets of the pick-ups of the BTP injection line. Are the ones entered in the trajectory measurement programme correct? This needs to be verified with J. Belleman.

S. Gilardoni is following-up the alignment issues with T. Dobers. A lot of the information is not yet available in the SU DB.

The first tests with the new QKE for the CT extraction should be performed in week 31.

G. Arduini asked when the optics and aperture information for the BTP line will be available. O. Berrig replied that they will be available in 1 week.

MDs (G. Rumolo)

Some more measurements have been done for the V-electron cloud instability in the SPS.

A.O.B.

The water leak from the roof in building 9 has been fixed

Next meeting

Monday, 6^th August 2007 at 09:00 in room 354-1-001

Agenda

General information (G. Arduini)

Results of the first experiments at 160 MeV at the PSBooster (M. Chanel)

Status of the understanding of the transition losses in the PS (S. Aumon)

Status of the machines (supervisors)

A.O.B.

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