Minutes of the LIS Section meeting held on 30^th April 2007.

Present: M. Aiba, G. Arduini, S. Aumon, O. Berrig, C. Carli, A. Franchi, S. Gilardoni, J. Jowett, M. Juchno, M. Martini, H. Medina, E. Métral, Y. Papaphilippou, G. Rumolo, B. Salvant, G. Smirnov, F. Zimmermann.

General information (G. Arduini)

Hubert Medina has joined the LIS Section. He is a trainee student from Clermont Ferrand University working on the Z-base impedance database under the supervision of E. Métral.

D. Quatraro has been selected as a Doctoral Student to work on the HEADTAIL code under the supervision of G. Rumolo. He should arrive at CERN in Autumn this year.

There will be a Group Meeting at the end of May or beginning of June to review the results of last year and the main objectives for this year.

PS2 design status (Y. Papaphilippou) - slides - link to PS2 WG web page -

Y. Papaphilippou is contributing to the study of a replacement of the PS with a new machine: two options are presently discussed, a normal conducting one (PS2) and a super-conducting one (PS2+). The latter would allow to achieve an higher extraction energy and therefore could open the way to the construction of a super-conducting 1-TeV injector for the LHC (SPS+).

The issues of the PS2 extraction energy and the overall impact of the PS upgrade on the performance of the injector chain are still being debated. G. Rumolo is studying the energy dependence on the vertical electron cloud instability which is the major limitation for the performance of the SPS as LHC injector in order to provide input for the upgrade studies.

The minimum PS2 injection energy is determined by the maximum acceptable space charge tune spread for the maximum bunch intensity presently considered for the PS2 upgrade (i.e. 4.1×10¹¹ p). The minimum value is 4 GeV.

C. Carli noted that it is not yet clear how ions can be injected in the PS2 and he noted that the solution with only one RF system (40 MHz) in case of a direct injection of an SPL into the PS2 would not be compatible with ions.

The circumference presently considered for the PS2 is approximately twice the PS circumference. One of the preferred option is C_SPS=77/15 C_PS2 =5.1333 C_PS2 =11 C_PS

This solution would favour a 5-turn extraction from PS2 to SPS for the fixed target beam. G. Arduini noted that one of the main constraint imposed by the SPS for the fixed target beam is to have a high filling factor of the SPS circumference in order to minimize the beam local density. After the meeting it was noted that the proposed circumference ratio would imply a re-design of the beam dump kicker in the SPS as the present kicker is approximately 1 microsecond (~half the revolution period of the PS).

The study of the lattice has been based on the following basic assumptions:

• 200 main dipoles

• Maximum dipole field of 1.8 T

• Dipole length ~ 3 m

• C_PS2 ~2 C_PS

J. Jowett noted that the constraint on the PS2 circumference and the initial requirement of having a transition gamma similar to that of the PS imposes a weak focusing.

Several lattice options have been considered (see also summary table in the slides):

• FODO (n2,n3,n4)

• FMC (Flexible Momentum Compaction - S.Y. Lee et al. paper ) with FODO

• Doublet (n1,n2,n3,n4),

• Triplet and TripletRO (=Reverse Order, i.e. MB, ..., MB, QF, QD, QF, MB,... , MB) (n1,n2,n3,n4)

• FMC2 (=FMC with DOFO) (n1,n2,n3,n4,n5),

• FMC module with FODO, missing dipole

where n1=total number of dipoles, n2=number of dipoles per half-cell, n3 and n4=phase advance in degrees for the H and V plane, n5=drift length in m between elements.

J. Jowett mentioned that the choice of the transition gamma is heavily affecting the optics and the motivations for the choice of the value of the transition gamma should be reviewed because of their implications on the tunability of the optics and on aperture. A closer look on the beam manufacture (longitudinal) is desirable taking into account that the PS2 should also serve as ion injector.

Additional information on the lattice studies by J. Jowett can be found here and a movie showing the transition from real to imaginary transition gamma (J. Jowett) is shown here. 

For an explanation of the scaling laws used in slide 9 see (W. Bartmann slides)

Injection and extraction schemes based on simple FODO structure with 90 degrees phase advance. No study has been conducted yet on the sections matching the arcs with the 2 long straight sections. Only a "missing-dipole" dispersion suppressor has been considered.

In slide 11 extraction kicker should be read as extraction septum. In particular for the 5 turn extraction with capture in 5 stable islands the phase advance between the equivalent octupole exciting the resonance and the extraction septum should be 45 degrees. O. Berrig noted that enough aperture should be provided for the separation of the islands just before the extraction. S. Gilardoni pointed out that the requirements on the island separation could be reduced by a careful design of the septa (or by a combination of electrostatic and magnetic septa).

G. Arduini asked whether there is at present any preference between the real and imaginary transition gamma. Y. Papahilippou replied that a choice has not been done yet but there is a preference for a real transition gamma. E. Métral suggested to look at the AGS experience at BNL as they are crossing transition with high intensity.

TMC Instability in the SPS : HEADTAIL simulations and MOSES calculations (B. Salvant) - slides

B. Salvant presented the results of the benchmark of the HEADTAIL and MOSES codes triggered by the need of better understanding the nature of the fast vertical instability observed in the SPS.

In order to compare the HEADTAIL simulations with the results from MOSES dedicated techniques for the spectral analysis of the bunch centroid motion as estimated by HEADTAIL have been applied (SUSSIX code).

The agreement between the MOSES and HEADTAIL codes is excellent and for the first time the mode analysis has been applied to the simulation data provided by HEADTAIL. This shows clearly that the vertical instability as simulated by HEADTAIL is a Transverse Mode Coupling Instability. The agreement between MOSES and HEADTAIL in the case of a broadband impedance without space charge makes us confident that HEADTAIL can be used to simulate the more realistic cases in which space charge and more realistic impedance models are taken into account.

G. Arduini asked whether there are plans to apply the analysis technique to the data measured in the SPS for the fast vertical instability. B. Salvant replied in the affirmative but he noted that the signal might be much less clear due to the decoherence of the signal due to non-linearities or space-charge.

Y. Papaphilippou noted that the adverse effect of decoherence could be palliated by using the signal of several Beam Position Monitors.

G. Arduini noted that the standard BPMs might not have enough bandwidth and might distort the bunch centroid motion.

C. Carli noted that some of the lines observed in the analysis of the HEADTAIL data do not appear in MOSES. G. Arduini suggested that it could be useful to look at the pattern of the oscillation corresponding to these modes once isolated from the rest of the spectrum.

F. Zimmermann asked whether the thresholds for the onset of the instability for the round and flat chamber have been compared. After the meeting B. Salvant produced a slide comparing the case of a round and flat chamber. This slide compares the imaginary part of the tune shift for the round chamber and for both planes of the flat chamber. From that one can observe that:

1. there is an instability in the horizontal plane for a flat chamber, and its threshold is about twice that of the vertical plane.
2. the threshold in the round chamber is slightly below the threshold for a flat chamber, but more data points for the flat chamber should be taken to be more conclusive.

Additional information can be found at (RLC meeting 20/1/06):

http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/Meetings/2006/2006.01.20/H&VTMCIThresholdsInaFlatChamberWithHEADTAIL_RLC_20-01-06.pdf

http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/Meetings/2006/2006.01.20/Minutes%20of%20the%20RLC%20meeting%2020_01_2006.txt (point (5) of the minutes).

Status of the PSB (G. Arduini for M. Chanel).

The major problem is the orbit acquisition:  beam position is read even without beam. The problem is not yet understood and this is delaying the beam-based realignment of the PSBooster.

Status of the PS (S. Gilardoni, Y. Papaphilippou)

Main beams (SFTPRO,EAST, AD) have been prepared (at least at moderate intensity. Some more work is required for the optimization of the slow extraction, but for the time being the beam is not extracted to the East because of a vacuum problem in the East Area.

There is a problem with the controls of the PFW (operated in 5-current mode simulating the old 3-current mode of operation) generating frequent trips of the power supplies. A possible solution has been identified and could be implemented soon.

The tune measurement is not working (it was at the start-up). the problem seems to be related to the DSC. This is required for the tune and chromaticity measurements required to measure the matrices for the correction of tune and chromaticity.

The BI/SW expert (M. Ludwig) has asked the operator not to use the wire scanners because there is an "unspecified" problem. The wire of the scanner V75 might be broken. an access might be required to verify the integrity of the wire.

Next meeting

Monday, 14^th May 2007 at 09:00 in room 354-1-001

Agenda

General Information (G. Arduini)

ACCSIM-ORBIT benchmarking - latest results and follow-up of actions (M. Aiba, M. Martini)

Status of the machines: PSB-PS-SPS (M. Chanel, S. Gilardoni, Y. Papaphilippou, E. Métral)

A.O.B.

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