Minutes July 18th, 2008
Present: Rama Calaga, Fritz Caspers, Klaus Hanke, Elias Métral, Diego Quatraro, Federico Roncarolo, Giovanni Rumolo, Benoit Salvant, Elena Shaposhnikova.
Agenda
- Elena Shaposhnikova
"Longitudinal Impedance Measurements in the SPS" (minutes)
- Diego Quatraro
"HEADTAIL upgrade: new features and options" (slides, minutes)
- Benoit Salvant
"Status of SPS impedance studies" (slides, minutes)
- Elias Metral
"SPS impedance budget follow up" (slides, minutes)
- Rama Calaga
"Some RHIC and SPS MDs" presented at the AB seminar (slides)
Minutes (thanks to Elias and Giovanni)
Presentation by Elena Shaposhnikova :
- Elena compared the SPS modelled and the measured slope (quadrupolar oscillation frequency Vs bunch intensity) between 2001 and 2008. The agreement is reasonable. The model took into account the kickers and the RF cavities (analytical estimates, that should be imported in Zbase). The experiments done on 2 consecutive days yielded 2 distinct parallel lines with a similar slope. It was observed that the bunch distribution was parabolic-like at low intensity and gaussian-like at higher intensity. Klaus will try to get a gaussian distribution out of the PSB at both low and high beam intensities.
- Elena showed analytical computation of the slope as a function of bunch length, but Giovanni mentioned it may be also interesting to have the slope as a function of longitudinal emittance.
- Elena mentioned that space charge should have a non negligible effect on the frequency shift, and they would like to make measurements at higher energies (55 GeV for instance for which there is a beam available this year).
- A longitudinal instability was observed in the SPS, and Elena said it is a quadrupolar instability.
- The beam is longitudinally unstable in the PS between 4 and 7 1010 p/b.
Presentation by Diego Quatraro:
- Diego presented the work performed to take into account the linear optics in HEADTAIL.
- The new upgrade was successfully benchmarked with a 1 kick approximation for the 20 SPS kickers case. It was also applied to the localization of impedance sources.
- HEADTAIL is now able to use the dipolar and quadrupolar transverse wake fields created by ZBASE for each element.
- It would be interesting to run MAD-X from HEADTAIL so that MAD-X gets the tunes and chromaticities directly from the HEADTAIL input file. It would also be interesting to add several families of impedance sources in the same HEADTAIL simulation.
Presentation by Benoit Salvant:
- ZBASE was updated to import the beta functions from a MADX created twiss file. The ZBASE user can choose whether he wants to weigh the wake fields by beta or not.
- Next steps are: (1) including the other sources of impedance available and (2) include the calculation for the longitudinal impedance. Also, being able to run MADX from ZBASE could help prevent mistakes.
- Further analysis of the TMCI 2007 measurements, yielded losses and growth rates patterns that are very similar to HEADTAIL simulations. This is again more evidence in favor of the existence of a TMCI in the SPS.
- Both Rama and Benoit found a "tune shift Vs intensity" slope that lead to a vertical impedance of 17 to 18 MOhm/m in November 2007, which are quite far from the 22 MOhm/m measured by Helmut in the beginning of the 2007 run, and again in 2008. This difference should be investigated.
Presentation by Elias Métral:
- Elias showed Helmut's transverse impedance measurement preliminary results and reminded which contributors could create a positive coherent tune shift with intensity. For instance, a PS kicker measurement with a single wire shows a negative imaginary part of the horizontal impedance up to 0.8 GHz, which can lead to a positive tune shift.
Author: Benoit Salvant CERN
AB/ABP-LIS