Summary notes of the LCU meeting on 27/08/2007

Present: IA, MA, RB, HB, RDM, MG, JJ, AK, PL, VP, SR, FeRo, TW

Report from meetings

• SLM -> MG
  MG reminded that he sent us an email on the forthcoming TSC (student selection committee) on the 10th of September. Anybody considering to take a student is encouraged to contact him before this date.
• ICC -> MG
  The leak detected in Q17.R8 will be fixed by replacing the whole magnet. A spare magnet will be assembled and Yannis selected the main quadrupole for the new assembly in order to minimize the beta-beating.
  Sector 7-8 has been warmed up and it was found that several RF-fingers stick into the aperture.
  The cool down of sector 4-5 had to be stopped in July due to a vacuum leak. It was decided to skip the 80K plateau completely after cool down startup.
  Status of the LHC commissioning will be discussed in a talk by Lyn Evans scheduled for the 13th of September. 

Simulation of coupled-bunch instabilities in the LHC -> AK (pdf file)
AK reported about coupled-bunch simulations, based on the work he did for his thesis about 3 years ago. He wrote a program MULTI-TRISIM, extending the existing single bunch code TRISIM3D. The program, as its predecessors, is written in FORTRAN77 and currently requires the 32 bit option to run on recent 64 bit machines like lxplus.
The simulation is done in the frequency domain. A classical rigid-bunch tracking is implemented, with one bunch being represented by macro-particles. The tracking is performed using energy dependent transfer matrices and kicks for transverse non-linearities (only octupoles for the time being), and the impedance.
The simulation of the impedance includes resistive wall with inductive bypass and Yokoya factors as important for the flat SPS-chambers as well as narrow resonances from cavity like structures. The program uses FFT, convolution and zero-padding techniques to reduce the computing time which would otherwise scale with the numbers of bunches squared and become rather impractical for the LHC.
His simulations for the LHC include octupoles and predict that the resulting Landau damping allows to stabilize the multi-bunch instabilities expected in the LHC when the transverse feedback is turned off at top-energy to limit emittance increase by noise. It appears that the program of AK will be very useful for the LHC and that better documentation would be highly desirable. In addition, it seems clear that some extensions will be needed to perform a detailed study, such as:

• simulation of transverse damper
• simulation of generic transverse non-linearities to take into account magnetic errors
• simulation of beam-beam effects to take into account tune spread
• the possibility to use impedance models from databases, such as, e.g., ZBASE

Preliminary results from optics studies for IR2 -> JJ (web pages)
JJ reported on the current status of his work on LHC aperture model and IR2 matching (squeeze and un-squeeze sequences) as required for heavy ion and proton operation in the LHC, respectively.
The work is done for LHC optics V6.501. A new version of the complete aperture model merging the new information in the aperture assignment files for V6.501  into the complete continuous model that was built for V6.500 was generated. He showed results of  a comparison changes between V6.500 and V6.501. MG recalls that the aperture model for V6.501 is indeed the aperture model of the machine as built and it is the outcome of the magnet allocation at the MEB. The information from the measured geometry was stored in the tolerances. Details can be found in the LHC Project Report 1007 by JBJ. This means that the only difference between the V6.500 and V6.501 aperture models should be in the tolerances. Further work is also required on aperture and it was suggested that this should be followed up involving John, Stefano, Thys and Samy.
A new matching of the squeeze in IR2 is required for V6.501. Un-squeeze to increased beta* has also been requested for proton operation. JJ reported that he started on both the squeeze and un-squeeze in V6.501.

AOB
LHC collimator impedance measurements -> FeRo
FeRo reported about the status of the laboratory measurements of impedances which are relevant for benchmarking of the estimates for the LHC collimator impedances. Using a preliminary setup with network analysers, coils and graphite plate samples, it appears that measurements of the impedance in the relevant frequency range of about 1 kHz to 1 MHz are in fact feasible and should allow to verify important assumptions like the inductive bypass. The work is progressing well. Little progress is expected to happen in September due to other activities. SR said that a proper collimator test stand has been made and might be available for some time for various tests and measurements. MG states that any decision in this sense should be taken in agreement with RA.

Last update: 27-August-07

MG & HB

Back to LCU Minutes