From: "Francesco Ruggiero" To: "Bruno Zotter" ; "Daniel Schulte" ; "Elias Metral" ; "Frank Schmidt" ; "Frank Zimmermann" ; "Gianluigi Arduini" ; "Helmut Burkhardt" ; "John Jowett" ; "Robert Gluckstern" ; "Werner Herr" ; "Luc Vos" ; "Maxim Korostelev" ; "Alex Koschik" ; "Eric D'Amico" ; "Walter Wittmer" ; "Gregory Penn" ; "Lifshitz Ronen" ; "Elena Benedetto" Cc: "Jean-Pierre Riunaud" ; "Karlheinz Schindl" ; "Louis Rinolfi" ; "Michel Martini" ; "Oliver Bruning" ; "Roberto Cappi" ; "Charles Hill" Subject: Minutes of LHC Collective Effects meeting 23/05/2003 Date: Monday, May 26, 2003 9:11 PM ---------------------------------------------------------------- Minutes of the ABP-LCE team meeting of 23.05.03 present: GA, OB, TdA, AK, EM, FR, DS (secretary), LV excused: WH, JJ, FZ ---------------------------------------------------------------- 1) Follow-up of pending actions and matters arising --------------------------------------------------- Christian Rathjen has sent new drawings of the recombination chambers in IR8 and preliminary information for IR2, where the available space is limited. ACTION -> LV will send a (positive) reply concerning IR8 ACTION -> FR will ask Bruno Spataro to perform MAFIA calculations for the following list of items: - recombination chambers at IR2 - tanks for the OTR screens and wire scanners - geometric impedance of LHC collimators ACTION -> LV will attend the ATLAS workshop at CERN on 2-3 June and contribute in connection with the impedance of Roman pots. AK found that the LHC resistive wall impedance gives rise to a multi-bunch instability with a rise time of 590 turns. As a simplified model, he used the wakefields of the baseline collimators with a radius of 2 cm for the whole ring. He will repeat the calculations using a better model. ACTION -> AK will perform new LHC simulations and discuss with FR concerning future developments of his PhD thesis work. 2) Follow-up on tune shifts of higher-order modes ------------------------------------------------- EM presented an update of the high order head-tail mode calculations to clarify the large effect of the inductive bypass and the important difference between real and imaginary parts of the tune shifts. For the new results he used 3564 bunches with regular spacing and assumed collimators made of graphite with a radius of 2 mm. He showed that the difference of the impedance between the case with and without inductive bypass is noticeable at a frequency omega = 1e7 Hz, but very small at omega = 1e9 Hz. Consequently he suspected that the differences found in his previous calculations were due to the low frequency content of the sinusoidal modes, corresponding to parabolic bunches, whose spectrum has low frequency "lobes". He verified this by removing this part of the spectrum. FR remarked that this may explain why only even-order modes had large tune shifts. EM then repeated the calculations using Hermitian modes, corresponding to Gaussian bunch distributions and having no low frequency lobes. For the case with no inductive bypass, he found that the imaginary part of no mode with m>0 exceeded the limit of 1e-4 that can be stabilized with the Landau octupoles. As FR pointed out the real part of the tune shift for the m=1 mode was ~2e-4 and for the m=0 mode it almost reached 1e-2. LV agreed that this may be too high, but a limit needs to be established. Adding the effect of the inductive bypass reduced all the imaginary tune shifts, now below 1e-4 for all modes, although the real tune shifts were still 1e-3 and 2e-4 for the m=0 and m=1 modes, respectively. EM has repeated the calculation for a chromaticity Q'/Q = 0.1, corresponding to a chromatic frequency of 220 MHz, which he considered close to the maximum one might have to deal with. In case of no inductive bypass the imaginary tune shift of mode m=1 reached 5e-4 clearly exceeding the limit; the real part also reached 1e-3. Again the effect of the inductive bypass cured the problem for all the imaginary tune shifts, but the real tune shifts were still ~1e-3 and ~2e-4 for the m=0 and m=1 modes, respectively. It remains to be understood why in the latter case mode m=0 is unstable without inductive bypass and stable with bypass. In a discussion it was concluded that the large real part of the impedance can be a problem if one wants to run with no feedback system but only the Landau octupoles. As mentioned above the actual limit for this part of the impedance still needs to be consistently established. ACTION -> EM and LV will continue to work on the stability of high order modes and will check the limit of 100 MOhm/m currently assumed by the LHC collimation team for the maximum acceptable collimator impedance. 3) New developments for the ECLOUD code --------------------------------------- DS presented some recent progress of the electron cloud code development. By removing some bugs he was able to show that different methods of solving the equation of motion in a dipole field now yield similar results. One of the bugs had to do with the beam field calculation with image charges and the substraction of the field by a pencil round beam. DS also found that with elliptical boundaries it seems possible to reproduce qualitatively the measurements of the electron cloud density as a function of the dipole field, which were carried out at the SPS. Previous simulations showed a behaviour different from the measurements. A quantitative analysis remains to be done, as does an investigation of what allowed to obtained these results. The speed of the code was increased for dipoles/drifts by 30% to a factor 3. ACTION -> DS and FZ will follow-up the ECLOUD code developments, document different (diverging?) versions and bug fixes, and systematically compare SPS measurements with simulation results using a consistent version of the code. 4) LHC Impedance Database ------------------------- OB reported on the progress of reviving ZBASE. The data base has been successfully ported to the new tcl/tk version. Most external binaries are functional on linux. Also a windows port is well advanced, the tcl-scripts have been modified where necessary, and a number of binaries is available on windows. However more work remains, in particular to ensure the access to MAFIA, which is not available on all platforms. Since the manual is old, OB suggested a new one should be written, mainly by a potential user of the impedance data base. ACTION -> LV volunteered to lead a mini-team with TdA and EM to test ZBASE and start with a practical (and useful) example, i.e. with the impedance of the LHC beam screen. ACTION -> Hiroshi Tsutsui will help finalizing the impedance interpolation routine when he visits CERN in a few weeks. 5) New LHC Design Report ------------------------ FR showed a list of chapters and keywords for the new LHC Design Report that we will have to write by September 2003. Further discussions to agree on the work sharing within the LCE team will take place in the next weeks. Documenting the LHC impedance budget and instability thresholds can be done in parallel with feeding information on the LHC impedance catalog in ZBASE. -> The next meeting of the LCE team will take place on FRIDAY 6 JUNE. Attachments: presentation by EM (pdf), slides by DS (ps), and list of chapters and keywords for the new LHC Design Report (pdf). -- E-mail: Francesco.Ruggiero@cern.ch, Location: Building 9/1-008 Address: CERN, A&B Division, CH-1211 Geneva 23, Switzerland Telephone: +41 (22) 767 3726 or 767 5272, TeleFax: +41 (22) 783 0552 WWW: http://wwwslap.cern.ch/~rgo/