From: "Frank Zimmermann" To: "Frank Zimmermann" ; ; "Francesco Ruggiero" ; "Daniel Schulte" ; "Elias Metral" ; "Frank Schmidt" ; "Gilbert Guignard" ; "Jacques Gareyte" ; "Werner Herr" ; "Luc Vos" ; "Alex Koschik" ; "Bruno Muratori" ; "Tommaso D'Amico" ; "Rita Paparella" ; "Walter Wittmer" ; "Gregory Penn" ; "Lifshitz Ronen" ; "Maxim Korostelev" ; Cc: "Jean-Pierre Riunaud" ; "Karlheinz Schindl" ; "Louis Rinolfi" ; "Michel Martini" ; "Oliver Bruning" ; "Roberto Cappi" ; "Charles Hill" ; "Gianluigi Arduini" ; "Helmut Burkhardt" ; "Daniel Brandt" ; Subject: Minutes of LHC Collective Effects meeting 14/02/2003 Date: Friday, February 14, 2003 9:12 PM Dear colleagues, the minutes of the 14.02.2003 ABP-LCE meeting are included below. Transparencies of five presentations (Francesco Ruggiero, Luc Vos, Elias Metral, Daniel Schulte, and Frank Zimmermann) are attached to this email. Best regards, Frank --------------------------------------------------------------- minutes ABP-LCE meeting 14.02.03 present: Td'A, GA, EB, WH, EM, BM, DS, FR, LV, FZ --------------------------------------------------------------- (1) follow up on action items ----------------------------- ACTION FR -> "get access to lxplus classes for WH, DS, FZ" new action item; by next week ACTION FR -> "ask for afs disk space (for beam-beam a few GBs & electron cloud about 5+10 GBs scratch)" remains to be done; by next week ACTION JJ -> "Report on tracking with imperfections and beam-beam in the coming weeks" John is in Japan and could not report FR underlines that DA with imperfections and beam-beam for the two LHC commissioning scenarios with 25 and 75 ns bunch spacing (see enclosed beam parameters) are needed before Chamonix. ACTION FS and WH -> "inform LHC optics team of different MAD-X results on different lxplus machines" done; but several new problems encountered due to change of operating system; FS and Eric McIntosh are following this up ACTION AK -> "2 bunch calculation, with improved SPS impedance model, also including space charge, res.wall, by end of February" absent ACTION FR WH -> "FR will send relevant information to beam-beam team for LHC commissioning options for Chamonix" done ACTION FZ DS -> "stricter handling of ECLOUD code; fix date to freeze the code, include input file and example; introduce version numbers, and documented changes; tracing of old versions; use cvs; time scale 2 weeks" done; new ecloud code web page: http://www.cern.ch/proj-ecloud-code containing source code, example input, example output, list of bugs, for versions 0.0 and 2.0 (version 1.0 should be recalled from archive); ECLOUD users at BNL, CERN, DESY, GSI, IHEP, INFN, TJNAF were informed; suggestion to add list of changes in different versions and short explanation of new features in future versions ACTION LV -> "Review and document specs for Ti coating of TDI absorber (a few microns), and check consistency with proposed specs for collimator coatings." done; new estimate of res.-wall wakes (report by LV later in the meeting, see below) ACTION FR LV -> "Review boundary conditions for non-continuous coating, e.g. stripes of carbon. Review allowable distance between collimator slices, in case collimator jaws are realized in discrete slices: sawtooth surface." pending ACTION FZ -> "Check transverse wakefield with proposed tapering angle" done; wake field formulae from Yokoya and Stupakov for round and flat shallow tapers were reviewed by FZ. EB is implementing these wakes in the HEADTAIL code to study the possible incoherent emittance growth over 1e6 turns LV recalled study by A. Wagner on round tapers and the Yokoya formula. ACTION FZ -> "Review SNS studies on local e- cloud at collimators" done; J. Wei/ BNL-SNS was contacted; they anticipate 3 primary sources of electrons: injection stripping foil, collimation, and multipacting (everywhere); clearing electrodes and monitoring system near injection point, solenoids at collimators; estimate of electrons due to collimation from Pivi/Furman -> this step still to be understood (by us) to get estimate for LHC (see next) ACTION FZ FR -> "Estimate order of magnitude of problem for the LHC" study ongoing, in connection with previous item ACTION FR -> "Create LCE web page with updated action items" new; end of February (2) LHC collimators: resistive impedance - rough estimate (FR) A simple estimate by FR shows that the transverse resistive wake of the collimators is a potential problem. It scales linearly with the length, with the square root of the resistivity, and with the inverse cubic power of the aperture. Assuming 20 m collimator length, 2x1.8 mm gap, and 100 times the resistivity of cold copper, the impedance would be 10 times larger than for the arc beam screen. (3) LHC collimators: resistive impedance - detailed calculation (LV) LV showed the transverse impedance for different collimator materials as a function of aperture. There is a steep increase towards small apertures as expected. 100 MOhm/m is the present total budget. Old system contained 20-cm long Al primaries and 50-cm long Cu secondary collimators; its impedance would be 2.1 and 2.5 MOhm/m at injection and 13.8, 16.8 MOhm/m at top energy; a new system gives about 841 and 1017 MOhm/m assuming carbon; 10 times higher feedback gain would then be required. the limit on the feedback gain is set by noise-induced emittance growth. The increased impedance might be ok, in the presence of beam-beam tune spread. (4) Discussion on Collimator Philosophy (FR et al.) FR suggests to relax the requirements on a single system. For example, special protection collimators could be placed in front of the triplets (FZ suspected that these would become the primary collimators in such a case); a larger beta function reduces the impedances as 1/sqrt(beta); also mis-firing of the kickers could be handled by special devices and not necessarily by the cleaning collimation. ==> A later discussion with JBJ clarified that some additional protection of the triplet aperture (by tertiary collimators) would allow us to relax the requirements on the collimation efficiency and thus to open the collimators by 2-3 more sigmas. Next week, there will be a special meeting of the collimator project, where the latest resistive-wall wake estimates will be presented by FR and/or LV. (5) LHC collimators: geometric impedance (FZ) Reported above under action items. Elena Benedetto of the Politecnico in Torino has joined the LCE team for electron-cloud studies. In particular she will pursue a plasma approach to the electron cloud. She has already started some simulations with the HEADTAIL code. (6) Electron cloud simulation code developments (DS) DS presented lots of new results. Simulated electron distribution is now completely symmetric, after adjustments to space-charge routine. Code speed much increased for dipoles and quadrupoles. For these two magnetic fields, different ODE solvers were implemented, and compared. For dipoles the equations of motion can also be solved directly analytically, and the corresponding simulation agrees with that of the numerical ODE solution. Results for vertical motion only (strong field limit) show some differences, and the more exact calculations including cyclotron motion do not seem to converge to this limit. A large difference in the build up is seen for 100 G and 1 T fields, possibly in conflict with the SPS observation. (7) Discussions with VAC&RF on LHC/SPS e-cloud aspects (FZ) Two meetings were held. One with C. Benvenuti et al. pointed out the importance of pumping by the beam and by the electron cloud for pressure calculations; another meeting with GA, FC, rf group, et al, has decided the position of the rf coupler (in BA2 and BA3 near start of the arc). (8) Incoherent longitudinal tune shift and Landau damping (EM) EM presented stability diagrams for different distributions (parabolic, Sacherer, elliptic, etc.). The stability limits are quite different, especially in the direction towards capacitive impedances. Tune spread and coherent tune shift both enter the calculation. More results will be presented by EM in an ABP forum tentatively scheduled for February 26. (9) beam-beam: preparation for Chamonix (FR, FZ, WH) FR presented two possible commissioning scenarios for the LHC. One has 75 ns spacing, with almost the nominal charge per bunch and nearly nominal emittance; the second has 25 ns spacing with less charge and 3-4 times smaller emittance (which might be challenging in itself); he raised the question how these two scenarios compare with respect to long-range beam-beam effects. FZ presented a simplified weak-strong simulation for the diffusive aperture. The 25-ns case resembled the nominal LHC situation with a clear diffusive aperture at about 6 sigma, the 75-ns did not show any evidence for strong diffusion up to at least 10 sigma. It looks much preferred. ==> ACTION JJ -> confirmation by detailed tracking. In case JJ is unable to provide results in time: ==> ACTION FR -> Manpower allocation and time schedule to be discussed with LOC and group leader. FR mentions a scenario with 43 single bunches, a few of which should be displaced by 75 ns to provide luminosity in LHCB (WH). (10) AOB rehearsal of RP's beam-beam presentation (Monday 17 Feb at 14:30, 354/1-001) ABP Forum by EM on Landau damping (Wed 26 Feb) other volunteers for an ABP Forum are encouraged to apply. WH asked about participation in workshops and conferences; FR responded that there are no news, but likely tight control by the management; Chamonix participation also is not transparent, which could be improved. Attached: slides by FR, LV, DS, EM, FZ