From: "Francesco Ruggiero"
To: "Daniel Schulte" ; "Elias Metral" ; "Frank Schmidt" ; "Frank Zimmermann" ; "Gianluigi Arduini" ; "Gilbert Guignard" ; "Helmut Burkhardt" ; "Jacques Gareyte" ; "John Jowett" ; "Werner Herr" ; "Luc Vos" ; "Maxim Korostelev" ; "Alex Koschik" ; "Bruno Muratori" ; "Eric D'Amico" ; "Walter Wittmer" ; "Gregory Penn" ; "Lifshitz Ronen" ; "Elena Benedetto"
Cc: "Hans Braun" ; "Jean-Pierre Riunaud" ; "Karlheinz Schindl" ; "Louis Rinolfi" ; "Michel Martini" ; "Oliver Bruning" ; "Roberto Cappi" ; "Charles Hill"
Subject: Minutes of LHC Collective Effects team meeting 28/03/2003
Date: Friday, March 28, 2003 5:08 PM
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Minutes of the ABP-LCE team meeting of 28.03.03
present: TdA, WH, JJ, EM, BM, FR (secretary), LR, FS, DS, LV
excused: FZ
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The joint ABP Forum/CLIC meeting was too long and several people left
the meeting room (or did not show-up).
ACTION FR -> discuss with Hans Braun to optimize the schedule/length
of future joint ABP Forum/CLIC meetings
ACTION LCE team (especially students) -> attend LCE team meetings or
propose alternative dates
The LCE team meeting was largely devoted to discuss LHC collimator
impedance and beam stability.
1) Report from last LHC Collimation meeting
-------------------------------------------
LV reported about the LHC Collimation meeting of last Friday 21 March,
that he attended with EM. Berillium seems to be ruled out for mechanical
stress reasons, at least with the current assumptions on the dump kicker
failure scenarios. The idea of leaving the collimators open, i.e. with
injection settings, during the whole ramp was questioned. If they are
closed at some intermediate energy the implications for impedance and
beam stability may be serious. Bernard Jeanneret thinks that this will
not be necessary.
It was agreed that collimator impedance studies should be somewhat
decoupled from LHC Collimation meetings: LCE team representatives will
report relevant new results on impedance and beam stability when they
become available. On the other hand, collimator impedance has top
priority for the LCE team.
JJ recalled that the LHC Machine Advisory Committee had suggested to
consider graphite with some carbon fibers for the collimators. Fritz
Caspers has recently proposed a new graphite foam. LV remarked that in
his beam deflection measurements Mark Seidel had used a rather high
value of the resistivity for graphite. FR speculated that this may be
due to baking at 400 C.
2) Follow-up of last LTC meeting
--------------------------------
FR summarized the actions and future studies on collimator impedance
following the last LTC meeting (see enclosed list). LV remarked that the
frequency dependence of his transverse impedance model Z(omega) depends
on the "inductive bypass" effect (compatible but not tested by Seidel's
measurements), which in turns depends on collimator material and
aperture. After some discussion it was agreed to use the baseline
collimator apertures and lengths provided by Ralph Assmann.
ACTION LV -> ask the LHC Collimation team to give an official value for
the resistivity of graphite collimators. Use baseline
graphite collimator design for injection and squeezed
optics and transmit information on collimator transverse
impedance Z(omega) (plots/tables and Mathcad file) to the
RF group and LCE team.
ACTION FR -> compute coherent tune shifts of higher order head-tail
modes and compare to stability curve with LHC octupoles.
3) Detuning due to Long Range Beam-Beam Collisions
--------------------------------------------------
WH has computed particle distributions in tune diagram Qx,Qy induced by
long range beam-beam collisions, assuming pseudo-parabolic beam
distribution in x-y betatron amplitudes (see enclosed ps file).
ACTION LV -> use WH's results to estimate stability limits
(possibly with some help by FR).
4) MAFIA and/or GDFIDL simulations
----------------------------------
It was recognised that 1-2 young members of the LCE team should start
learning MAFIA asap. On a shorter time scale, external experts will be
asked to perform MAFIA simulations in time domain to confirm analytic
estimates of resistive and geometric collimator impedance. FR said it
would be already interesting to check resistive wall impedance for a
short high-resistivity section (say 1 m long) in a smooth beam pipe.
This would decouple resistive and geometric wakes, and confirm that
resistive low-frequency wakes do not travel with the beam. This has
significant implications for the collimator impedance, since a
multi-bunch instability requires at least coupling from one bunch to the
next: if next bunch interacts with the wake of previous bunch a few
metres away from collimator, the radius changes by a factor 10 and the
deflecting kick is
3 orders of magnitude smaller!
ACTION DS -> ask Igor Syratchev to launch some MAFIA runs,
if compatible with his CLIC duties.
5) References on Localized Resistive Wall Impedance
---------------------------------------------------
FR listed some relevant references on "localized resistive wall
impedance" (other references may be found by a simple Google web search)
and suggested that they be distributed among LCE team members, who will
report on them and follow-up their implications for the LHC.
ACTION AK -> Read paper by G. Duncan on "Localized multi-bunch modes in
accelerators" (PRST-AB 2, 094401 (1999).
Report at the next LCE team meeting.
Use simulation code to confirm localized multi-bunch modes
predicted by G. Duncan with localized resistive wall
impedance and sufficient octupole detuning.
- EPAC02 paper by V. Danilov et al on "Closed orbit instability" was
already discussed by FZ. Conclusions for LHC, especially with large
collimator impedance, are still missing.
- EPAC00 paper by M.M. Karliner et al on "Calculation of transverse
resistive impedance for vacuum chambers with arbitrary cross sections"
highlights that unstable slow wave has a phase velocity 100 times slower
than the speed of light and suggests how the impedance should be
estimated. No changes for thick wall, but significant differences for
thin resistive wall (see BINP codes under Collective web page).
- DESY report 94-068 by A. Piwinski on "Impedance in Lossy Elliptical
Vacuum Chambers" is available on the web. It is quoted and used by
M. Seidel to estimate resistive impedance for the collimator body.
6) AOB
------
- Electron Cloud simulations are in standby.
EB has some preliminary results on emittance growth.
ACTION EB -> report at the next LCE team meeting.
- FS asked for a volunteer to follow-up the MAD-X Plotting Module.
- Next week EM will attend a workshop on Space Charge simulations.
He will report in a forthcoming Forum.
--
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/