From: "Francesco Ruggiero"
To: "Bruno Zotter" ; "Daniel Schulte" ; "Elias Metral" ; "Frank Schmidt" ; "Frank Zimmermann" ; "Gianluigi Arduini" ; "Helmut Burkhardt" ; "Hiroshi Tsutsui" ; "John Jowett" ; "Robert Gluckstern" ; "Stephane Fartoukh" ; "Werner Herr" ; "Luc Vos" ; "Maxim Korostelev" ; "Alex Koschik" ; "Eric D'Amico" ; "Walter Wittmer" ; "Gregory Penn" ; "Lifshitz Ronen" ; "Elena Benedetto" ; "Elmar Vogel"
Cc: "Jean-Pierre Riunaud" ; "Karlheinz Schindl" ; "Louis Rinolfi" ; "Michel Martini" ; "Oliver Bruning" ; "Charles Hill" ; "Wolfgang Hofle" ; "Andre Verdier"
Subject: Minutes of LHC Collective Effects team meeting 31/10/2003
Date: Thursday, November 06, 2003 9:55 PM
Minutes of the ABP-LCE team meeting on 31.10.03
present: WH, JJ, AK, FR, DS, EV, LV
excused: EB, EM, FZ
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(1) Pending Actions
-------------------
ACTION -> Check sign of real part of collimator impedance (DS).
DONE: Sign is ok (decelerating), but impedance values are questionable.
A resonable agreement between GdfidL results and theoretical predictions
is obtained only for much higher values of the wall resistivity.
DS has now started using HFSS.
ACTION -> EM will clarify ambiguity in collimator orientation.
ACTION -> EM will check possible longitudinal 'Yokoya factor' relating
round and rectangular cases.
ACTION -> Check discrepancy by a factor ~3 on collimator impedance
(EM+LV) and put numbers into ZBASE (EM).
(2) Round table discussion
--------------------------
LV has estimated a modest impedance Z/n=0.6 mOhm for each of the ~10
Roman Pots in the LHC, assuming a half aperture of 0.8 mm corresponding
to 10 sigma for a low intensity beam with 1 micro-m normalized
emittance, and for a chamber with an inner radius of 30 mm. At high
intensity the half aperture should become 5 mm and the impedance will be
reduced to 0.3 mOhm, always per pot, to be compared to an LHC impedance
budget of about 80 mOhm and a threshold impedance of 260 mOhm. With a
few microns of Cu on SS, the resistive impedance is negligible.
ACTION -> DS will cross-check these estimates by numerical simulations
and follow-up the impedance of Roman Pots with Roger Perret.
For the proposed Roman Pot test in the SPS, LV assumes a half aperture
of 5 mm and a chamber with inner radius of 80 mm, yielding an impedance
of 1.2 mOhm, to be compared with the total SPS impedance of 7-9 Ohm.
EV is working on the suppression of damping for some selected
multi-bunch modes with the feedback system. This is possible with a
Field Programmable Gate Array (FPGA) box, although data transport is
limited to 75 MHz and a special circuit has to be removed to exceed the
80 MHz needed for the feedback system (i.e. 2x40 MHz). EV will discuss
with experts in the EP/ED division and decide about investing some
reasonable time in learning VHDL (Very high speed integrated circuit
Hardware Description Language) to implement his filter on the XILINX
chip. In the best scenario this could be tested with beam in 2004.
EV reported about SPS MD's on the feedback system with Wolfgang Hofle.
- A problem of synchronization for the Tektronics scopes can be solved
with ADC's, but no device is available for a quick test.
Software layers (to be provided by the CO group?) are needed in case
we
buy the ADC's off the shelf. The RF group is looking for a special
scope
where the timing can be shifted.
- The simple formula to estimate decoherence of injection oscillations
works only for small oscillation amplitudes below ~1 sigma.
- RF amplitude modulation to induce bunch-to-bunch synchrotron tune
spread and increase beam stability has been re-evaluated by Elena
Chapochnikova for the 800 MHz RF system. New threshold computed by EV:
an amplitude modulation of 22% is needed to stabilize longitudinal
coupled bunch modes.
- LHC beam with 75 ns bunch spacing seems to be unstable.
AK shows a review of his past activities and MD contributions.
With Fixed Target beam, 5 ns spacing, 2100 bunches, 10^13 total proton
intensity, feedback switched off and on again after 7 msec (300 turns):
the horizontal BPM signal shows that "nothing happens" during the first
50 turns and then an instability grows with an exponential rate of about
500 turns. This result may help damping injection oscillations in the
LHC, with limited emittance growth inspite of the resistive wall
instability. In the vertical plane the instability is much faster
(150-200 turns). Simulations with random injection offsets of +-1 mm for
each bunch
predict a linear growth. LV suggest to measure also sum mode with the
BPM's to get absolute scale instead of arbitrary scale in "bits".
ACTION -> AK will repeate simulations with a constant injection offset.
Other activities of AK include:
- Preparation of a draft report on the transverse resistive wall
wake function with Inductive Bypass.
- Ambiguity in the Fourier Transform of the collimator impedance found
when using "FullSimplify" in Mathematica (a Mathematica bug?).
- Machine precision problem for wake-function with Inductive Bypass.
DS suggest to use a Taylor Serie expansion.
- AK needs to check if it is possible to transform from impedance to
wake potential for triangular bunch distributions.
- Problem of fonts when exporting Mathematica plots to eps format.
JJ has a simple solution that does not require special set-ups or
single user Mathematica licence.
ACTION -> JJ will explain how to produce eps plots with Mathematica.
ACTION -> FR will read and approve the draft note by AK.
--
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/