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
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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
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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/