Minutes of the ABP-LCE team meeting on 21.11.03
present: EM, WH, AK, EM, FR, DS (partially), EV, FZ
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(1) LHC Impedance & SPS Electron Cloud (DS)
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DS discussed with Bruno Spataro. They will compare
Gdfidl and MAFIA calculations for taper impedance.
Taper angle so far was too large for test purposes.
DS presented simulation results for 25-ns and 75-ns
bunch spacing, and delta_max=2.2. He cut off particles
whose vertical kinetic energy is lower than 20 eV,
so as to model the effect of the detector bias voltage.
The resulting flux rates (factor 20 ratio between the
two bunch spacings) and the simulated energy spectrum
in excellent agreement with SPS measurements.
The simulated stripe position does not change, although
a third central stripe appears for the larger spacing.
This is in contrast to the experiment. It was questioned
whether the actual bunch parameters (bunch length, profile)
perhaps were not exactly identical for the two cases.
FR recalled that GA mentioned a BCT calibration problem.
The bunch length or central beam density likely changes
due to the different rf gymnastics in the PS.
(2) E-cloud presentation in LTC (FR)
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FR reported that a request had been made to present
a comparison of measurements and simulations for the
SPS as well as a discussion of implications for the LHC
to the LTC on November 26. This date was deemed to be
too early and it was proposed to postpone the presentation
to December. A meeting was held by GA, FR, DS, FZ, and
Miguel Jimenez to discuss parameters and measurement
results. The beam parameters for the 75 ns spacing
are needed as simulation input. Beam lifetime and emittance
growth are important aspects that will also be
simulated for realistic parameter sets.
Measuring the detuning along the bunch train would provide
quantitative information on the net cloud density.
The decay of the cloud for variable batch spacing
would allow probing the amount of elastic reflection
experimentally.
FR pointed out that there is a problem with the
distribution lists of LHC reports and project notes.
Several announcements were not received by him,
and neither by FZ.
(3) Minutes of Last LCE Meeting and Pending Actions
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ACTION -> EM will check possible longitudinal 'Yokoya factor' relating
round and rectangular cases.
Still to be done. FZ recalled that a factor Pi^2/8 had been
assumed by K. Bane.
FR points out that we should make use of EdA's experience
and work with Zbase as much as possible to detect possible
problems. He would like to see an example of a successful
use of Zbase, including an analytical formula and Tsutsui's
interpolation routine.
Zbase should be edited from scratch, e.g., by creating
a new model called LHC2004. Only validated impedance items
should be entered.
ACTION -> Present example of Zbase usage (young Zbase users).
There is a request by JPK for updated numbers of higher-order
CB head-tail growth rates. Updated impedance could be used in
conjunction with S. Berg's program. Narrow-band modes,
resistive-wall impedance and collimators are important
ingredients to this calculation. FR will prepare a preliminary
reply.
ACTION -> CB head-tail growth rates for updated LHC impedance
(LCE team)
ACTION -> Check discrepancy by a factor ~3 on collimator impedance
(EM+LV) and put numbers into ZBASE (EM).
Solved, no discrepancy in rise time left (see below); numbers still
to be put into Zbase.
ACTION -> DS will cross-check these estimates by numerical simulations
and follow-up the impedance of Roman Pots with Roger Perret.
Ongoing.
ACTION -> AK will repeat simulations with a constant injection offset.
In progress.
ACTION -> FR will read and approve the draft note by AK.
AK's note has been revised and will be reread by FR.
(4) Collimator Impedance Calculation for 9 Options (EM)
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EM presented tune shift and stability boundary calculations for
all nine LHC collimator options and both transverse planes.
He noted that option 7 is identical to option 8 (excel files
by RA were identical). The plot for option 8 shown last time
were wrong. There is a large variation between the various options,
and no. 3 or 2 look particularly promising.
EM has sent the results to RA, and also asked him which figure
should be included for the design report. He is still waiting
for an answer.
(5) TMCI simulations for SPS Measurements (EB)
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EB and GR perform simulations which explore TMC
instabilities in the SPS, for comparison with
this year's experimental observations. By adjusting
the impedance to 10 MOhm/m the simulated losses occur
after 3 ms, resembling the experiment. EB is conducting
a scan of the chromaticity, so as to find the threshold
of instability.
FR suggested that the model be upgraded to include
at least two resonators, such that the effect of the
kicker impedance can be studied. A question to be
addressed is the importance of kicker shielding.
The simulation results with a single resonator
can be compared with EM's theory, that is not easily
extended to several resonators.
ES is performing MOSES calculations that can also
be compared. The SPS impedance model to be used
in future simulations is to be agreed on by EM, EB,
HB, et al.
FZ mentioned that ES discovered a missing factor
(4 pi) in LV's recent paper.
(6) Electron-Cloud Instability Simulations (EB)
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Emittance growth simulations converge if the
number of interaction points is 5 or larger.
Boundary conditions are included, but this is
not essential for the convergence.
Simulations will be extended to include the transverse
damper and a high chromaticity, so that the
results can be compared with the machine study of
August 5, where a beam lifetime of about 20 minutes
was observed.
Typical simulations cover 2000 turns and take 8 hours
CPU time. It will not be easy to reach the minute
time scale. FR recommended to develop a strategy
for approaching this time scale.
ACTION -> EB, FR, DS and FZ will further discuss electron
cloud instability studies and simulation strategies
(7) News from LHC TR (WH)
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According to present planning, LHC will not have
a bunch-by-bunch luminosity measurement, or only
with an accuracy of 10-20%. This does not fulfil
our specifications and will not be adequate.
A relative precision of a few percent is required,
especially for machine studies.
The poor resolution results from the choice of
technology and radiation damage concerns.
WH will contact E. Bravin to discuss this matter.
FZ pointed out that also no single-bunch tune
measurement is foreseen. He thinks a bunch-by-bunch
tune monitor would be extremely valuable for
routine operation and studies.
WH mentioned that there will be no PLL at the
the LHC start up, and that the general lack of
man power in EST was deplored.
(8) RF Amplitude Modulation in the SPS (EV)
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EV and TB performed a successful study of rf
amplitude modulation. Extrapolating from the
Sacherer criterion, the bunch-to-bunch synchrotron
tune spread needed for stabilization was estimated
to be a quarter of the incoherent spread induced
by the higher harmonic rf. This estimate was
roughly confirmed in the MD. Measurements were
performed for intensities varying from 3e10 to
1.1e11 per bunch. The modulation depth was scanned
in rough steps.
The beam was stable for a 31% modulation.
Only at the high intensity on the flat top a small
phase motion developed, but this also is the
case when the 800-MHz system is operated in
bunch-shortening mode, which is the usual
stabilization scheme. The possible reduction of
kicker heating with this alternative rf modulation
scheme remains to be demonstrated. It is not entirely
clear whether the bunches are actually shorter in the
bunch shortening mode and, if yes, by how much.
A possible application of rf modulation to the
LHC is under consideration.
Enclosed: Slides by EM and EV.