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Minutes of the ABP-RLC team meeting of 14.10.2005
present: EB, EM, FR, DS, FZ
web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/
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(1) Announcements
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FZ was contacted by Tanaji Sen concerning the expected orbit
distortion at the IP, which is needed for Mokhov's simulations.
FZ estimated it should be at least 10-15 sigma, and he told TS to
contact OB or WH. TS is waiting for a response.
Following a suggestion by FR, FZ has asked OB, RdM, JPK and TS
to provide baseline optics versions in MAD format to be posted on the
HHH web site. No file was received so far.
Elena Shaposhnikova has asked FZ for considerations on the effect of
an increase in SPS injection energy to 40-60 GeV on the electron cloud,
for a report to the PAF on 17.10.
She assumes that the bunch length remains the same. FZ did not
expect a change in the electron density and a reduction in the
multibunch growth rate. Single bunch instability thresholds and
incoherent tune shifts should stay about the same as at 26 GeV/c.
In response to this reply, ES suggested simulations by GR, DS or EB.
EM reported on a meeting with Luca Bruno, Jean-Michel
Fraigne, AG and himself on TCDD and TDI. Follow up meeting for TCDD
will be held on 07/11/05 with results by AG. 4 Questions are to
be answered for TDI.
Guillaume has sent to EM an update of the LHC collimators parameters.
EM will reiterate the LHC impedance and stability calculations
with the new parameters.
(2) Follow Up of Fast Instability at SPS Injection (EM)
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EM revisited the fast instability in the SPS at injection.
In addition to results presented previously, he now showed
animated time evolutions of both the measurement and two
theories (BBU and TMCI). The frequency of the instability is
about 1 GHz in all cases. The instability seems to travel
in opposite directions in the theoretical results compared
with the measurements. The initial offset should be verified.
EM also presented FFTs of the signals which were obtained
by adding zero over most of the revolution period,
and Fourier transforming the whole data set, which
results in a periodicity of 40 MHz.
For the TMCI case EM showed results obtained for coupling of
modes 3 and 4, as well as for a single mode.
The latter does not resemble the measurement.
EM also presented curves of HEADTAIL phase shifts
for various chromaticities. With nonzero chromaticity
there is an apparent traveling instability, which for
mode 0 inverts its motion after half a synchrotron period
(which happens to be roughly equal to the time when
beam loss occurred in the experiment).
DS suggested to normalize the signal by the Gaussian
bunch shape before applying the FFT.
EM concluded that both theories reproduce the measurement
if he assumes Q=1, 1 GHz resonant frequency, and a shunt
impedance 10 MOhm/m.
=> ACTION EM will clarify the offset at the first turn and the
offset at which beam loss occurs.
(3) Impedance of PS Kickers for New CT (EM)
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Measurements were done by FC and TK as reported earlier.
The only theory which describes the type of geometry (combination
of ferrite and conducting boundaries) is Hiroshi Tsutsui's.
EM almost reproduced trhe curve shown by HT, but not quite
(assuming 4A4 ferrite as did HT).
The ferrite of the real kickers is 8C11.
The theoretical longitudinal impedance agrees well with
the measurement. The measured vertical impedance is
three times larger than predicted (both real and
imaginary part). The horizontal impedance has a
sign opposite to the prediction.
At high frequency the transverse impedance seems
to approach the value of the Burov-Lebedev theory.
EM noted that the kicker ferrites are blocks
with a period of 24 mm (19 mm ferrite followed by 5 mm
of Al), which is not taken into account by the theory.
In the worst case the PS impedance will go up by 15%, with
consequences for the nTOF beam. Its longitudinal
emittance would need to be increased from 2.3 eVs to 2.5
or 3 eVs. Since two 200-MHz cavities have been removed
from the PS, it is not clear whether such an emittance
is feasible.
Recent news from MG is that instead of 4
only 3 new kickers will be installed.
(4) First Results with HFSS for the LHC Collimator (EM)
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EM could reproduce most of HT's results. HFSS does not
give any impedance below 1 MHz (this might be due to
length of model considered), and several high frequency
points are off scale. DS suspected that a remedy might
b to reduce the mesh size, as he had encountered a similar
problem with GdfidL.
EM's simulations at the moment consider a wire with radius
0.5 mm or two wires with 2 mm distance.
(5) Crab Cavities (FZ)
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FZ compared diffusion rates simulated by Ohmi with
the Sen-Zorzano theory and found a better agreement
than reported by Ohmi at HHH-2004. The proper
relationship between action and amplitude diffusion
needs to be taken into account.
FZ derived an analytical formula for the luminosity
degradation by a non-zero R22 matrix element between
the crab cavity and the IP and illustrated it for
three crossing angles. Another formula describes
the effect of the crab rf frequency. For a crossing
angle of 1 mrad, 400 MHz is acceptable, but 800 MHz
is not, unless the bunch length can be reduced, e.g.,
by decreasing the longitudinal emittance.
A formula by Yuri Alexahin describes the emittance
growth due to random beam-beam offsets, including
decoherence. The tolerance for 1% emittance growth
per hour in the LHC is a random offset of 1.5 nm,
if a feedback is active, and 0.6 nm without feedback.
Presentations at Nanobeam'02 from RHIC and the
Tevatron indicated orbit motion much larger than this,
and large vibration amplitudes of the low-beta
quadrupole cold masses.
FZ asked whether such vibrations could be measured
on one the LHC low-beta quadrupoles, in order to
get a feeling for the magnitude of this effect in
the LHC.
FZ mentioned that EV is developing a phase stabilization
system for the XFEL with a tolerance of 0.02 ps, i.e.,
tigher than the SuperLHC requirement.
(6) Highlights from the FNAL IR Upgrade Workshop (FR)
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Tevatron reduced beta* from 35 cm to 28 cm, increasing the
peak luminosity by 10% and end-of-store luminosity by 5%
(hourglass effect). Further decreases in beta* are excluded
by a dramatic increase in Q'' and in the corresponding
chromatic tune footprint. This should be one of the
criteria for choosing the LHC IR layout.
Crystal extraction with 78+/-12% efficiency has been
demonstrated at the Tevatron, giving a boost to the
SPS programme.
Quench limits, radiation damage, and dynamic heat load
of the IR magnets were discussed. A stainless steel
absorber of 5 mm thickness, and correspondingly larger
magnet coil aperture reduces the peak power deposition
by a factor of 2.
A magnetic TAS would need an integrated field of 15-20 Tm
to be effective.
A symmetric optics for flat beams was presented
by John Jonstone. The tune footprint is much larger
than for the nominal LHC. (SF is working on an
asymmetric flat-beam optics, using the present
magnet configuration).
Helical quadrupoles were proposed to spread the
debris uniformly in azimuth.
=> ACTION: the RLC team should provide a proposal
for aperture and field requirements. Chromatic
performance of all IR solutions should be evaluated.
Miguel Furman now finds results in agreement
with ECLOUD when re-diffused electrons are not
included and a factor ~2 more LHC heat load
with re-diffused electrons.
Xialong Zhang has sent a draft report about recent
ecloud measurements at the Tevatron indicating an
emittance growth rate of 28pi/hr. This is an important
input/benchmark for our LHC predictions.
FR described the roadmap for the LHC upgrade
studies, modeled after the ILC GDE. In mid-2006
a reference design report will be finalized.
Alternative designs and their risks should be
included, with proper definitions.
(7) AOB (FR)
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Following up on the last APC meeting, FR suggested to
put some emphasis on beam dynamics studies at CTF-3.
Demonstration of understanding the drive-beam stability
limits is important.
Posted on the web: Slides by EM, FZ, and FR; email by EM.
Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/