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Minutes of the ABP-RLC team meeting of 11.11.2005
present: EB, GR, EM, FR, RT, FZ
excused: WH, DS
web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/
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(1) Minutes of last meeting, actions, announcements etc. (FR)
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RA is awaiting our assessment for the design of the phase-2
collimators. Some comment was already received from Fritz Caspers.
A revised copy of the CDR is available on the SLAC LARP web site:
http://www-project.slac.stanford.edu/ilc/larp/
We should comment especially on Section 3.2.4, RF contacts
(Approval by CERN experts is required before proceeding)
and Section 3.2.8, Unresolved issues (Acceptability of RF parts
(CERN RF experts), central groove in jaws (smooth track for central
aperture stop), strain-relieving grooves in jaws, do the grooves
require a tapered lead-in, what is the acceptable range of taper
angles for the jaw ends, heat generation in thin RF parts).
GR and RT will perform their induction interview with FR
next week. Goal is to set objectives for the first 6 months.
Follow-up on LHCb crossing scheme => WH will report next week
(2) Follow-up on SPS TMCI threshold (GR, EM)
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GR performed simulations at 26 GeV/c, 40 GeV/c and 60 GeV/c with
and without space charge. The bunch length was assumed to be
the same at all energies, so was the emittance, following
instructions from Elena Shaposhnikova. Unlike for earlier
simulations, in this new set of studies the bunch distribution
was always matched to the rf bucket. The result is that the
instability threshold increases in proportion to eta, which
equals momentum compaction factor minus 1/gamma^2.
For zero chromaticity, the threshold increases from 0.7e11
at 26 GeV/c, over 1.5e11 at 40 GeV/c to 1.9e11 at 60 GeV/c
without space charge. With space charge the threshold is
higher by 0.2-0.3e11. In conclusion, a higher injection
energy is beneficial for the SPS TMCI.
EM showed that the simulation results are consistent with
his theory, where the threshold depends only on eta and on
the longitudinal emittance. It is the distance from the
transition energy which matters.
Comment by EM:
"EM showed that the simulation results are consistent with his theory..." =>
I think GR also said this.
FR suggested to explore changing the lattice so as to
reduce the transition energy. FZ recalled that studies with
a different transition energy were reported in the SPS SWG
around 2000.
(3) Electron cloud in the SPS (EB, FZ)
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Concerning electron cloud effects at higher energy,
EB is in the process of studying electron-cloud instabilities
for a situation with stripes. This is of interest for the
SPS, LHC and the SPS with higher injection energy.
Presently, rectangular boundary conditions give rise to a
non-monotonic dependence of emittance growth as a function
of bunch intensity. EB will report in the next meeting.
FR asked FZ for a written discussion of the instability
scaling with energy.
FR also requested a review or prediction of incoherent
electron-cloud effects in the LHC.
GF will visit CERN for one week, probably in the week
of the CARE'05 meeting.
EB will also perform simulations for the two SPS
working points.
EM asked whether only injection losses or also the
lifetime was improved at the new working point.
(4) Comparison of TMCI theories (EM)
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EM showed that his theory, the one of Laclare and
the MOSES code make consistent predictions of instability
thresholds (4 different cases and SPS, respectively).
All three theories construct solutions to the Vlasov equations.
The solution procedures differ and so does the extrapolation
to higher currents.
Comment by EM:
"EM showed that his theory, the one of Laclare and the MOSES code
make consistent predictions of instability thresholds (4 different
cases and SPS, respectively). All three theories construct solutions
to the Vlasov equations. The solution procedures differ and so does
the extrapolation to higher currents." => My theory should be in this
case Laclare's theory (I did not add anything at this point, I just
re-did the computations and implement them myself with Mathematica,
comparing the results I obtained with the ones published by Laclare).
Then, I mentioned that there exist several ways to solve the same
equation, and I presented the one by Laclare, which is very fast but
does not give the imaginary part. To have it, one has to decompose the
modes onto a normalized orthogonal basis composed of the solution
modes at low intensity... (this is the classical method for TMCI).
(5) Report from FP420 Meeting (EM)
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EM attended a meeting organized by K. Potter on the FP420 experiment,
which includes a movable chamber in the dispersion suppressor
420 m from IP1 or 5. He commented on the wake field aspects
of this chamber, including trapped modes and resistive wall.
The resistive wall impedance is complicated by the fact that
the chamber is one-sides. The Piwinski theory may be used
for such a situation. The foreseen distance of 3 mm from the
beam is closer than the specified 20 sigma. This will be
clarified by the experiment's proponents.
In general, a copper surface is bad at the 8 kHz frequency,
since it leads to the image current flowing near the beam.
A postdoc at the Cockcroft institute is calculating
the impedance. He will join our group next year.
EM will send pertinent information to him.
(6) Highlights from Nanobeam'05 and KEK news (FZ)
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FZ reported on the NB'05 workshop and on KEK.
See his slides. He will give a more detailed report
on the Nanobeam workshop at the CLIC seminar on
Friday the 18.11. At KEKB lower feedback gain
increased the luminosity presumably limited by the feedback
noise, the new crab cavity optics reduced it, peculiar tune
shifts with current are observed for pilot bunches,
synchrobetatron sidebands are seen and can be used to
detect incoherent tune shifts.
(7) E-cloud benchmarking (FZ)
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ECLOUD simulations for KEKB show that the result is
independent of integration routine, and only weakly affected
by the choice of the SEY model. Benchmarking for the ILC
damping ring version "OCS" shows good agreement between the ECLOUD
code using the Hilleret model, CLOUDLAND and POSINST.
However, with the scaled Cimino-Collins model (benchmarked at the
SPS) the results are hugely different for the ILC OCS ring.
(8) Linear instability due to LHC finite crossing angle (FZ)
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Computing the eigenvalues of the 6x6 transport matrix
as a function of beam-beam tune shift shows that linear
stability is lost for about 10 times the nominal LHC crossing
angle. The threshold varies with tune by a factor of 3.
This may restrict LHC upgrade options. Similar calculations
can be done to investigate the effect of one or two crab cavities
per IP, a case which was analyzed by Chao and Hoffstaetter.
Posted on the web: Slides by GR, EM, and FZ.
Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/