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Minutes of the ABP-RLC team meeting of 10.06.2005
present: EB, UD, AG, WH, EM, FR, FZ
web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/
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(1) Minutes of last meeting, pending actions, announcements
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=> ACTION => compare the size of the longitudinal geometric wake with
RW longitudinal wake from A. Koschik (FR)
STATUS: PENDING, FR will talk to A. Koschik
=> ACTION => attempt to derive a general nonlinear theory including
the inductive bypass (EM)
STATUS: PENDING, PhD student being looked for.
=> ACTION (EM+FZ): compare and understand the different predictions
of the Burov-Lebedev theory calculated by FZ and EM.
STATUS: A. Koschik has not yet done any comparison with this
notebook. FZ found that the generalized Piwinski formula
agrees with EM's result in the limit of small transverse emittance.
FR has discussed collaboration items with Ingo Hofmann. Ingo plans
to organize a mini-workshop on 'trapping and beam loss' due to
space charge and also due to electron cloud in November.
FR mentioned that there are other workshops such as Coulomb'05
in Senigallia. Ingo will likely attend HHH-APD LHC-LUMI-05
workshop in Arcidosso.
Independently of the discussions with Ingo, a bilateral
CERN-GSI collaboration agreement is being finalized, which
in addition to joint magnet development includes beam dynamics
activities.
(2) Impedance estimates for TCDS and TCDQ absorbers (AG)
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The TCDS and TCDQ protect septum and quadrupoles at the extraction
point. Each consists of two sections of about 3 m length, separated
by a tapered region. The total number of tapers is 4 for both
TCDS and TCDQ. Graphite blocks are placed between
the stored beam and the extracted beam. The side of the graphite
facing the stored beam is copper coated. In the TCDS case, a second
graphite block is located at the other side of the extracted beam.
AG computed the impedance for the un-tapered devices and for two
different types of taper.
For the TCDQ, taper type 1 has a tapering angle of 5 degree and
the graphite block is closer to the beam; taper 2 is shorter,
has a tapering angle of 15 degree, and the wall is further
away from the beam.
Numerical calculation by GdfidL gave a Z/n of 203 microOhm without
taper, 146 microOhm for taper 1, and 90 microOhm for taper 2.
The total impedance is higher by a factor of 4 as there are 4 tapers.
It was decided to recommend the taper 2 with the lowest impedance.
For the TCDS, also two options, taper 1 and 2, were considered,
but somewhat different from those for TCDQ, and both of the same
length. In this case, the Z/n per transition is 82 microOhm
without taper, 60 microOhm for taper 1 and 35 microOhm for taper 2.
It was decided to accept either of the two taper options, since
the impedance is small compared with that of TCDS.
Trapped modes were not found to be an issue, as had been
feared. Some trapped modes were identified, but these were
unrelated to the splitting of the beam pipe into two halves.
One additional concern are the slots between individual
segments, forming these devices. There are about 8-10 slots,
of 1 mm length, which interrupt the image current flow
on the side of the graphite (or other material) blocks.
=> ACTION: EM and/or AG will analytically/numerically estimate
the slot impedance (AG has already performed a numerical
estimate: ~30 microOhm per slot, these results will be discussed
in a meeting with the equipment groups next monday morning)
(3) Tune shift with generalized Piwinski formula (FZ)
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FZ showed the tune shift computed from the generalized
Piwinski formula for the benchmarking example with Elias.
The tune shift varies substantially with emittance.
In the limit of low emittance the result is the same as
Elias'. For large emittances it can be almost a factor
of 2 difference, explaining the collimator measurement
in the SPS. So, the two curves included in Helmut's PAC
paper were most likely correct.
(4) Reports from meetings and ongoing activities
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FZ presented an update of electron-cloud simulations at
the MAC. The predicted heat loads are more optimistic
than earlier, but the uncertainty is at least a factor
of two. Observations with COLDEX, at RHIC and in DAFNE
are not well understood, and point to a lack of
understanding of the scrubbing.
The incoherent emittance growth due to electron cloud
could be responsible for the poor beam lifetime seen
in the SPS with LHC beam. If e-cloud induced tune shifts
should be less than e.g. 1e-4, the acceptable e- densities
are of order 1e8 m^-3, hence 30000 times lower than the
fast-instability threshold or the heat load limit.
M. Harrison asked for beam lifetime predictions in the
LHC (or SPS) based on these simulations.
=> ACTION: EB will perform such simulations for a dipole
field.
FZ contacted Kay Wittenburg for the impedance of the
LCH wire scanners (vessel copied from DESY). Kay has not
yet responded, but Ferdi Willeke remarked that the
impedance had not been calculated and that DESY never
encountered any problems. The same wire scanners are
also used for the electron beams with much shorter
bunch length.
In contact with J. Koopman and AG, EM is setting
up HFSS calculations of the wire scanner.
Last minute: after the RLC meeting Lyn Evans has (urgently)
requested a plot of the head-tail growth rate as a function
of chromaticity at injection energy for nominal current.
EM and FZ will use an updated LHC impedance model to produce
this plot, if possible by next monday. FR has already provided
some qualitative information based on an impedance model not
including the collimators.
=> ACTION: produce a plot of HT growth rates as a function of
chromaticity at injection energy for nominal current (EM and FZ).
Posted on the web: Slide by FZ
Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/