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Minutes of the ABP-LCE team meeting of 04.06.04
present: EB, AB (A. Burov), AK, WH, DK, EM, DM (D. Moehl, partly),
FR, ES (E. Shaposhnikova), DS, FZ
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(1) Pending Actions & Reports from Meetings
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ACTION => Clarify relation between loss factor & synchronous phase shift.
PENDING. EM ? (Energy loss depends on t and theta)
ACTION => Clarify why B. Zotter's results are an order of
magnitude larger than the results from Vos,
Burov&Lebedev. BZ will give a presentation in one of
the next LCE meetings. AK plans to repeat the derivation
with field matching and four em potentials.
PENDING.
ACTION => Clarify the origin of the difference in graphite
resistivity.
EM and FR received independent information that
for the TCDQ (?) the same graphite resistivity as
for the collimators could be assumed, i.e., a factor
5 less than previously stated. FR pinpointed the
lack of professional attitude, since the resistivity
of the material used should be known, and our
estimates cannot be based on arbitrary (and wrong)
numbers.
DS suggested to have clear rules which level of trapped modes
is acceptable and which not. FR suggested that DS presents a
proposal for such criteria.
FR reported on two meetings. There was a meeting on the impedance
of TOTEM Roman Pots. The conclusion is that F. Caspers will measure
the impedance in the lab, before a decision is made to run GdfidL
simulations. Based on the results we will determine whether the
beam pipe at the Roman Pots should be Cu coated or not.
A second meeting with Sergio Calatroni was on the RF contacts for
the collimators, that will ensure a smooth conduction path from
the vacuum chamber across the graphite jaws for the longitudinal
beam image currents. Our target is not to exceed a contact
resistance of ~1 mOhm and this goal will be met for the LHC
collimators. The SPS prototype, however, will have sliding RF
contacts directly on graphite (instead of a special Cu case)
and the contact resistance may exceed somewhat our specification,
but will still be impossible to measure. The additional question
of sliding vs flexible RF contacts connecting the jaws to the
collimator tank is discussed below (see point 4).
(2) Impedance Issues: Collimators and Kickers (A. Burov)
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AB first described the derivation of the analytical formulae.
The long-wave approach is used, so that only the electro-static
potential and the axial magnetic vector potential need to be
determined. The vector potential is the more difficult part.
Its value and derivative divided by mu_r must fulfill
continuous boundary conditions at the chamber wall.
The calculation and results for various specific cases
were presented by A.B. and Valeri Lebedev at EPAC 2002.
The impedance computed for the collimators is in close
but not perfect agreement with the calculation by L. Vos
(SL-20000-010 AP). The difference in the effective
impedance is small. For the SPS kickers, however, AB
questioned two main assumptions made by L.Vos and H. Tsutsui
in LHC Project Report 234, namely that the dissipative
terms can be neglected and that the excitation of waveguide
modes is the main effect. AB's result differs by a factor
of 4-5 from the earlier calculation. AB also pointed out
that, for a flat chamber, there is a factor 1.6 amplification
in the vertical tune shift, arising from adding instability
wakes and detuning wakes. AB predicts that the real part of the
kicker impedance has a maximum of 3.5 MOhm/m at 1.8 GHz.
It was thought that HB's measurements indicated a frequency
around 0.6 GHz. However the beam-based measurements
in the SPS are not very sensitive to the frequency.
(3) Collective Effects in the Tevatron (AB)
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For a long time a head-tail instability was observed
whose growth rate was 4 times larger than predicted
from the impedance model. AB found that the discrepancy
was due to the impedance of the two Lambertson magnets.
A Lambertson magnet is 20 m long, and consists of
laminated iron plates, which increase the effective
impedance. An experiment with bump confirmed that
the Lambertson was a source of large impedance.
The Lambertson was shielded with stainless steel,
which reduced the growth rate.
It was hoped that the chromaticity (xi >~ 1 at top
energy) could now be reduced, but it is still high
in order to keep the beam stable in case of
chromaticity drifts, which occur for unknown reasons.
AB's calculation was based on a macroparticle
simulation, not on a TMCI calculation with MOSES.
(4) Collimator Contacts (DS)
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DS discussed the two options of connecting the collimator
jaws to the tank: rolling contacts, based on a flexible
conductor, or sliding contacts.
The rolling contact is more stable mechanically, but it
gives rise to a cavity, where trapped modes are excited.
The trapped modes are smallest with sliding contacts and
using stainless steel instead of copper.
With rolling contact, a single mode at a single jaw
would induce a betatron frequency shift of 1.6 Hz which
is 16% of the total budget. DS also showed results for
a pessimistic addition of the effect of all e.-m.
modes, and the tune shifts for various headtail modes.
The LCE team decided that stainless steel and sliding
contacts be used, and that the other solutions are
unacceptable from the impedance point of view.
(5) E-Cloud Benchmarking for Electron Energy Detector (FZ)
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FZ simulated the flux and energy spectrum for the detector
of J.-M. Laurent. Cutting at vertical momenta of 30 eV
he obtained both the full energy spectrum and the
energy spectrum associated with the vertical particle
motion. The maximum energy is the same in measurements
and simulations. The dependence on the number of batches
is about ok. The full energy spectrum looks similar to the
measured one, though in the experiment only the vertical
spectrum is measured. There are clearly details in the
6-dimensional acceptance of the detector, which are important
for the shape of the energy spectrum and which are neither
known nor included in the simulation.
(6) EPAC Papers
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EM mentioned that the paper on the inductive bypass will be
completed with the help of A. Burov and AK next week.
FZ finished two papers. Simulations were run for determining
proton beam emittance growth for the QCD explorer, using
a modified version of HEADTAIL. The emittance growth is
25-100 times smaller than the most pessimistic estimate
but still larger than tolerable.
For the ECLOUD code comparison paper, emails were sent out
soliciting contributions and revised simulations from
all coauthors. Four coauthors responded that they would
consider the request. Meanwhile the ECLOUD simulation
result has been updated already.
FR suggested that E-cloud simulations be repeated for
insertion-region quadrupoles using the latest version of
the code. He also recommended that FZ shows the slides
presented at the last LTC meeting to the LCE in the
next team meeting.
(7) Beam-Beam Simulations (DK)
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DR has set up linear errors for the beam-beam simulations,
with the help of WH. This includes multipole errors,
misalignments, and orbit correction.
(8) AOB
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FZ reported that the BBLR installed last week was removed
again, after two vacuum leaks were detected in one device.
The leaks occurred after 1 or 2 days of operation, and
they are believed to be due to an etching applied
after brazing in the workshop. One of the LSS2 will be
modified and completed (without etching) to be ready
for installation instead of the leaking one.
Noel Hilleret has offered his help and advice to JPK.
Attached: Slides by AB and FZ.