Minutes of the ABP-LCE team meeting on 28.11.03
present: GA, EB, WH, JJ, AK, EM, FR, DS, EV, FZ
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(1) 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.
EM verified that the longitudinal Yokoya factor is 1 in
the two limits of a square and of two infinite plates.
It is slightly smaller for intermediate values. So all
previous calculations are exact. FZ confirmed after the
meeting that the factor pi^2/8 mentioned by Karl Bane
referred to a collimator impedance in the transverse
plane.
After the meeting FR pointed out the calculation in his
paper "Resistive wall impedance as derivative of the electric
capacitance for a beam pipe of arbitrary cross section"
published as CERN-SL-95-95-AP, which is also found at
http://prola.aps.org/abstract/PRE/v53/i3/p2802_1
The longitudinal loss factor here is the same for a beam
travelling in the centre of a circular pipe of radius
b or parallel to an infinite metallic plane of equal
resistivity at a distance d=b. Moreover, a square pipe
has the same longitudinal impedance as the inscribed
circular pipe. Finally, for a beam travelling in the
centre of a rectangular pipe, the resistive loss
reaches a minimum value about 6% lower than that
corresponding to a square pipe when the ratio between
the sides of the rectangle is around 1.33.
EM asked about the factor of 4 Pi found by E.S. This
was related to LV's paper on the SPS impedance.
ACTION -> Find ES' email (FR)
Done. Email to LV is enclosed.
ACTION -> Present example of Zbase usage (young ZBase users).
EM has started discussions with TdA on ZBase. The ZBase
directory is on afs.
ACTION -> FR will check the access rights
The ZBase program can be executed on linux and on windows.
Young users are supposed to put numbers and formulae into
ZBase rather than do offline calculations, e.g., length,
resistivity, gaps, cavity trapped modes etc.
The ZBase documentation is NOW linked to the collective
effects page and can be accessed directly at the web address
http://bruening.home.cern.ch/bruening/ZBASE/zbase.html
AK has tried the ZBase program and found that most
of the values are old.
ACTION -> CB head-tail growth rates for updated LHC
impedance (FR)
Pending.
ACTION -> DS will cross-check Roman-pot estimates by
numerical simulations and follow-up the impedance of
Roman Pots with Roger Perret.
DS again discussed Roman Pots with Roger Perret. Drawings
have changed since the last time. DS obtained drawings and
explanations. Thickness of coating not yet well defined
(about 10 micron Cu?). Two cases need to be calculated, with
pots in and out.
GA mentioned that a prototype will be ready for installation
into the SPS by the end of the year.
ACTION -> AK will repeat simulations with a constant
injection offset.
Done.
ACTION -> FR will read and approve the draft note by AK.
Done.
ACTION -> EB, FR, DS and FZ will further discuss electron
cloud instability studies and simulation strategies
Done. List of actions was established.
Presentation at LTC in 2 weeks or (preferred)
early January before Chamonix.
Computer problem with batch cluster.
Linux is being installed on a computer in EB's office.
(2) Collimator Impedance and Stability Limits (EM)
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Final data are available for IR7 (no information
on IR3 yet). The data contain collimator positions
and settings for 6 and 7 sigma aperture and the two
beams. With 7 sigma aperture the two beams may be
marginally stable both at injection and top energy.
These results are without any coating. IR3 might
slightly change the picture.
FZ asked whether it is coincidental that the aperture
in units of sigma is the same at injection and at top
energy.
FR mentioned a request to study the stability of
the TOTEM beam, with 1/3 bunch intensity, 1/4 emittance
and 2 times tighter collimators, 156 bunches.
Rough estimate indicates some margin. The experiments
also would like to decrease the collimator gap by
another factor of 2.
ACTION -> Stability Analysis for TOTEM Beam (EM)
ACTION -> Revise Thresholds for Single-Bunch
Instabilities (EM?)
(3) Progress Report on GdfidL, ZBASE, and Multi-Bunch
Instabilities (DS+FZ+EM+EV+AK)
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EM discussed the Yokoya factor as mentioned above,
using CAS CERN 95-06, and papers by Zotter, Gluckstern,
Palumbo et al. FR mentioned two related papers,
by Kurennoy and himself.
DS reported that the new version of Gdfidl including
the Napoly option is slow. It is being worked on.
I.Syrachev also started running Gdfidl and they are
now sharing the computers.
Comparison with B. Spataro is on the way. There can
be pseudo-trapped modes at the collimators. Results
depend on the boundary conditions chosen.
AK performed resistive-wall instability simulations
with random and constant initial offset. No Yokoya
factor was taken into account. Then the expected
ratio of growth rates in the two planes is 52, while
with Yokoya factor it would be about 2. The simulated
vertical growth rate is about 40-45 turns for
both types of initial offsets. The horizontal
growth rate for a constant offset is 2000 turns.
AK notes that there always is a 'latency' time until
the exponential growth sets in. FR remarks that this
depends on the initial bunch offsets: if they happen
to correspond to the most unstable mode, no latency
time is expected.
(4) Progress report on Electron Cloud Studies (EB+DS+FZ)
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EB summarized meeting with FR, Ds, and FZ.
The HEADTAIL code will be speeded up with the help
of DS. The results poorly converge with the number
of IPs, which is ascribed to the integer part
of the tune. Number of macroparticles and slices
will be varied to check the sensitivity of the
results. Experience at the SPS suggests a large
chromaticity suppresses the instability.
EB is now performing simulations for SPS conditions
including damper and a large chromaticity.
FR raised the question whether an analytical
model can be constructed. This is already being
worked on, but explicit solutions are difficult
when nonlinear motion is taken into account.
DS suggested to seed the instability with a
well-defined initial condition and to look at
a uniform profile.
The analog or digital noise of the damper
will be quantified and included. EV pointed
to a paper by WH.
EB also mentioned ongoing collaboration and
comparison of HEADTAIL with QUICKPIC code.
Contact is Ali at USC.
FZ showed simulation results for e-cloud build up
at LHC injection. There is a resonant like
dependence with maximum heat load occurring for
about half the nominal charge, and becoming
negligible above 7e10 per bunch. The simulation
considers gas ionization. The resonant dependence
(which needs to be confirmed for elliptic
chamber geometry) was seen for three types of field.
The dependence on the secondary emission yield was
also explored, for a dipole field.
For preparing a baseline parameter set for the
SPS MDs, accurate input numbers and several data
files were provided by GA. FZ showed the apertures
and computed beam sizes at 14 detector locations.
Complete sets of numbers are now available for
the two bunch spacings at injection and top energy.
The transverse beam emittances are a factor 2
different for the 25-ns and 75-ns spacings, which
might have some impact on the simulations. The
bunch lengths from TB are smaller than previously
assumed, but could have a systematic error.
FR and EM remarked that the LHC beam parameters
should be the same for the two bunch spacings.
(5) Q-meter Specs and Spectra with Beam-Beam
Collisions (WH)
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WH has restarted coherent beam-beam simulations,
extending them to multiple bunches. An increased
forest of pi modes was expected and is seen.
Aim of the study is to respond to a question
from BDI (JPK) and to compare single- and
multi-bunch tune measurements. The preliminary
results confirm that excitation and measurement
of a single bunch is clearly preferred.
If several bunches are excited due to limited
bandwidth, it would still be best to excite
regular, rather than PACMAN, bunches to measure
the response of one of the excited bunches only.
(6) AOB
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FR mentioned that Fritz Caspers is studying the
possibility of a Schottky monitor in the LHC.
Impedance implications should be estimated.
EV will look at a related paper by McGinnis
from FNAL.
ACTION -> Study McGinnis paper and impedance of
of a Schottky monitor for the LHC (EV)
Enclosed: Slides by EM.