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Minutes of the ABP-LCE team meeting of 22.10.04
present: EB, AG, WH, TP, FR, DS, EV, FZ
excused: EM
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(1) Minutes of last meeting, pending actions & news
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ACTION -> EM will check with BT experts and/or with FC
whether 4A4 ferrite data is available at higher frequencies.
Otherwise some analytic model will be used to extrapolate
the available data to higher frequencies.
STATUS: Pending.
ACTION -> Estimate emittance growth time and beam lifetime
from gas scattering in the SPS (FZ)
STATUS: GA provided some vacuum data for 270 GeV/c.
Data at injection will be recorded during two 26 GeV/c coasts
in November.
ACTION (EB) -> Plot results as a function of effective phase
advance. Perform tune scan with a constant electron density
Simulate the situation of the recent SPS experiments
STATUS: Tune scan ongoing.
ACTION -> Estimate whether/which measurements can distinguish
between resistive-wall theories (EM)
STATUS: FZ presented some results on this topic (see below)
ACTION (FZ) -> Organize meeting of VB, JMJ, Ds, FR, FZ
to discuss outstanding simulations.
STATUS: No common free time slot could be found in week 43.
Meeting is now planned for week 44 without FR.
ACTION -> EM and/or FZ will review Landau-damping mechanisms.
STATUS: pending.
ACTION -> Finer tune scan near resonance for constant density (EB)
STATUS: pending.
ACTION -> EB will redo this comparison either without
synchrotron motion, or with QS, Vrf and alphaC all
scaled by a factor 2 as well.
STATUS: pending.
ACTION -> Study sensitivity of TMCI threshold for LHC beam
to various impedance parameters (EM).
STATUS: pending.
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.
STATUS: Bruno Zotter will give a short talk mid or end November.
He also recommended to move the tune closer to an integer
to detect the low-frequency part of the collimator
impedance in the SPS experiment.
A similar suggestion was made by FZ during last week's
MD. However, this could not be realized, since, approaching
the integer from below - for an unstable situation -
implies crossing the half integer, which according to GA
is impossible at 270 GeV/c.
FR announced that he will be absent for the next two weeks.
He suggested to hold a meeting without him on October 29.
(2) Preliminary results of the SPS collimation test:
trapped modes (FR)
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FR reported that Fritz Caspers and Tom Kroyer have
observed trapped modes, when the collimator jaws were
moved inwards. The mode power was more than 20 dB
above the noise level. This means FC has won a bet.
(3) Preliminary results of the SPS collimation test:
measured vs. estimated impedance effects (FZ)
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FZ compared predictions for tune shift, orbit deflection,
head-tail growth rates, and multi-bunch instabilities, with
preliminary measurement results. The predictions were
calculated for the Burov-Lebedev impedance and for an
approximation to Bruno Zotter's calculation result.
Both predicted and measured tune shifts are of the order of
1-2e-4 in good agreement. However, the predicted dependence
on the gap size is an inverse cubic one, while the measured
dependence appears about inversely proportional to the gap,
characteristic of a geometric impedance.
FZ studied whether the nonlinear part of the wake field
together with the finite size of the beam could explain
the difference. He computed a Piwinski form factor as
a double integral over source and test particle
distributions cut off at the collimator gap. The result
was that the nonlinear part of the wake should enhance
and not lower the tune shift.
Orbit deflections of a few 10ths of a microrad were
expected. No deflection was observed by Ralph Steinhagen
with a measurement resolution of about 1 microrad.
To get a deflection above the noise level, the beam
center should be closer than 200-300 micron to the
jaw, at which point a large beam loss would occur.
The expected change in the head-tail growth rates
from closing the collimator is small, and comparatively
largest at low chromaticities. Measurements were
taken by G. Arduini and FZ. Within the error of the
measurement the effect of closing the collimator was
either invisible or with a sign opposite to expectation.
All these single-bunch results were calculated to be
the same for Burov-Lebedev's and Zotter's impedance.
The two impedances make very different predictions
for multi-bunch growth rates. For a uniformly fille
dring, a few mutlibunch modes should have up to 100 times
larger growth or damping times. In the SPS experiment,
4 trains of 48 bunches were stored. There was no
obvious change when the collimator was closed.
(4) Weak-strong simulations of LHC crossing schemes (FZ)
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FZ presented the results of weak-strong beam-beam simulations
for the effect of long-range collisions alone and the combined
effect of long-range and head-on collisions. He compared
results for xx, xy and yy crossing at the two main IPs,
scanning the vertical tune over a total range of 0.05.
The 0 amplitude tune was kept identical for each of the
three schemes.
Simulations were repated for both the nominal crossing
angle and integer tune, as well as for a reduced crossing
angle of 200 microrad and also for a horizontal integer
tune of 63 instead of 64. In addition, start amplitudes
were chosen along the lines x=y and y=0.
In all cases, except on the coupling resonance, where
the invariant calculation may encounter problems, the
xy crossing exhibits a smaller diffusive aperture. The
difference in diffusive aperture varies between 1 and
more than 6 sigma depending on the tune and the scheme.
In several cases, the diffusive aperture for equal-plane
crossing is above 9 or 10 sigma.
Frequency maps illustrated the qualitative difference
between the crossing scheme. With xy crossing long
wings expand in two directions, with equal plane
crossing only in one. Besides, more resonances are
excited with xy crossing.
The frequency map results appear robust against
small tune changes.
A tentative explanation is that xy crossing
cancels the dodecapole and 20-pole terms between
IPs, leading to a different 'folding' of the
frequency map. It also excites twice the number
of resonances.
(5) Preliminary results of beam-beam simulations (TP)
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TP presented results of a strong-strong beam simulation
with 4 bunches colliding against 4 bunches at 8 IPs.
Varying the intensity of one of the 8 bunches can
produce an additional line in the frequency spectrum
of multi-bunch oscillations, for a certain range of
relative intensity. It remains to be investigated
whether this new line reflects a growing or damping
phenomenon.
This study attempts to understand in a systematic
way, earlier observations, made when all bunch
intensities were varied randomly.
(6) LCE contributions to CARE HHH-2004 workshop
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FZ is coauthor on 3 posters, addressing vacuum
effects in LHC, nonlinear collimation, and
electron-cloud instabilities.
FR recommended to DS and FZ to consider a poster
on certified heat load predictions, which will
no longer be subject to change. This can be
a collection of results published earlier, with
possibly a few additional ones covering the 75-ns
upgrade option. FR suggested that the 12.5-ns option
would also be looked at.
DS was invited to be a secretary and participant
in the panel discussion on electron-cloud effects.
WH chairs the beam-beam panel discussion.
TP may present a poster on beam-beam simulations.
DS pointed out that present limits from heat load
for LHC are delta_max=1.1 for 100% reflectivity
of low-energy electrons, delta_max=1.2 for 50%
reflectivity and delta_max=1.3 for 0% reflectivity.
These numbers are not very different from what
was published in the past.
EV suggested to look at limits from longitudinal
coupled-bunch instabilities. He may present a
poster on this topic.
(7) AOB
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EV performed an error calculation for the measurements
at the beginning and end of the batch for the CGNS
extraction tests. The rms value is not a good measure.
Considering the bandwidth and digitization accuracy
he estimates the error to be about 20%. This means the
measured performance is at the edge of the specified
limit of acceptability. Therefore, he is now waiting
for a response from CNGS.
Attached: Slides by TP, FZ