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Minutes of the ABP-LCE team meeting of 13.10.04
present: EB, WH, EM, TP, FR, DS, EV, FZ
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(1) Review of Outstanding Actions & News & New Actions
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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: EM has sent email to E. Gaxiola. No response yet.
ACTION -> Estimate emittance growth time and beam lifetime
from gas scattering in the SPS (FZ)
STATUS: pending.
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: First part done (report below). Tune scan ongoing.
FZ reported that the collimator impedance measurements had been
performed Monday night with the TOTEM beam. The BBQ detector
observed a tune shift of 2e-4 or 10 Hz frequency shift for the
smallest gap and a bunch intensity of 8-9e10.
The corresponding full gap size was estimated to be 1.0 mm or
1.9 mm using two different calibrations. Assuming that 1.9 mm
is correct, the tune shift appears consistent with theoretical
prediction. If the gap was smaller, the impedance would be
smaller than predicted. No effect was seen on the orbit, as
expected.
It was discussed whether one can or not discriminate the
theoretical predictions of Burov-Lebedev-Vos and Zotter
from these measurements or from similar measurements with
a high-intensity beam, or from growth rate measurements.
ACTION -> Estimate whether/which measurements can distinguish
between resistive-wall theories (EM)
Earlier ACTION -> Update and benchmark ECLOUD code (DS, FZ)
DS reported that a common version of ECLOUD is available;
it has been successively benchmarked for the example of
an SPS cold dipole field.
After further benchmarking, the new version will be made
public at the time of the HHH-2004 workshop. FR suggested
to also post 'beta versions' on the web, with some
warning.
VB has confirmed earlier heat-load measurements for COLDEX,
this time with a fully isolated detector. The measured heat
load is consistent with that measured independently on a
warm detector, and, according to simulations, with a conditioned
surface. Why no conditioning is observed remains unexplained.
Absolute electron flux for the quadrupole detector should
still be benchmarked.
After the meeting FZ discussed with A. Grobner, who is
convinced that ions must be included in the model, to
obtain the correct survival times of electrons.
ACTION (FZ) -> Organize meeting of VB, JMJ, Ds, FR, FZ
to discuss outstanding simulations.
(2) Update on HEADTAIL Simulations (EB)
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Replacing the electron cloud by a broadband resonator for
an electron density of 2e11 m^-3, i.e., below the "TMCI"
threshold, and a chromaticity Q'=2, no emittance growth is
seen over the usual time scale of a few tens of ms.
The conclusion is that the BB resonator is not an adequate
model for the slow emittance growth phenomenon. EM pointed
out that the BB resonator may show some growth on a longer time
scale. FZ commented that for positive chromaticity all low-order
modes may be stable, as explained in a lecture by J. Gareyte,
and that modes of higher order might be Landau damped.
ACTION -> EM and/or FZ will review Landau-damping mechanisms.
EB re-plotted her earlier results as a function of the
fractional tune between interaction points, assuming that
the emittance growth scales linearly with the cloud density.
There was some evidence for a 3rd order resonance. It was
recommended to perform a finer scan near one of the resonances,
for a constant electron density.
ACTION -> Finer tune scan near resonance for constant density (EB)
EB is performing a tune scan on the coupling resonance.
FZ thought that the results on the coupling resonance
may be difficult to interpret, since any random coupling
from the cloud could lead to large emittance transfer
between planes and leave the eigenmodes undefined.
EM recalled that GA found an increased threshold
close to the difference resonances for the initial
SPS tunes, but enhanced emittance growth after tune
inversion.
EB also performed simulation for a density of 4e11
and, with two kicks per turn, for a density of 8e11,
with twice the betatron tune. In the horizontal plane
the second cases shows twice the emittance growth
as might be expected. In the vertical plane, the emittance
growth for the two cases is the same, however.
DS noted that the synchrotron motion was not scaled.
ACTION -> EB will redo this comparison either without
synchrotron motion, or with QS, Vrf and alphaC all
scaled by a factor 2 as well.
(3) Beam Stability and Impedance Model for the SPS (EM)
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EM reviewed the complete impedance model for the SPS,
starting from the inductive impedance model of L. Vos,
which he represented by a broadband resonator with
resonant frequency 1.3 GHz and Q=1, and adding the
impedance of the MKE kickers.
Assuming this model of the combined transverse impedance
for different years, and considering beams of different
longitudinal emittances he computed the TMCI threshold
with MOSES.
For the nominal LHC beam the threshold was found to be 1.7e11
in 2000, 1.2-1.3e11 in 2003/04, and 1.05e11 in 2006. These
numbers refer to Qs=0.006 and an rms bunch length of 0.75 ns.
The 30% increase in inductive impedance from the kicker
reduces the threshold by about the same factor. FR remarks
that the threshold current reduction from 1.7e11 to 1.05e11
seems larger than 30%.
For the low emittance beam the threshold should be
5.5-6.5e10, and for the very low emittance beam 3-4e10.
These numbers agree perfectly with the observed intensity
thresholds for beam loss after injection.
FR asked why this limitation for the LHC beam was not
recognized before, e.g., in Chamonix. Possible answers
are that the threshold had been computed separately
for the kickers and the remaining impedances, instead
of for their combination, and that a different resonator
frequency had likely been considered.
FR recommended to study the sensitivity of this result
to changes in resonator frequency, Q, and impedance.
EV suggested to introduce error bars, possibly even
including an error bar for the theory.
ACTION -> Study sensitivity of TMCI threshold for LHC beam
to various impedance parameters (EM).
Attached: Slides by EM