Minutes of the ABP-LCE team meeting of 22.08.03
present: TdA, WH, AK, EM, FR, DS, EV, LV, FZ
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(1) Comments to last meeting and minutes
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There was a translation problem in the slides of AK
distributed after the last meeting. Corrected slides
will be attached to this email.
FR suggested that results on photon scattering be published
as an ABP note and possibly sent to contact persons at the
experiments.
He also mentioned a second study by DA and FZ for a QCD
explorer based on colliding the CLIC beam with LHC
superbunches. Estimated luminosity is 1000 times higher
than the minimum needed. This study could become a combined
CLIC-LHC note.
(2) Outstanding Action Items
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A) Status of Gdfidl (DS)
It was found that the mesh size must be chosen much finer than
the skin depth in order to get the correct impedance. Recent
results for the longitudinal impedance look promising and
consistent with HT’s paper. Typical CPU time is 0.5-1 day for
one calculation. Further simulations will be run with Even
finer mesh. The geometric impedance will be looked at
afterwards. It might be difficult to compute the resistive wall
impedance for the taper.
ACTION -- > DS, continue great work with Gdfidl.
B) Tune Dependence of CB effective impedance (EM)
Using Sacherer approach the tune dependence was computed
with and w/o inductive bypass. W/o bypass a strong dependence
was found in agreement with the classical res.wall theory.
With inductive bypass, the growth rates and frequency shifts
are independent of the fractional tune, which is ascribed to
the broadband character of the impedance.
C) Landau damping from octupoles (EM+LV)
Using the Berg-Ruggiero formula, EM computed the
stability diagram for a 2-m long effective graphite
collimator. The bunch distribution was considered pseudo-
parabolic. The rms tune spread was estimated to be about
1.8e-4 in both planes assuming present values for the
maximum octupole strengths and beta functions. The tune
spread differs by a factor of 2 from earlier calculations
based on D. Mohl’s paper. This factor of 2 had been
discovered by S. Berg. EM's calculation shows that mode=0
is not stabilized by the octupoles, but all higher-order modes
are. However, the collimator model was pessimistic by about
a factor 3.
Using a more realistic model summing over all actual
collimators, but considering a 1-dim. Situation and a
Gaussian distribution, LV showed that also the m=0 mode is
stable, both with and without the copper coating.
The coating moves the LHC complex tune shift vector
further inside the stability boundary, for a Gaussian
distribution, and it is thus recommended, especially,
in case the real distribution is not Gaussian.
LV points out that the tune shift induced by octupoles
warrants a second look, and he referred to an old note.
ACTION -> Verify factor 2 of SB&EM (LV)?
D) Inductive bypass impedance (EM)
EM distinguished between inductive and non-inductive regimes and
presented approximate impedance formulae for the various cases.
E) Space-charge tune spread versus Landa-damping of HT modes (EM).
EM presented data from the PS, where HT modes of different orders
were clearly observed in agreement with Sacherer approach and
resistive-wall theory. Turning on octupoles could stabilize the
beam. The corresponding tune spread was a factor 1000 smaller
than the space-charge tune spread. Therefore, the space charge
tune spread Is not very effective in providing Landau damping.
This is consistent with Mohl&Schonauer (1974) and Mohl (1995),
who predicted a small effect only when magnet nonlinearities
are also present.
The factors 2 from S. Berg and a factor 2 potential enhancement
by space could hint at a discrepancy by a factor 5—6 between
the measured and theoretical instability threshold in the PS (LV,FZ).
It was suggested to repeat the experiment, possibly at two different
energies. (ACTION?)
LV rcalled data by HB at high intensity in the SPS, which showed
no sign of decoherence and no instability.
F) RF amplitude modulation & CB transverse feedback (EV)
A frequency spread of 6.5/s is required, which corresponds to
20% rf amplitude modulation of the 200-MHz system. The 800 MHz
system Will be switched off for preliminary tests, done together
with T. Bohl (MD next week?).
Improvements for the transverse CB feedback are under study. Possibilities include a reduction of the amplifier noise using
an integrator, and a modal analysis utilizing information
from two successive turns.
G) Electron-cloud status (FZ)
Three sections for the LHC design report were completed, on
electron cloud, sybchrotron radiaton, and gas scattering. Response
from the vacuum group concerning gas desorption is still missing.
In the last APC meeting several interesting talks were held on
electron-cloud measurements in the SPS, by members of the vacuum
group. These measurements shall be benchmarked against simulations.
Preliminary studies indicate that the code does not reproduce the
(sometimes) observed 2-stripe structure for a field-free region.
Also, strong multipacting is simulated (by GR) for a maximum yield
as low as 1.0 for a rectangular chamber as used by the NEG test
stand. This could be related to the assumed angular dependence
of the yield, for which we only have old data for copper, but not
for the NEG coating.
Recently the SPS LHc beam experienced a poor lifetime of
20 minutes, Which could be due to electron cloud.
M. Furman had asked for electron-cloud working topics as part of
the US LHC collaboration, and FZ has sent him a few items,
mostly related to the SPS studies.
ACTION-- > See if ALS could measure photon backscattering
on sawtooth (FZ)?
H) Multi-bunch simulations (AK)
AK presented two movies illustrating the meaning of the different
multi-bunch mode numbers. Several examples showed how one can
extract the amplitudes of differerent modes from simulated pick-up
data. The next step will be to apply a sliding window and to extract
growth rates of different modes.
(30 AOB (FZ)
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Yesterday’s LRBB MD was successful, clearly revealing diffusion
on the wire scans. There was a request from FNAL to participate
in a review of their BBLR project. It was declined and
instead suggested that sombody from FNAL come to CERN and join
SPS BBLR experiment.