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Minutes of the ABP-RLC team meeting of 13.05.2005
present: EB, UD, AG, WH, Dobrin Kaltchev, Andrea Latina,
EM, TP, Stefano Redaelli, FR, DS, FZ
web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/
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(0) New Team Member
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DS introduced Andrea Latina, a new member of the RLC team and EUROTeV
fellow. Andrea will work on integrated simulations for CLIC.
(1) Minutes of the last meeting and pending actions
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=> ACTION => compare the size of the longitudinal geometric wake with
RW longitudinal wake from A. Koschik (FR)
STATUS: PENDING, FR will talk to A. Koschik
=> ACTION => attempt to derive a general nonlinear theory including
the inductive bypass (EM)
STATUS: PLANNED
=> ACTION => check sensitivity to centering with Piwinski theory (FZ)
DONE, see presentation below
=> ACTION => Implement analytical formula of Giuliano in
HEADTAIL to see whether his result can be reproduced (EB).
DONE, see presentation below
=> ACTION => Repeat simplified simulation for a much narrower
e-cloud distribution (GF)
DONE. A model with 10 times smaller e-cloud, corresponding
to 100 times higher tune shift (Delta Q ~4) at the beam center,
produced growth of the beam core, as seen in HEADTAIL.
A tentative explanation by Giuliano is that the particle motion
in the core becomes linearly unstable for a finite number
of kicks and a large tune shift.
(2) Update of the LHC collimator impedance (EM)
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So far 19 collimators were considered, all made from carbon.
EM presented an updated calculation for 42 collimators
including TCDQ (8 m), TDI (4 m) etc., some of which are made
from different materials or a combination of materials,
occasionally with coating. A question from the collimation
team is whether the length of the 4 primary collimators
could be increased from 0.2 m to 0.6 m.
Stability calculations were done for top energy. First, for
19 collimators only, the theories of Zotter, Vos and
Burov-Lebedev are not identical. The BL prediction for
the complex tune shift is 10-20% higher than the other
two. In the following only the Zotter formula was used.
Going from 19 to 42 collimators increases the tune shift
by 10-15%. Increasing the length of the primaries from
0.2 to 0.6 m further enlarges the tune shift by 5-10%.
This is a small change, and therefore our answer to
Ralph is that the change is acceptable.
The LHC beam is still expected to become unstable at about
half the nominal intensity.
FR recommended to repeat the same calculations at injection,
and also to use a more accurate model for the TDI, with coating
layers of finite thickness.
The number of primaries appeared to be less than expected,
but SR confirmed it after the meeting.
(3) Effect of nonlinearity on PS recoherence (EM)
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EM showed analytical calculations based on the Minty-Chao-Spence
paper for the effect of chromaticity and detuning with amplitude
on the decoherence, recoherence, and emittance growth in the PS.
If the detuning with amplitude is too strong, the recoherence
of the chromatic part is suppressed. The actual detuning with
amplitude in the PS is not known.
The measurements show a recoherence frequency which is quite
different from the synchrotron frequency. FZ remarked that
this could be an effect of linear coupling.
FR and EM mentioned that it was theoretically impossible to
produce the TOTEM beam and yet CERN did it.
At the moment, there is no solid basis to argue why a feedback
is necessary other than to ensure more reproducible results.
The HEADTAIL simulations including space charge, but without
nonlinear detuning and without headtail damping, show a 7-8
ms decay time of centroid motion, while in the experiment it
was about 2 ms. The headtail damping time is estimated to be
about 5 ms.
(4) Update on the SPS collimator impedance measurements (FZ)
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FZ showed a revised comparison of the tune shifts measured in
the SPS collimator experiment with theoretical predictions.
The predictions include the actual intensity, bunch length,
and emittance for each data point. The resistivity in the
model was reduced from 14 microOhm-m to 10 microOhm-m.
The Burov-Lebedev model agrees well with the measured curve
except, arguably, for the point at largest gap opening,
where the tune shift is small, close to the error of the
measurement.
The coherent tune shift derived from the nonlinear Green
function wake field in the Piwinski-Bane-Irwin-Raubenheimer
theory was revisited and a further integration over one of
the driving particle coordinates was added. The prediction
of the Piwinski theory is also very close to the measurement,
with even the point at large gap in better agreement.
The Piwinski formula allows taking into account an offset
of the closed orbit from the center of the jaws. A 100 micron
offset increases the predicted tune shifts by ~10%.
There is a puzzle why the Burov-Lebedev theory calculated
by FZ agrees well at small gaps, while that of Elias
shows a difference by a factor 1.5-2.0 (and a better
agreement for the smallest tune shift at the largest gap).
DS pointed out that the difference between the predictions
looks like a constant factor of 2.
=> ACTION (EM+FZ): compare and understand the different predictions
of the Burov-Lebedev theory calculated by FZ and EM.
(5) Update on slow emittance growth (EB)
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EB implemented in HEADTAIL the potential of a Gaussian
cloud approximating the model of Giuliano. HEADTAIL simulations
with this potential show a halo growth in qualitative agreement
with Giuliano's result. The magnitude of the rms emittance
growth is a factor 10 different, which might be due to remaining
differences in the force experienced by the protons.
EB also commented on Giuliano's recent simulations with
linear instability.
=> ACTION => FR will call Ingo Hofmann to discuss the
collaboration with Giuliano Franchetti on the emittance
growth due to electron cloud below 'TMCI' threshold.
(6) Reports from other meetings and AOB
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FR reported from a meeting with the vacuum and BT groups,
where it was decided to revisit the impedance in the LHC
injection region. These were designed purely by rule of
thumb ('15 degrees law'). C. Rathjen will help identifying
the most critical regions. AG will participate in this
task. The goal is to remove or damp 'cavity' modes.
Plots of the vacuum chamber layout in the LHC injection regions,
from C. Rathjen, are linked next to these RLC minutes
on the web site. They will be discussed in a
meeting scheduled on 30th May.
FR has discussed with Paolo Chiggiato who is collaborating
with Peter McIntyre for the installation of a proof-of-principle
getter coated stripe that can also be DC-biased to suppress
electron cloud in an LHC dipole.
FZ mentioned that he got no response yet from Laurent Tavian
on the question of the modified beam-screen cooling capacity
and its reduction as a function of luminosity. FR encouraged
him to take a telephonic contact.
FZ pointed to ongoing microwave-absorption experiments, the
latest one using a magnetron, by Fritz Caspers and Tom Kroyer.
These experiments have highlighted the dependence of the
absorption on the presence of a magnetic field and on the
radial electric field.
Posted on the web: Slides by EB, EM (2x) and FZ
Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/