Minutes of the ABP-LCE team meeting on 30.01.04
present: EB, WH, AK, EM, FR, DS, EV, FZ
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(1) Minutes & Pending Actions
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Zbase:
O. Bruening's Zbase tutorial was a great success.
Young Zbase users are encouraged to intensify their
usage of the code and to report incurred error
messages to TdA.
ABP-RF-BDI meeting on beam-beam diagnostics with feedback:
WH is organizing this meeting. A first preparative meeting
with W. Hofle and EV is planned. Afterwards BDI will be
involved.
There is a question from Gerhard Schneider as to whether
a 100-micron coating should be applied to the warm BPMs
in LHC. The preliminary assessment by FR is ambiguous,
as the positive effect is small and resonant modes may
be less damped. FZ will try to answer this question with
the help of the team. FR suggests that the LCE team
should take the initiative and initiate this kind of
studies. Answers to requests should be quick.
He mentioned that according to FC the impedance of a strip
line etc. depends on the outside circuit.
(2) Rigid-Bunch Collimation Code (EV)
-------------------------------------
EV studies a faster scheme of tracking with collimators,
where multi-particle tracking is replaced by a tracking of
3 numbers (rms size, cut-off and intensity). The beam
size damps exponentially with a pre-calculated damping
time. This simulation scheme could be used for the
CNGS feedback simulations. It was remarked that
centroid oscillations might complicate the picture
(no scraping after the first turn unless the coherent
tune shift is significant). Decoherence effects and
feedback response times would also be important.
FR states that the goal should be to have a valid
physical model.
(3) Stability for Beam Collimation at 6 Sigma (EM)
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Berg-Ruggiero formula underestimates the stability,
because it has a cutoff at 3.2 sigma. LV's Gaussian
model overestimates the stability. A more reasonable
model is a 15th order distribution function, devised
together with Andre Verdier, which is consistent for
6 sigma collimation. The stability calculation provides
the expected result. With the good setting of the
octupoles, the stability boundary is between the other
two curves. However, for the bad setting the
distribution is more stable than the Gaussian.
In the imaginary tune-shift direction the stability
is about the same as before, but in the real tune-shift
direction a factor 2 is gained with respect to the
Berg-Ruggiero distribution. This is important for
TOTEM.
The 2nd order distribution function is kept as
the pessimistic baseline. Real tails could be larger
or smaller than estimated by any of these distributions.
(4) Collective Limitations for TOTEM Beam (EM)
----------------------------------------------
The angle of observation should extend down to
less or equal 10.1 microrad. FR asked if the
crossing angle can be controlled with this precision.
EM showed that there is a Landau-damping limit on the
quantity (Nb/eps_N G^3), where Nb is the bunch population,
eps the normalized emittance and G the gap size in
units of sigma. The minimum angle that can be provided
at pilot-bunch intensity is 5.25 microrad.
(5) Electron Cloud in LTC (DS)
------------------------------
DS will present e-cloud simulation results next
Wednesday. There is one open question: discrepancy
between original and elliptical boundary (solved
after the meeting).
DA will review the status of benchmarking for the SPS.
WAMPAC1 is in good agreement. WAMPAC3 is not in good
agreement, but results are invalid according to the
vacuum group. Stripe position and response to magnetic
fields are consistent. The e-flux on the wall is off
by an order of magnitude, but the heat load and mean
energy seem to fit. So there is a puzzle to be solved.
The electron energy spectrum may disagree in some
cases (Private communication by JMJ).
The team goal is to reduce the discrepancy.
FR will ask Laurent Tavian for an updated official
cooling capacity (LT should confirm plot provided
by FZ; ACTION FZ: send plot to FR; ACTION FR: contact
LT).
We could attempt to predict scrubbing time.
(6) Emittance Growth from Electron Cloud (EB)
---------------------------------------------
Benchmarking of HEADTAIL and QUICKPIC is ongoing.
Various simulations were run for LHC at injection,
with different boundary conditions, two treatments
of the rf voltage, with and without dipole field,
and varying number of interaction points up to the
continuous limit in QICKPIC. There are discrepancies
by a factor of 2 for nominal inentical input between
the two codes. The variation of any of the
other parameters gives comparable or larger
changes. The reason for the discrepancy should be
understood. The continuous QUICKPIC simulation
shows no emittance growth inside a magnetic field.
The 1-kick approximation is not adequate, but
after 5-8 kicks the code converges.
FR recommended to include simulations for 5-8 kicks
in the comparison. Also one should compare
simulations at lower density (below the threshold
of fast instability).
FR and DS will meet on Monday to discuss LTC presentation.
(7) Chamonix Follow Up (FR)
----------------------------
MKE impedance modelling is an important issue that
we need to pursue. FC will measure the impedance of an
LHC kicker prototype as soon as this becomes available.
Steve Myers asked for a revision of the overall
LHC impedance model.
(8) Gdfidl (DS)
----------------
The resistive problem cannot be solved in the time
domain. It is better to compute the eigenvalues and
afterwards add a small resistive component.
FZ mentioned that Gdfidl could also be used for
computing the impedance of the BBLR.
DS explained that Gsfidl can compute any type
of impedance (e.g., instability and detuning
terms), depending on the processing by the user.
Enclosed: Slides by EM.