Minutes of the ABP-LCE team meeting on 13.02.04
present: EB, WH, AK, FR, DS, EV, FZ
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(1) Minutes & Pending Actions
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Old ACTION: FZ will inform Gerhard Schneider.
Done. FZ also pointed out that the calculated
impedance will change if the magnetic
permeability is largely different from 1.
There was no immediate answer as to what the
actual value is.
(2) Geometric Collimator Impedance (DS)
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DS presented new Gdfidl calculations of the
LHC collimator impedance. He considered 3
different geometries for the collimator.
Longitudinal trapped modes exist inside the
tank. The strongest mode is at 120 MHz.
Assuming copper as material, the total
heating obtained by summing over
all modes and resonances is 300 W in the
worst case that all resonances interact
with the beam. 1 kW is acceptable.
Transverse trapped modes are located
on the jaw. The predominantly driven coupled-bunch
head-tail mode is m=4, due to the frequency
of the resonator. The growth rates or imaginary
tune shifts for m=4 are of order 5e-6.
For higher resistivity the tune shift is reduced.
FR asked how the m=4 values enter into the
stability diagram.
ACTIONS (mainly DS):
=> clarify geometry of trapped modes
=> compute longitudinal impedance
=> practical advice for collimator design
=> redo estimate for larger gap
=> understand stability diagram for m=4
=> what can be measured in the SPS?
=> perform full simulation for the SPS (AK)
(3) SPS losses during CNGS beam extraction (EV)
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The particle losses in the 2nd batch after
extracting the first batch for CNGS was estimated
by integration, for various apertures and
residual-kick amplitudes. The kick experienced
by the first bunch of the second batch is 21%
and in total 2e-4 of the second batch would
be lost. If a gap of additional 50 missing bunches
is introduced, the fraction of lost beam is only
1e-8.
Beam in the gap was recently identified as another,
perhaps larger problem. This is under discussion
with W. Hofle.
Plans for future work: documentation of damper simulation
application to LHC
rf voltage modulation for LHC
FR recommended a presentation by EV.
(4) Review of resistive wall formulae and measurements (FZ)
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FZ contacted F. Caspers. His measurement results are
summarized in AB-2003-051, written with A. Mostacci,
and U. Irizo. The measurements are consistent with
the formula by Burov&Lebedev and also, in the low
frequency regime, with an inductive bypass formula
that is different from L. Vos'.
Overall, applying the various existing expressions
to the warm LHC BPMs gives predictions that vary
by more than an order of magnitude.
=> The same formulae should be applied to the measurement
of Fritz and to the collimator computed by H. Tsutsui.
=> Some unclear expressions should be discussed with
F. Caspers and A. Mostacci.
(5) AOB
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LHC Design Report.
The revised LCE chapter for the LHC design report is
finished and available from FR.
The arc SR heat load may still need to be updated
(peak values to be replaced by averages). The figures
showing LHC cryogenic capacity for electron-cloud
heat loads were updated. The numbers have not yet
been confirmed by L. Tavian.
EM has updated res.-wall rise times at injection.
This may have implications for the feedback damping
times.
RA has reduced the number of collimators by 30%.
This change will not be part of the design report.
WH has worked on MKI kicker specifications for CNGS.
These are related to the required stability on the
target. The maximum acceptable ripple is of order
+/-1.5-2%.
WH is working on a report on tune spectra with
beam-beam. He will send a figure to JPK for
the tune-meter specification.
WH, EV and W. Hofle held a meeting on feedback
measurements with beam-beam. WH will do simulations
for beam-beam. EV's feedback simulations may be
integrated.
FR mentioned a question on the feasibility of
a window in the feedback system, that would allow
measuring the tune of a single undamped bunch.
He recommended discussions with BDI to make sure
that the tune can be measured at injection
into the LHC.
We should have a meeting on MD plans for 2004.
These may include collimator study, single-bunch
collective effects, e-cloud, BB compensation.
Enclosed: Slides by FZ.