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Minutes of the ABP-RLC team meeting of 20.01.2006
present: FC, UD, WH, TP, FR, GR, RT, FZ
web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/
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(1) Minutes of last meeting, pending actions
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There was no comment on the minutes of the last meeting.
GR completed the action on the appropriateness of
perfectly conducting boundary conditions in HEADTAIL.
The electron oscillations inside a bunch represent
a typical frequency. The corresponding skin depth is
about 20 micron for stainless steel, much less than
the pipe-wall thickness. Therefore, the boundary conditions
seem well justified. Much lower frequencies could
penetrate through the wall, but they are not relevant
for the time scale of the HEADTAIL simulations.
FZ pointed out that the effect could be a little
larger for ECLOUD build-up simulations, and that
perhaps the resistive-wall wake field could
affect the long-time survival of electrons in a
large gap.
FR recalled an observation of electron cloud
behind a mask at LEP.
FC suggested to make use of the experience with
rf traps.
(2) Impedance of PS kickers and PS impedance budget (EM)
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The longitudinal impedance of two types of new PS kickers
was measured vs. transverse offset with single and two wires
in the laboratory by FC and TK.
Using the Panofsky-Wenzel theorem the transverse impedance
is extracted from the measurement. The 1 wire method is
expected to provide the combination of driving and detuning
impedance, the 2-wire method the pure driving impedance.
EM compared the two results and observed a good
agreement for the real part of the driving impedance
from 400 MHZ to 1 GHz. At lower frequencies, where the
experimental error of the 2-wire measurements is expected
to grow, the curves differ significantly. For the imaginary
impedance, the results are different by an order of magnitude
or more, over the entire frequency range (the 2-wire method
giving a much higher value).
Based on these results, the transverse impedance of the new
kickers corresponds to about 10% of the total PS impedance
estimated from beam measurements.
EM has made a more detailed presentation of these
meaurements and their consequences at the APC on 08.12.05.
(3) PS fast instability at transition with Gaussian and flat bunches (GR)
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GR presented transverse instability simulations for the PS
for Gaussian and flat bunches. The instability for the Gaussian
develops most strongly at the peak and in the second half of the
bunch, in good agreement with experimental observations. The
frequency of the instability in the bunch head is much lower
than the frequency in the back (250 MHZ compared with 750 MHz).
The synchrotron motion is almost frozen for the conditions considered,
close to transition. The simulation for the flat bunch serves as
a benchmark and demonstrates that the instability depends
mainly on the local beam density.
(4) SPS fast instability at injection with round and flat chambers (GR)
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The SPS impedance is about 20 MOHm/m, much larger than 3 MOhm/m for
the PS. GR compared TMCI threshold simulations with a flat and a round
vacuum chamber. The frequency of the instability is affected by the
vacuum chamber.
(5) H/V TMCI intensity thresholds for flat vs round chambers (EM)
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With the flat chamber the instability threshold is increased
slightly in the vertical direction (from 2.8 to 3.5e10) and by more
than a factor of two in the horizontal one (to 8x10), with a low
longitudinal emittance. The simulated increase can be fully understood
by the Yokoya factors for a flat chamber. The instability is not
entirely suppressed by the flat chamber in the horizontal plane.
FZ pointed out that the last aspect may be inconsistent with
analytical calculations by Danilov, Karel Cornelis and others.
Since the threshold is much higher in the horizontal plane than
in the vertical, coupling can be employed to increase the vertical
threshold. GR and EM still study this possibility for the SPS.
(6) New interpretation of SPS collimator impedance tests (FZ)
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FZ combined the time dependence of the resistive-wall wake field
from Burov and Lebedev with the nonlinear dependence on transverse
position from Piwinski. The resulting formula predicts tune shifts
which are in nearly perfect agreement with those measured at the SPS
in 2004 over the full range of gap sizes.
(7) Collective effect aspects of "Chamonix" presentations (WH+FZ)
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WH's presentation discusses the effect of the experimental magnets
of the 4 experiments. Coupling from solenoid is noticeable only
in the case of CMS, where at injection the induced coupling
is of order kappa~0.005. A local correction of coupling by the
skew quadrupoles at the triplets may be a possibility that
will be further studied. The effect of the dipole fields in LHCB
and ALICE is significant. WH's talk does not include any
collective effects.
FZ showed a quick preview of his Chamonix presentation on
beam measurements in the first two years of LHC commissioning.
A proposal from BDI is to cut certain functionalities (like
bunch-to-bunch position and luminosity measurements, or timing
synchronization for wire scanners) as well as a few
instruments (like matching monitors) for the first year.
collective effects include IBS, which requires longitudinal
blow up, already in year 1, setting up the beam-beam
collisions, for example by a beam-beam coupling monitor,
and impedance measurements.
FR mentioned that the measurement and compensation of the
Laslett tune shift could be necessary.
(8) AOB
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The presentation by EM on single and coupled-bunch instabilities
at LHC top energy vs Q' was postponed to a later meeting due to
lack of time.
Posted on the web: Slides by EM, GR, and FZ
Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/