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Minutes of the ABP-LCE team meeting of 16.07.04
present: WH, EM, TP, FR, ES, DS, FZ
excused: EB
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(1) Reports from EPAC04 (FR and FZ)
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FR reported that the DG presented the CERN strategy
and CERN priorities in the opening talk. He sent
a message to the linear-collider community by suggesting
that one laboratory should take the lead in order to
be successful. He also mentioned that P. Lebrun gave a
talk on LHC construction and compared the challenges
with those in space industry.
FZ summarized the projects presently drawing most attention,
several highlight presentations, and some new ideas.
See his attached slides for details.
(2) Contact Resistance of Collimator Jaws (EM)
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EM reported that the contact resistance of the rf fingers
was measured to be 200 mOhm per contact. There are 15
fingers in parallel, which would imply 30 mOhm.
It needs to be checked how this relates the usual
beam impedance and budget
==> ACTION EM & FR?
An exchange of emails with Sergio Calatroni, explaining our
target contact resistance for the LHC collimator jaws (~1 mOhm)
and the situation for the SPS prototype, is enclosed.
(3) Electron-cloud simulations for RHIC, LHC, SPS (FZ)
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FZ showed new simulation results for the cold arcs
of RHIC where recently pressure rise due to electron
cloud was observed, but no heat load (resolution
limit 5 W over 100 m). The simulation shows that
a delta_max of 1.9 or larger is required to generate
multipacting for the RHIC parameters. The heat load
simulated over the first turns is extremely small.
==> ACTION FZ, further simulations for larger delta_max
and a larger number of turns
FZ summarized a number of surprising statements from
the open session of the LHC MAC. There was confusion
as to whether the COLDEX experiment was shielded by
cold baffles (FZ thinks not, FR and DS think yes).
A simulation for the e-spectrum detector with different
values of elastic reflectivity confirms that the
electron cloud decays in a gap for all values of
low-energy reflectivity between 0 and 1, only the slope
of the decay and final level differs.
Simulating the LHC at top energy, FZ could confirm
recent results by DS for a dipole field, but some
difference remains for a drift.
==> Compare simulation for quadrupole (FZ and DS)
There is a question which pressure rise beyond the nominal
value would still be acceptable in the LHC. The vacuum
group reported a margin by 2 orders of magnitude,
but FZ (and partly FR) were left with the impression
that basically no margin exists in the cryogenic system
(based on a presentation by L. Tavian).
The question of pressure rise and acceptable vacuum levels
for ions was studied by John Jowett. A related email exchange
between JJ and FR is attached. The web site of John's EPAC
paper is
http://cern.ch/jowett/epac2004/
though FR has been unable to print this paper.
(4) SPS Kicker Impedance (EM)
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EM described ongoing work with AB from FNAL. Two papers
are in preparation - one describes a correction to the
previous theory where a term for the ferrite is added,
and the other on the effect of kicker shielding by a thin
0.35-micron Ti coating, which reduces the peak kicker field
by 1% (EM and AB will also calculate the effect on the kicker
rise time in response to a question by FR after the meeting).
(5) Longitudinal loss factor (FR)
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ES has asked for an update of the longitudinal loss
factor in the LHC including the effect of the electron
cloud. A few degree synchronous phase shift may matter
for particle losses in the longitudinal plane.
The effect of the electron cloud was estimated during
the meeting by FZ and DS. Typical longitudinal wake
fields simulated in 2002 by FZ and GR were of order
of a few V/m. Multiplying with 1-A beam current gives
a few Watt per meter, which is consistent with the
simulated heat load as it should be. The additional
voltage due to the energy loss is then of order 100 kV,
small compared with the total rf voltage (8-16 MV).
The energy loss in the kickers and due to higher-order
modes might also be relevant as pointed out by
Francesco
==> ACTION?
(6) Simulation of Resistive-Wall Instability with Simulink (EV)
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EV described a simplified faster way of computing the transverse
wake field of a batch over successive turns. For the first few turns,
he computes the wake for individual bunches. Later he groups
128 consecutive bunches and takes the average wake-field amplitude.
For still larger distances the groups are chosen larger.
He demonstrated on line a simulation of instability and
damping by feedback for different tunes.
How the method and results compare with those of AK
has not been studied.
(7) AOB
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Tatiana Pieloni has joined the LCE team from today. She will
work with WH on coherent beam-beam effects in all senses.
EM is working with various colleagues (HB, GA, ES, EB) on modeling
the TMCI instability in the SPS for the Benzheim workshop.
FZ suggested that the calculations by K. Cornelis, A. Burov and
V. Danilov should be verified (no - horizontal? - TMCI instability
in a flat chamber). FR recommended to also continue follow up
on LHC impedance.
DS reported preparation for LINAC04.
FZ and DS mentioned ongoing electron-cloud simulations for
the rf structures of linear colliders. The correct modeling
of the electric field appears challenging.
Attached: slides from FZ
email exchange of JJ and FR
email exchange of FR and Sergio Calatroni