Minutes of the ABP-LCE team meeting on 05.09.03

present: GA, WH, AK, EM, FR, DS, EV, LV, FZ
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(1) Collective Effects Chapter in LHC Design Report (FR)
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FR reported that the collective effects chapter is now
in good shape, following a week of intense discussions 
with LV and EM on stability diagrams and impedances.
A prime result is a plot of the complex tune shifts
expected from the collimators superimposed on the 
stability diagram. The nominal LHC beam is unstable,
if the graphite collimators are not Cu coated. A beam 
at half the nominal intensity would be stable. 
With copper coating also the nominal beam is predicted 
to be stable.

The broadband (BB) impedance is only 1.32 MOhm/m, small
compared with the space-charge impedance of -6.3 Mohm/m.
Since the BB impedance is so low, there is little 
natural compensation of the latter, e.g., by inductive
vacuum-chamber components. This puts us in the situation
of operating in a regime for which no generally accepted
theory is available. FR suggested a later mini-workshop
devoted to tune shifts and Landau damping from space
charge. It was also discussed to pursue measurements
at the SPS and/or PS, in order to validate or falsify existing 
theories. FR speculated that the reason for the missing 
consensus in the community is a lack of experimental data.

With one exception, the impedance part of the design
report contains only numbers, but no formulae.

FZ mentions that the space-charge effect including
conducting boundaries could be simulated with the
latest version of HEADTAIL. (contact GR? possibly
in collaboration with EB) 

There will be two or three weeks of iterations before
the design report is finalized. The team members
should implement changes to the tex file as recommended
or appropriate, in response to eventual feedback from
other groups.


(2) suggested contributions from US LARP (FZ, FR, et al.)
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FZ presented a list of possible electron-cloud studies
proposed to the US LHC collaboration, in particular
LBNL, and possibly SLAC. The suggestions mainly 
pertain to modelling recent SPS observations, to 
benchmarking POSINST and ECLOUD codes against 
each other, to electron/ion-microwave interaction
(F. Caspers' experiments), to the measurement
of surface parameters (secondary yield, diffuse 
photon reflection), and to instability studies.
FR added electron-cloud and beam-beam studies for
superbunches (for BNL and/or FNAL). Electron cloud
experiments could also be done for conventional beams
at the Tevatrom where it is seen without coalescing
(18.9 ns bunch spacing) and at RHIC (with 108 ns
and 216 ns spacing).
Other beam-beam experiments are of interest, and
there exists a list of proposals by WH.
Beam-beam simulations for the Tevatron are done
by A. Kabel (SLAC) and Qin (LBNL). 
The two could possibly run similar strong-strong 
simulations for the LHC.


(3) Feedback Simulation (EV)
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The previous plan was to use the information of two
successive turns, but since this generated ghost lines.
EV reconsidered the scheme and now prefers to use two
BPMs at orthogonal phases. He is constructing a 
simulation using MATLAB and SIMULINK. A discrete
description would save computing time. LV recommended
to search for papers by Zhabitski on feedback theory
using Z transforms, published about 15 years ago.
The main objective of EV's study is to compute 
coupled-bunch instabilities for damper optimization.
The basic idea is to implement special filters and damp
individual modes rather than single bunches.
FR encouraged EV to develop the damper into a tool
for actual beam diagnostics.

(4) AOB
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Due to the holiday on Thursday and the absence of the
team leader, there will be no meeting next week.