-------------------------------------------------------
Minutes of the ABP-RLC team meeting of 29.04.2005
present: EB, UD, AG, WH, EM, TP, FR, FZ
web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/
-------------------------------------------------------
(1) Minutes of the last meeting and pending actions
----------------------------------------------------
=> ACTION => compare the size of the longitudinal geometric wake with
RW longitudinal wake from A. Koschik (FR)
STATUS: FR will talk to A. Koschik
=> ACTION => attempt to derive a general nonlinear theory including
the inductive bypass (EM)
STATUS: PLANNED (?)
(2) Reports from other meetings & discussions
---------------------------------------------
EM reported from the Collimation Working Group meeting. The collimation
team has asked for the sensitivity of the tune-shift predictions to
the centering of the beam between the collimator jaws.
=> ACTION => check sensitivity to centering with Piwinski theory (FZ)
Further data analysis by Stefano Redaelli is needed for the
actual centering in the experiment. But, roughly, a positioning accuracy
of 50 micron is expected for each jaw, allowing for a shift in the
beam centering by up to 100 microns.
The DC resistivity of the collimator graphite was measured to be
10 microOhm-metre in all directions.
Bench measurements on a collimator prototype will start in 2 weeks.
EM mentioned a discussion with Fritz Caspers on the low-frequency
impedance, and a confusion between thick-wall and thin-wall formulae.
He points out that there are two different effects: inductive bypass
and current redistribution. EM plans to still examine the possibility
of an inductive bypass effect for the longitudinal impedance starting
from B. Zotter's general approach. Luc Vos' paper had suggested that
there is no inductive bypass longitudinally.
FR related a new explanation of the SPS microwave experiments
in terms of a 'magnetron effect' by Fritz Caspers. FZ added that
a lab experiment of the magnetron effect is foreseen for next
week.
(3) Pressure rise induced by the electron cloud in the LHC (FZ)
---------------------------------------------------------------
The question arose whether e-cloud scrubbing would be needed in the LHC.
Effects possibly requiring scrubbing are the vacuum pressure increase,
the heat load, fast single-bunch instabilities, and long-term
emittance growth. FZ investigated the vacuum pressure evolution
by discussing with Vincent Baglin. The vacuum model goes back
to W. Turner. In the presence of electron cloud the vacuum pressure
is directly proportional to the electron flux and to the desorption
yield. Both factors decrease with time. The surface coverage
of the beam screen is predicted by Noel Hilleret to be 10% of
a monolayer in the worst case. The gas density can exceed the 100-hr
lifetime limit for a short time, for nominal electron flux,
but will drop below the 100-hr limit after 17 hours of nominal
operation. Synchrotron radiation alone would lead to a rapid
improvement of the vacuum to a level corresponding to 10000 hr
beam lifetime. A combined calculation of SR and e-cloud induced
pressure rise is not available. A delta_max of 1.5 required to
store the nominal intensity, could be reached after 4-12 hours.
A few weeks of TOTEM beam operation could contribute to the
scrubbing of the beam screen by synchrotron radiation and
photo-electrons.
FR remarked that the initial gas densities employed in the
model calculation could be far too optimistic. He also suggested
that the emittance growth may set a tighter limit than the
beam lifetime. FZ pointed to a recent report with B. Jeanneret
for HHH-2004 where the emittance growth was estimated.
(4) Electron cloud in the quadrupoles (FZ)
------------------------------------------
FZ presented a comparison of simulated e- trajectories in a
quadrupole field using 5 different integration routines.
All methods exhibits a slow drift of e- towards the two diagonals.
The simulated heat load indeed is concentrated in a narrow
region at the diagonal. Two reasons for the drift could be
identified: one is the combined effect of elastic reflection
plus the geometry of field lines and chamber boundary,
the other is an artificial effect arising from the algorithm
used to determine the secondary emission point on the surface
in the case of particle loss. The artificial effect might be
removed by some modification of the e-cloud code.
FZ mentioned that the space charge forces are not calculated
correctly for a narrow e- distribution on the diagonal.
AG and FR suggested that an analytic estimate should be
possible, assuming a uniform electron distribution along
the diagonals.
(5) Electron-cloud induced emittance growth below the threshold
of the TMCI (EB)
---------------------------------------------------------------
In collaboration with G. Franchetti (GSI), EB explores a model
of emittance growth based on a trapping/detrapping mechanism
similar to that successfully used to reproduce the results of
space-charge experiments (see G. Franchetti's talk at HHH-2004
workshop). Giuliano considers a simplified model of the
e-cloud distribution, where the cloud density increases
linear in z and has a transverse Gaussian distribution with an
rms size equal the beam size divided by sqrt(2). He finds
an emittance growth which occurs mainly in the tails (halo
formation). HEADTAIL uses a more realistic e- distribution
which is strongly pinched during a bunch passage, giving
rise to a narrow spike at the center and rings moving
radially outwards. The HEADTAIL simulation primarily shows
a beam-core blows up, which in addition is faster than the
growth observed by Giuliano's code. K. Ohmi at KEK followed
an approach whose basic model is similar to Giuliano's,
and where the emittance growth is attributed to Arnold
diffusion.
Several team members remarked that differences in the
electron distribution could explain the difference in the
simulated emittance growth between HEADTAIL and Giuliano's code.
FZ mentioned that another difference is the use of an analytical
formula for the e- force by Giuliano and the interpolation
on a grid in case of HEADTAIL.
=> ACTION => Implement analytical formula of Giuliano in
HEADTAIL to see whether his result can be reproduced (EB).
=> ACTION => Repeat simplified simulation for a much narrower
e-cloud distribution (GF)
FR suggested to look for the position of resonance islands
in phase space. He also remarked that the model of the
e-cloud field is not symplectic, since the transverse kick
is taken to change with longitudinal position, but there
is no corresponding longitudinal kick.
(6) AOB
-------
Following a request by Volker Mertens, there will be a meeting
with the LHC Injection WG to discuss the overhall vacuum chamber
layout around the injection zones.
Posted on the web: Slides by EB and FZ (2x) and movies by EB (2x)
Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/