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Minutes of the ABP-RLC team meeting of 13.01.2006
present: UD, AG, TP, FR, GR, DS, RT, FZ, Theo Demma
excused: WH, EM
web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/
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(1) Minutes of last meeting, pending actions (FR)
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The list of open actions was briefly reviewed.
Progress still need to be made on the wake field for
two beams, beyond the literature survey.
For the question of bunch time distance at the tertiary
and other two-beam collimators, AG has obtained a MAD-X
file from Stefano Redaelli, which includes all collimator
positions. Numbers for TCTV are already available.
For the other collimators they will be known soon.
(2) Transverse Impedance of TDI (AG)
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TDI features 4-m long jaws plus transitions.
The jaws are closed for injection and can afterwards
be opened. The jaws presently are not tapered.
The half gap in closed position was taken to be 8 mm.
The jaw consists of a Cu frame containing a BNi
ceramic, which on the side of the beam is coated
with Ti. There are 15-mm long slots on the
upstream and downstream ends of the jaw, and a
1-mm wide slot on either transverse side.
The transverse impedance was computed by GdfidL.
The BB impedance is of order 40 kOhm/m for a gap
of 16 mm. Many modes are found, the lowest at frequencies
of 10-20 MHz. The instability rise time corresponding
to the simulated modes is of order 1000 turns,
for a shunt impedance of 30 MOhm/m.
AG considered two options, namely either
closing the downstream and upstream slots, or
closing all slots. He recomputed the impedance
for these two cases. This helps at low frequency,
but the magnitude and number of higher-order modes,
above 200 MHz, are little affected.
The full gap is now known to be 8 mm instead of
the 16 mm assumed in the calculation above.
This will increase the BB impedance from 40 kOhm/m
to about 320 kOhm/m, which is 16% of the total LHC BB
impedance budget (2000 kOhm/m) . Also Landau
damping is likely to be lost for this impedance.
AG concluded that (1) tapering the ends of the
jaw is necessary, and (2) ferrites must be added
to damp the HOMs. This information should be
transmitted to the responsible person.
The data for the TDI were obtained from Luca Bruno.
(3) Preliminary results on LHC electron cloud maps (Theo Demma)
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Theo Demma, from the University of Sannio, visited
CERN for 1 week, as part of CARE HHH. He reported
about the application of electron-cloud maps to the LHC.
The parameterization of electron-cloud build-up simulations
by cubic mappings goes back to U. Iriso and S. Peggs,
who demonstrated its viability for RHIC.
A longstanding question by FR was whether the same
recipe works for the LHC.
TD described the underlying idea and showed an early
example from RHIC, before presenting his first successful
implementation for the LHC, answering FR's question.
The cubic map contains a linear term representing the
growth, a parabolic term, describing saturation due to
space charge, and a third-order term.
The map describes the evolution of the average
electron density between two successive bunch
positions.
More precisely, from the simulation the coefficients
of 4 maps are fitted, describing the evolution from
an empty or full to an empty or full bucket, respectively,
yielding a total of 12 coefficients. The same coefficients
can then be used to predict the build up for other
bunch patterns. The method works, as long as one does
not change the bunch or surface parameters.
An open question is whether the explanation of first-order
phase transitions in the pressure, as observed in RHIC,
requires the invocation of ions.
FZ pointed out that the equilibrium density should
be computed from the 1-turn map and not from the map
between two bunches, allowing for a much richer
spectrum of solutions. Fitting another type of
function might be a further method of generating
first-order transitions. This was exemplified in a
paper by Thieberger, Iriso and Peggs, where
a modification was introduced to represent ions.
FZ suggested that a similar functionality could
be possible for an electron cloud alone.
He pointed out that we have never tried to look
for other steady-state solutions in electron-cloud
simulations. A possibility would be to start with
a high density of electrons and observe whether
the same equilibrium is reached.
(4) Review of EPAC06 abstracts
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The team went through the proposed EPAC abstracts
not related to CLIC. The proposed list is posted
on the team web site.
(5) AOB
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FZ has given a preview of his Chamonix presentation
on 'beam measurements in the first two years' to a
mixed group from BDI, controls, rf and BT. He received
some critical questions, and feedback. As an outcome,
he will be sent a list of functionality options for
each instrument by Tuesday afternoon. FZ proposed
to present the outline of his talk to the RLC team
prior to Chamonix, for comments and corrections.
FZ mentioned an email by Tanaji Sen, who announced
that Elliott McCrory will visit CERN in the coming
week for discussions on the application of the FNAL
CHEF code to LHC beta beating and optics checks.
Tanaji also asks for topics of possible involvement
of FNAL in 2006 SPS machine studies. He has
forwarded Tanaji's email to OB and MG.
RT has created an optics web site for the LHC
upgrade. Optics files from Riccardo de Maria
and Tanaji Sen are already posted. Another optics
is expected jointly from Oliver and Riccardo,
and a last one from Jean-Pierre Koutchouk and Ezio
Todesco.
Posted on the web: Slides by AG and TD
Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/