Minutes of the ABP-LCE team meeting on 07.11.03
present: EB, WH, AK, EM, FR, DS, EV, LV
excused: FZ
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(1) Minutes of Last LCE Meeting and Pending Actions
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Concerning the minutes of the last meeting, EV remarks that the special
chips have been already removed to operate the FPGA module at 80 MHz.
Further comments by W. Hofle can be found in the attached email.
(1a) Follow-up of LHC collimator impedance
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ACTION -> EM will clarify ambiguity in collimator orientation.
DONE: Excel tables for collimators layout provided by RA give angle
between the x-axis and the direction perpendicular to the jaws (see
enclosed slides by EM).
ACTION -> EM will check possible longitudinal 'Yokoya factor' relating
round and rectangular cases.
ACTION -> Check discrepancy by a factor ~3 on collimator impedance
(EM+LV) and put numbers into ZBASE (EM).
Partially DONE: discrepancy traced back to the use of different
collimator layouts. LV's estimates are in excellent agrement with EM's
automatic collimator impedance computations for "Phase 1" baseline
collimator layout (see enclosed slides by EM). Results still need to be
put into ZBASE.
(1b) Impedance of Roman Pots for SPS test and for the LHC
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ACTION -> DS will cross-check these estimates by numerical simulations
and follow-up the impedance of Roman Pots with Roger Perret.
Partially DONE: DS has contacted Roger Perret and will cross-check
impedance estimates of Roman Pots. He is also running GDfidL to compute
geometric collimator impedance.
ACTION -> AK will repeate simulations with a constant injection offset.
ACTION -> JJ will explain how to produce eps plots with Mathematica.
ACTION -> FR will read and approve the draft note by AK.
DONE
(2) Specs for Q and Q' meter: implications from Impedance and Beam-Beam
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J.-P. Koutchouk needs input from the LCE team to specify Q and Q'
measurement accuracy/functionality at the LHC.
In particular, limits on Q' at injection for various beam currents,
originally estimated by S. Berg in LHC-PR100, need revision in view of
changes of the impedance budget. The tolerance on Q' sharply depends on
the beam current, the machine impedance and the assumptions on Landau
damping. With the 1997 assumptions based on LHC-PR100, the tolerances
were estimated as follows at injection:
- Single bunch, up to 10^10 p: no constraint (Q'>-150).
- Nominal beam structure, 10% of nominal current: Q'>-15.
- Nominal beam: Q'>0 and in practice DQ'=±1 for Q'=2.
JPK has also a question concerning transverse spectra: should we expect
clean transverse spectra or can the spectra be significantly modified by
coupled-bunch modes or the beam-beam interaction, even when the beams
are separeted? He argues that if the spectrum could become like a
forest, for example following the large impedance increase, we would
have to invest in a 40 MHz bunch-by-bunch kicker that is not in the
baseline.
EV asks whether the feedback system has to be switched off during tune
measurements and remarks that even fast kickers will never have a
performance comparable to that of the feedback system. The Q-meter in
HERA WAS the feedback system itself.
WH has written the following draft text for further discussion:
We assume that the question concerns the excitation of single or
multiple bunches and that the Q measurement is done on a single bunch
only.
Indeed we must expect contribution from other frequencies in the single
bunch spectra due to the beam-beam interaction. However, simulations
show that they are heavily suppressed but strongly depend on initial
conditions. Furthermore, they should be independent of whether a single
bunch or multiple bunches are excited since on the time scale of a tune
measurement, the excitation cannot be coupled to the other bunches fast
enough. This is true for coupling through beam-beam interactions as well
as coupled-bunch modes. In case the simultaneous excitation of multiple
bunches becomes a problem, we suggest to evaluate the use of feedback
kickers to excite bunch oscillations for the tune measurement.
(3) Electron cloud effects with 75 ns bunch spacing
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Recent SPS observations with 75 ns bunch spacing and more than 1 bunch
train indicate electron cloud activity especially during the ramp. We
need to revise simulation results, also to confirm the dependence on
bunch length in the SPS and in the LHC, using debugged (and stable?)
code.
ACTION -> DS will launch new ECLOUD simulations.
EB simulations of emittance growth with lower electron density levels
and increased number of kicks indicate better convergence.
(4) Impedance of NEG coating of all warm sections
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The impedance of multi-layered vacuum chambers can be computed using a
Transmission Lines formalism developed by LV (see paper to appear soon)
and implemented in the Mathcad program available on the web at
http://wwwslap.cern.ch/collective/luc/Transmission-Lines.mcd.
For a resistive coating on a good conductor, such as the NEG coating now
foreseen for all Cu thick chambers in the warm LHC sections, the
impedance is given by the surface inductance mu_o*thickness divided (2
pi b).
LV finds a surface inductance per unit lenght of 5 pH/m/micron
corresponding, for a chamber radius b=40 mm and a 2 micron NEG layer
over 4 km of warm sections, to Z/n~2-3 mOhm. For comparison, the pumping
slots in the LHC beam screen correspond to a linear inductance of 10
pH/m and for 23 km of beam screen to Z/n=17 mOhm.
(5) AOB
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There are serious afs disk space problems for
/afs/cern.ch/eng/lhc/collective and /afs/cern.ch/eng/lhc/zbase.
Some 13 GB have been freed by FZ and the problem for "zbase" is solved.
ACTION -> FR will ask for additional disk space for "collective".
FR and the whole LCE team would like to warmly thank LV for his
important contributions to the study of collective effects in the LHC
and wish him a happy retirement.
Attachments: email by W. Hofle, slides shown by EM on Automatic
Collimator Impedance Computations and Landau Damping by Octupoles and
Space Charge.