Summary notes of the LCU meeting on 17/06/2008
Present: MA, HB, FC, SF, MG, WH, JJ, DK, PL, EM, AM, JN, TP, VP, SR, FR, FS, TW, SW, SY, FZ
Report from meetings
Recent results for LHC impedance activities -> EM (pdf)
EM gave a detailed review of the activities and issues dealing with the LHC impedance.
The analysis of the impact of the collimation system on beam stability is
reviewed. In particular, since last analysis two main findings occurred:
firstly, the performance of the transverse feedback seems to be sufficient to
dump the multi-bunch instabilities due to the machine impedance in collision.
Secondly, the analysis of the emittance blow-up induced by the noise in the
transverse feedback showed that this should not be an issue. This implies that
even though the ocutpoles are not enough to stabilise the nominal LHC beam, the
transverse dumper should be effective. In any case, it is commonly agreed that a
detailed study should be carried out. For this, it is planned to have a doctoral
student or a fellow working on this topic as from the second half of 2008.
It is also stressed that the transverse impedance (both real and imaginary
parts) of the LHC can be decreased by increasing the collimators' gap. The real
part of the transverse impedance is increased by reducing the resistivity of the
secondary collimators. This means that at top energy:
The use of Landau damping imposes to reduce the imaginary part of the collimator impedance (as this dominates the contribution from the rest of the machine).
The use of the transverse feedback to stabilise the beam would be improved by reducing the real part of the collimator impedance (in particular until ~ 20 MHz).
The work on the impedance of a collimator (inductive by-pass
effect) and on the evaluation of the impedance of the PIMMS will be presented in
two EPAC08 papers. The first, will report about the nice agreement between
theory, simulations, and measurements. In it recalled that the inductive by-pass
effect is crucial. The second paper considers the impedance of the PIMMS,
including also the assessment of the impact of non-nominal fingers (i.e.,
fingers with a bad contact).
In the framework of the Phase II collimation project, strong efforts are devoted
to the reduction of the collimators impedance by selecting special materials on
the basis of their conductivity etc.
In the framework of the activities related to the assessment of
the impedance of the components installed in the LHC ring in order to keep under
control the overall impedance budget, there was recently the analysis of the CMS
vacuum chamber. In spite this topic was considered already some time ago (O.
Brüning in 1995), recently it was found out that the chamber built is not
confirming with the feedback given by impedance experts in the past. A
re-evaluation of the situation showed that indeed there should be no problems.
Detailed numerical simulations to quantify the coherent loss from HOM will be
performed by R. Wanzerberg (DESY). Parenthetically, he will visit us in August
to help with the simulations of some LHC components in order to store the
contribution in the ZBASE database.
Recently a request of evaluating the impedance for the proposed triplet layout
for the LHC IR upgrade was issued. The impedance of the increased size beam
screen should be estimated, including also the analysis of the impact of a
higher temperature for the beam screen (40 K-60 K). While the resistivity of
cupper changes considerably with temperature in the range 5 K - 60 K, stainless
steel has a much reduced variation. A more detailed estimate will be prepared
and communicated to the LHC IR Upgrade Project leader.
Measurements of heavy ion beam losses from collimation -> RB (pdf)
This presentation will be given at the forthcoming EPAC08 Conference.
Collimation efficiency during
Pb82+operation of LHC predicted to be lower than ultimately required because of
fragmentation processes: protons are lost due to large angles imparted by the
interaction with the collimators. Ions are lost due to the fragmentation process
in the collimator and hence the dispersive motion.
Experiments in the SPS with protons and ions are meant to benchmark the
simulations (performed with ICOSIM).
Uncertainties are linked (among other sources) to the computation of the cross
sections for fragmentation.
Collimation experiment confirms qualitatively different loss patterns for
ions (dispersive) and protons (angular).
Simulations with ICOSIM + FLUKA reproduce measurements within estimated uncertainties, not only in terms of loss positions but also in absolute BLM signals.
Future work will focus on mitigation measures for LHC ion collimation such as: magnetized collimator, cold collimators, re-matching optics (increase dispersion), extra high-Z spoilers.
AOB
Last update: 2-July-08
MG & HB
Back to LCU Minutes