Special LHC Forum (6/03/1997)
MOTIVATION
SCHEDULE
WH stressed the importance of Pacman bunches and beam-beam induced orbit effects:
with 4 LHC experiments and under unfavourable conditions, the beams can be offset by
1 sigma at the IP. FR remarked that this calls for a self-consistent orbit
calculation and LV pointed out that nontrivial bunch-to-bunch local feedbacks
would be required to fix this problem. Possible interference between separation
scheme and coupling compensation should also be investigated. Finally, JPK remarked
that beam-beam related requirements for the LHC optics should be worked out.
The alternative beam-beam kick presented by JJ and implemented in WIGWAM should be
benchmarked against the usual Bassetti-Erskine formula with the Error function.
Existing beam-beam programs developed for LEP bunch trains can be modified and
applied to LHC, provided "collision schedules" are defined: EK prefers them in the
form of 3000x3000 matrices.
According to MM, observations in the SPS suggest that the beam-beam interaction
tends to equalize all bunch emittances.
JM said that injection oscillations should not be relevant for dynamic aperture with
separated beams, provided they are damped in less than a few hundred turns. However,
dynamic effects with a longer time scale could be relevant. WH suggested that tunes
should be corrected before performing tracking with beam-beam.
According to FR, the possible loss of Landau damping and the effect of feedback in
presence of beam-beam interaction should be tested using a simulation program.
A strong argument in favour of an increased beam-beam separation is the possibility
of measuring and controlling tunes during the ramp: RS suggests 12 sigma, i.e.
twice the present value. In presence of dynamic effects, this might become 10 sigma
(the target dynamic aperture). Even a more modest separation of about 10 sigma would
call for an increased aperture of the triplet.
F. Ruggiero
In parallel to analytic estimates based on perturbation techniques,
we need detailed simulation programs with a full description of the LHC optics.
This calls for systematic developments (e.g. of MAD) allowing
more flexibility to test new physics.
Telegraphic summary of the discussion
Tentative programme for Future LHC Beam-Beam Studies
luminosity reduction
emittance growth and tails
F. Ruggiero
Fri Mar 14 18:51:23 1997