E. Keil Eberhard.Keil@cern.ch
CERN3mm
The International Workshop on High Brightness Beams for Large Hadron
Colliders was attended by 53 participants from 10 institutes. Its goal
was to review and discuss the phenomena in the injector chain which
affect the brightness of the hadron beams for large hadron colliders,
in particular the LHC. The injectors for large hadron colliders were
presented in plenary talks by J. Maidment DESY on the HERA proton
injectors, S. Holmes FNAL on the Main Injector, W.T. Weng BNL on AGS
and RHIC, and Y. Mori Tokyo University on the Japanese High Intensity
Proton Synchrotron; J. Gareyte CERN presented the LHC requirements on
its injectors. K. Schindl, R. Cappi, and T. Linnecar, all from CERN,
discussed the present plans for the PS Booster, the PS and the SPS,
respectively. B Autin talked about PS-XXI, a new PS for LHC injection,
R. Garoby about a 2 GeV linear accelerator for LHC injection,
M. Martini about emittance measurements, and H. Schmickler about SPS
instrumentation in view of the LHC. Three working groups had been
asked to study specific aspects.
The working groups heard several contributed talks and had discussions on specific subjects. Their finding may be summarised as follows:
5mm
The working group on Intensity Limitations was chaired
by W. Chou FNAL. It studied the PSB, PS and SPS, but included comparisons
of bunch populations, emittances, phase space densities of other
machines. The proposed operation of the PSB at harmonic numbers 1 and
2 is unknown territory for high-intensity non-LHC beams. Injecting
H
ions appears not to be useful now, but may be useful
if the linac energy has to be increased.
There are no significant transverse intensity limitations in
the PS. However, the tight requirements on the bunch area are a
challenge. Another PS problem is the rise-time of the ejection
kickers, causing the loss of about three bunches in a pulse. Various
solutions were discussed. The SPS is close to the thresholds of the
microwave and longitudinal coupled-bunch instabilities. Avoiding them
requires longitudinal gymnastics and a controlled increase of the
bunch area. Eliminating components with large contributions to the
wake-field might help. The question was raised whether the performance
of the SPS might be improved by a lattice with variable transition
energy.
5mm
The working group on Emittance Preservation was chaired
by F. Willeke DESY. It noticed that the emittance budget is tight, and
that there is no room for blowup. It
compared achieved emittances in existing machines, and listed sources
of emittance blowup, e.g. accidentally rotated quadrupoles, vertical
kickers, poor field maps of stray fields along ejection trajectories,
and ripple on kicker magnets. Lattices with high or imaginary
transition energy have no transition crossing during acceleration, and
bunches better adapted to LHC requirements at ejection. The PS-XXI
proposal has these advantages. An alternative might be a new
synchrotron in the ISR tunnel. A proposal for a 2 GeV linac consisting
mainly of super-conducting LEP cavities would make it easier to achieve
the characteristics of the ultimate proton beam for LHC, and would give the
possibility to address new domains of physics at CERN, e.g. spallation
neutron source of energy amplifier. That proposal has to be balanced with
the possibility to upgrade the existing PS injectors. Fitting
techniques for finding sources of errors and diagnostics were also
discussed.
5mm
The working group on Active Emittance Control was
chaired by G. Lambertson LBNL. It wants to limit filamentation at
injection and the associated emittance increase with dampers and to suppress
the coupled-bunch instabilities with feedback. In the discussion of
transverse control in
the SPS, it made no allowance for non-LHC uses,
and assumed that the bunches on the kicker rise were eliminated by
deflectors in the PS, and that the kickers are rebuilt with 0.5%
ripple. Concerning the transverse feedback system,
it was proposed to change the shape of the electrodes in the
horizontal kicker, and to use the same kicker for the lowest
coupled-bunch mode which needs a similar voltage as injection
damping. In order to use the same system for higher modes, parts of it
must be rebuilt because the present system has resonances in the
operating frequency range. Feedback on longitudinal coupled-bunch
oscillations needs damped cavities as feedback kickers which are
available in the SPS. In the PS, no injection dampers are needed if
the injection kicker is rebuilt to 1% ripple and if the recombination
error of the four PS Booster beams is reduced. However, this particular
recommendation was strongly challenged by PS participants who insisted on
the need for injection dampers. No new feedback system
is needed since no vertical coupled-bunch instability is observed, and
the growth time of the horizontal coupled-bunch instability is rather
long.