Impedance estimates have been completed for several LHC components,
such as bellows, monitors, and experimental chambers. The merits and
drawbacks of a metallized ceramic chamber to reduce the injection
kicker impedance are currently being investigated. Stability studies
based on Landau damping thresholds with two-dimensional betatron tune
spread, for a detuning at 1 sigma of 10-4 in both planes
achieved by two octupole families, show that a pilot bunch with poorly
controlled chromaticity is stable against the head-tail instability
for currents as high as 10% of the nominal current. Multiple bunches
in the same current range remain stable without feedback for negative
chromaticities as large as -15.
Besides contributing to the conceptual design of the SPS as LHC
injector (intrabeam scattering and emittance control), the team
activity includes longitudinal beam dynamics studies based on
low-noise multiparticle tracking and transverse multi-bunch
simulations with partial filling for the SPS and LHC, to be further
interfaced to the existing impedance database. The multi-bunch
instability growth rate is found to be always smaller than that for
symmetric filling with the same total current, even for random bunch
populations and spacings. Ground motion effects and their impact on
closed orbit and emittance growth in collision are also being
investigated.
Surface resistance measurements for the copper coated LHC beam screen
at cryogenic temperatures indicate a beam-induced ohmic heating about
a factor two larger than previously estimated. For frequencies up to
1.5 GHz, the additional effect of an 8.4 T magnetic field is only 10
to 15%: an absolute measurement precision of a few per cent is
reached by comparing the quality factors of even and odd TEM modes in
a cylindrical structure with two inner conductors.
Synchrotron radiation from proton bunches in the LHC creates
photoelectrons at the beam screen wall. These photoelectrons are
pulled towards the positively charged bunch. When they hit the
opposite wall, they generate secondary electrons which can in turn be
accelerated by the next bunch if they are slow enough to
survive. Depending on several assumptions about surface reflectivity,
photo-emission and secondary-emission yields, this mechanism can lead
to the fast build-up of an electron cloud with potential implications
for beam stability and heat load on the beam screen. In view of the
tight deadline for the design of the LHC cryogenic system, a
crash program
has been set up to measure the relevant
physical quantities (by EPA irradiation tests and multipacting tests
in a superconducting magnet) and to validate analytic estimates and
numeric simulations.
Multipacting tests have been successfully performed with a coaxial
resonator. A simple and reliable technique, based on amplitude
modulation of the input signal, has been developed to detect
electronically the onset of multipacting. Preliminary results have
shown that the electron cloud build-up is not suppressed by a strong
dipole magnetic field, while a weak solenoidal field of about 50 Gauss
is usually sufficient to stop the multipacting. A substantial
decrease of the multipacting threshold is observed for a dipole field
intensity such that the electron cyclotron frequency is equal to the
resonant frequency of the coaxial cavity.
Computer codes have been developed, debugged and used to predict the
heat load on the LHC beam screen under several conditions and the rise
time of a multi-bunch instability associated with the electron cloud
wakefield. The results are in agreement with quasi-analytic estimates
of the critical secondary-emission yield, and indicate that doubling
the LHC bunch spacing would be an effective back-up solution.
Alternative cures, including low-emissivity coatings, clearing
electrodes, and an increased surface roughness, are under study
together with their possible impact on the impedance budget.
27/01/1998
Francesco Ruggiero@cern.ch