Jacques Gareyte

The Large Hadron Collider (LHC) in the LEP tunnel

After the remarkable start-up of LEP, the installation of a Large Hadron Collider, LHC, in the LEP tunnel will open a new era for the High Energy Physics. This report summarizes the main LHC parameters and subsytems and describes the more recent studies and developments.

Transverse mode coupling instabilities

Transverse mode coupling instabilities (TMCI) emerged between 1974 and 1980 as the main limitation of dense bunches in electron synchrotrons and storage rings. A two-particle model allows one to calculate the beam break-up (BBU) instability in linacs. Extending this to synchrotrons shows that the BBU instability is suppressed below a threshold intensity by synchrotron oscillations. The classical theory of head-tail modes, together with the general properties of coupling impedances, is used to show how single bunches become unstable when head-tail modes couple together: this is the TMCI threshold. Above threshold, observations in both proton and electron synchrotrons can be described by BBU theory.

Collective Effects at Very High Intensity in the CERN-PS

The CERN-PS beam intensity is being steadily increased (1.55 10’ 3 protons per pulse achieved). In addition, a change of harmonic number by debunchingrebunching is performed to allow a clean bunch-intobucket injection into the SPS. Many collective phenomena had to be studied in this context lately, and selected results of interest to machine designers or operators are reported here. Topics covered are resistive-wall instabilities with peculiar characteristics due to space-charge detuning and fast-decaying wakes, microwave longitudinal instabilities, and problems associated with strong cavity beam-loading.

The Damper for the Transverse Instabilities of the SPS

For beam intensities above 1012 protons per pulse in the SPS, collective transverse beam instabilities develop with frequencies between 15 kHz and 3 MHz because of the resistive wall effect of the vacuum chamber1). An active feedback system2) with an electrostatic deflector has been installed in the SPS for damping the resistive wall instabilities in both the vertical and horizontal planes. Measurements have been made to determine the threshold and growth rate of these instabilities. As a novel application, the damper can be used also for the excitation of small coherent betatron oscillations. A phase-locked loop tracks the beam oscillations and provides a continuous display of the betatron wavenumber Q during the cycle.

Acceleration and Storage of a Dense Single Bunch in the CERN SPS

First tests of the lifetime of a normal SPS beam stored for several hours at 200 and 270 GeV were encouraging. The natural logarithmic decay time is in excess of 24 hours. However, in the proton-antiproton scheme, 200 MHz bunches containing fifty times the normal design population of particles are to be injected into the SPS above transition at 26 GeV, accelerated and stored. Lacking the hardware to inject at so high an energy, we first injected bunches of 1011 protons at 10 GeV accelerating them through transition but found it difficult to pag transitionwith more than 40% of this design population . Nevertheless we report some interesting observations on head-tail and negative-mass effects which limited intensity during these tests.

Dynamic aperture and long term particle stability in the presence of strong sextupoles in the CERN SPS

The reduction of dynamic aperture produced by strong sextupoles has been measured in the SPS (Super Proton Synchrotron). The short-term aperture, inside which particles survive for at least one second, is well predicted by computer simulations based on the early detection of chaotic motion. Inside this aperture particles diffuse slowly to larger amplitudes, so that long-term stability is only assured in a smaller region of the phase space, the long-term aperture. Slow tune modulations increase the chaotic regions and could explain the observed diffusion phenomena. The criteria based on tune shift and smear which are used in the design of large hadron colliders are discussed.<<ETX>>

Acceleration and Stacking of Deuterons in the CERN PS and ISR

Deuteron acceleration in the CERN 50 MeV Linac has been tried out already 13 years ago followed by programmed acceleration in tne CPS up to about 100 MeV.

Beam Dynamics Experiments in the PS Booster

The main problems encountered on the way to 1013 ppp have been emittance blow-up and coherent instabilities. The observations and counter measures are described in the text.

Observation and correction of instabilities in circular accelerators

The correction of beam instabilities in circular accelerators requires a good knowledge of some basic concepts which form the basis of a widely accepted Standard Model. These concepts are introduced here in a simple semi-empirical way, and their relevance to the observation and understanding of collective phenomena is illustrated by practical examples.

Performance Limitations of the CERN SPS Collider

The SPS has now accumulated more than 10 months of operation as a proton-antiproton collider spread over 3 long physics runs. During this time the peak luminosity has been pushed up to 3.5×1029 cm-2S-1 and the luminosity lifetime to almost 30 hours. Different physical phenomena limit the machine performance at various times during injection and storage. The peak luminosity is governed by the number of bunches per beam and the emittance and intensity per bunch as well as by the horizontal and vertical beta values at the experimental insertions. Although the transverse emittances and antiproton intensity are defined by the injector chain, the proton bunch intensity is mainly limited by the microwave instability in the SPS itself. The low-beta insertions have been pushed to the limit of available quadrupole strength and chromaticity correction capability at the maximum storage momentum of 315 GeV/c (ßH = 1m, ßV = 0,5m). The luminosity lifetime in the first few hours of storage is limited by the transverse emittance growth of the dense proton bunches due to intrabeam scattering1,2. As the store progresses, the proton bunch decay rate increases due to the growth of the longitudinal emittance (also through intrabeam scattering), finally becoming the dominant contributing factor in governing the luminosity lifetime. The beam-beam interaction limits the usable area in tune space to a very small region free of 10th order resonances3. Consequently, the total tune spread must be kept below 0.025, allowing only 3 bunches per beam without separation at the unwanted crossings.