Daniel Boussard

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.

Control of Cavities with High Beam Loading

Beam loading of RF cavities is a subject of great concern for the design and operation of high current circular accelerators and storage rings. The steady state and transient perturbations of the accelerating voltage by the beam current lead to undesirable beam behaviour, like for instance the onset of instabilities or particle loss by lack of longitudinal acceptance. To some extent, it is possible to alleviate or even completely suppress these effects by a proper control of the RF power amplifier - cavity combination, using feedback or feedforward techniques. Several new developments in this field have taken place during the past years. For instance, the implementation of fast feedback technology, the use of cavity compensation schemes or digital filtering of signals in long delay feedback systems have resulted in considerably improved machine performance. Such techniiques will be reviewed in this paper and their performance and limitations will be presented.

Improvements to power couplers for the LEP2 superconducting cavities

Power couplers for the 352 MHz LEP2 superconducting RF cavities have been plagued by vacuum and electron outbursts which are attributed to multipacting. Processing of these couplers has been a lengthy operation which was often needed again after high power running even if only for a relatively short time. We report here on recent progress made in improved production methods of coupler parts and special treatment of surfaces, as well as practical tests and simulations of geometrical coupler modifications.

Production of Beams with High Line-Density by Azimuthal Combination of Bunches in a Synchrotron

Simultaneous acceleration of several beams in the same machine by several RF fields of slightly different frequencies was already considered a long time ago1,2). As they rotate at different frequencies, the bunches will periodically coincide in azimuth, thus producing beams of high local line density. Bunches rotating at different frequencies can be produced either outside the machine (e. g. in the injector) or even inside by appropriate modulation of the RF cavities. These techniques will be used in the p-p¿ project at CERN. A high line-density proton beam, necessary for antiproton production, is obtained in the CPS by azimuthal combination, either at the injection or at the ejection level. On the other hand, the antiproton bunches in the SPS must be azimuthally combined before storage in order to achieve the design luminosity. Computer simulation and RF manipulations of these procedures, as well as experimental results already obtained at the CPS, are presented.

RF power requirements for a high intensity proton collider. Part I

It is shown that, in a high-intensity machine, with a very small synchronous phase angle, RF power is required not for stability reasons but to avoid phase modulation of the beam. During transients where the cavity tuner is out of equilibrium, the RF generator must also compensate for the reactive power unbalance. The optimum coupling between generator and cavity must take into account these various effects to minimize the overall RF power requirements.<<ETX>>

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 LHC superconducting cavities

The LHC RF system, which must handle high intensity (0.5 A d.c.) beams, makes use of superconducting single-cell cavities, best suited to minimizing the effects of periodic transient beam loading. There will be eight cavities per beam, each capable of delivering 2 MV (5 MV/m accelerating field) at 400 MHz. The cavities themselves are now being manufactured by industry, using niobium-on-copper technology which gives full satisfaction at LEP. A cavity unit includes a helium tank (4.5 K operating temperature) built around a cavity cell, RF and HOM couplers and a mechanical tuner, all housed in a modular cryostat. Four-unit modules are ultimately foreseen for the LHC (two per beam), while at present a prototype version with two complete units is being extensively tested. In addition to a detailed description of the cavity and its ancillary equipment, the first test results of the prototype are reported.

Non integer harmonic number acceleration of lead ions in the CERN SPS

The project to accelerate lead ions in the CERN complex has been successfully completed and physics has begun. In the SPS, the final machine in the chain, the ions are accelerated from an energy of 5.1 GeV/nucleon to 160 GeV/nucleon using the existing 200 MHz travelling-wave cavities. The change in revolution frequency during acceleration is much larger than can be accepted by the untuned cavities when operated at constant harmonic number. A technique has been developed to overcome this limitation which takes advantage of the filling time of this type of cavity which is shorter than one turn. Fast amplitude and frequency modulation of the RF waveform allows the cavities to operate at a constant, optimum frequency during the passage of a batch of particles in the structure. This frequency is not a multiple of the revolution frequency and therefore during the gaps between batches the phase of the composite RF waveform is changed to maintain synchronism from turn to turn as the beam accelerates. The technique and hardware are described in detail together with the first operational experience.

Design of a ring rf system

ABSTRACT In a circular accelerator or collider, the excitation of coherent longitudinal oscillations (dipole and quadrupole) by RF noise, magnetic fluctuations and the like may cause indesirable effects, for instance emittance blow-up or even beam losses. The RF system architecture can be designed to damp these oscillations using feedback techniques. Th1!Se will be presented in this paper, both for the low intensity case and when cavity beam loading becomes important.

The Influence of RF Noise on the Lifetime of Bunched Proton Beams

In a proton-antiproton storage ring like the SPS (\(p\bar p\) project), RF noise limits the lifetime of the bunches1). The equations describing the r.m.s. increase of individual particle oscillation amplitude are derived for phase and amplitude excitation. The non-linearity of the phase oscillation, although small, in the long run transforms the coherent noise excitation into a random excitation of each particle. Computer simulations have confirmed this picture which leads to a macroscopic description of the phenomenon by a diffusion equation. The various RF noise sources in the SPS machine are analyzed and their relative effects compared. Careful choice of the loop parameters minimizes the RF noise influence. Finally, some experimental results obtained on the SPS machine are reported and compared with theory.