A Hilaire

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>>

Second Order Effects in the Sextupole-Corrected SPS Lattice

Chromaticity correction is essential in the SPS accelerator both for normal operation and for high density single bunch acceleration and storage, required for the CERN proton-antiproton collider project. Linear correction is presently achieved with one set of 36 F sextupoles and another set of 36 D sextupoles, regularly placed around the machine circumference. This lumped sextupole distribution induces non-linear variations of the machine tune with ¿p/p. These higher order effects which are also encountered in large electron machines cannot actually be corrected in the SPS, where they are well observable at least up to second order. Analytic calculation of the non-linear chromaticity, using Fourier expansions of the lattice functions, as well as computer simulation with the AGS program closely agree with the measurements performed at various energies. For the same reason, the transition energy depends upon the momentum deviation ¿p/p of the beam, which can lead to losses when beams with large ¿p/p are accelerated through transition, since no correct phase-jump timing can be found for all particles. Measurements of this effect in the presence of different chromatic corrections are presented and also compared with the predictions of the analytic calculations and of the computer evaluations with the AGS program.

The CERN SPS Low Beta Insertion Control System

Within the context of the CERN SPS ppbar colliding beam experiments there are 2 × 15 power supplies which have to provide the currents necessary to produce the two low beta insertions. Their control system is described for both the hardware and the software parts. The references use a combination of an 18 bit DAC and an integrator to produce a relatively smooth drive signal to the current controlled power supplies. These 18 bit devices must operate in a hostile environment (electrical noises, temperature changes). Results are given on their performances, especially on the accuracy achieved at the required settings (better than 10-4 at intermediate levels) and, more important, the repeatability (of the order of 10-5). Each set of 15 references is controlled from a dedicated computer. Each 60 msec point of each reference can be defined for a cycle length of up to 21.6 sec. The software driver is laid out such that during the pulsing mode (setting up) one can insert mini-coasts at any desired level or, during the coast proper at 270 GeV, one can adjust simultaneously the level of 1, some or all 30 references. The system also allows the setting up of a high beta in one insertion and a low beta in the other. It has transposed the theoretically calculated values so well that all the desired beta characteristics were achieved without having to add corrections due to inaccuracies in the reference control system.