[en] The future use of the CERN SPS accelerator as injector for the Large Hadron Collider, LHC, and the possible use of the SPS as a neutrino source for the Gran Sasso experiment are pushing the maximum intensity requirements of the accelerator much higher than achieved up to now. At the same time the requirements on beam quality are becoming far more stringent. The SPS machine, built in the 70's, is not a ''smooth'' machine. It contains many discontinuities in vacuum chamber cross-section and many cavity-like objects, as well as the 5 separate RF systems at present installed. All these lead to a high impedance, seen by the beam, spread over a wide frequency range. As a result there is a constant fight against instabilities, both single and multi bunch, as the intensity increases. A program of studies is under way in the SPS to identify, reduce, and remove where possible the sources of these impedances

[en] The intensity of leptons accelerated in the SPS machine is limited by a vertical transverse instability. The results of measurements of the thresholds for this transverse instability are compared with theoretical predictions for different broad band impedance models of the SPS. The threshold intensities found for the transverse instability and the position of the losses in the cycle enable the parameters of the broadband resonant impedance to be specified. (author)

[en] The average luminosity of the SPS collider could be improved if the slow blow-up of transverse emittances due to beam-beam and intrabeam scattering effects were to be reduced by a transverse cooling system. We examine the parameters of such a system and propose a technological approach which seems better suited to the case of a few bunches circulating in a large machine. (orig./HSI)

[en] After almost one year of operation two kinds of problems dominate the longitudinal behavior of the SPS beam. At high energy longitudinal coupled bunch instabilities occur. They are driven by the rf cavity impedance both on its fundamental and high-order passbands. Present cures include damping the high-order modes of the cavities, Landau damping techniques and feedback systems. At the injection energy a debunching-rebunching procedure is performed in order to change from the 9.5 MHz rf frequency of the injector to the 200 MHz of the SPS. Debunched beam instabilities driven by the cavity and vacuum chamber impedances (up to the GHz region) at present limit the rf capture efficiency

[en] The average luminosity of the SPS collider could be improved if the slow blow-up of transverse emittances due to beam-beam and intra beam scattering effects were to be reduced by a transverse cooling system. We shall examine the parameters of such a system and propose a technological approach which seems better suited to the case of a few bunches circulating in a large machine

[en] The future use of the SPS as injector for the LHC requires both a high intensity and a high quality beam. For many years the longitudinal single bunch instability was one of the most serious limitations to obtaining the nominal beam parameters. The main sources of impedance causing this instability were found a few years ago from measurements of the spectrum of unstable single bunches. Recently the major part of a programme to reduce this impedance was completed. Reference beam measurements with RF on and RF off have allowed the changes in bunch stability as well as in the coupling impedance to be seen

[en] A continuous measurement of the betatron tune has been implemented in the SPS pp^- collider with hardware normally used for the detection of Schottky noise. Suitable values for the tunes and chromaticities are thus more easily achieved in a machine where the strong space charge effects of the two counterrotating beams severely reduce the available space in the resonance diagram. The basic idea is to monitor the frequency of one of the betatron lines together with the revolution frequency and to deduce the tune according to a formula provided in this paper. The betatron lines which naturally appear as noise signals adding incoherently are strongly enhanced by exciting the beam at the required frequency with a kicker. The whole system then forms a closed loop in which an oscillator excites the beam producing a coherent signal at the Schottky detector, this signal providing the reference to lock the oscillator frequency. The excitation, 50 W at 10.7 MHz causes particle loss (about 3%/cycle) and emittance blow-up (about 10%/cycle) restricting the use of the apparatus to setting-up periods

[en] The CERN SPS machine has been extensively upgraded to prepare it for its future role as the LHC injector. These changes include a significant impedance reduction programme and new beam-control and feedback systems. Nominal beams have recently been produced at the extraction energy, 450 GeV. To achieve this intensity it was initially necessary to inject higher intensities due to particle loss at capture and low energy. These intensity-dependent losses reached 30% for matched, but not optimum, capture. Studies of different phenomena leading to these losses have increased the transmission efficiency to ∼95%. We describe the steps taken to minimize losses, pointing out mechanisms involved and presenting unresolved issues

[en] Optimisation of pick up and detection techniques for observation of ''Schottky'' signals in bunched beams is discussed. Methods for distinguishing coherent and incoherent motion are described

[en] We describe a new method used in the CERN SPS accelerator to measure the longitudinal impedance in the frequency range 100MHz to 4GHz. Single high intensity proton bunches were injected and their spectrum observed during slow debunching. The presence of different resonant impedances leads to line density modulation at the resonant frequencies. This instability reaches some maximum modulation amplitude which was recorded as a function of frequency for many bunches. Using sufficiently long bunches the SPS impedance structure was observed and previously unknown sources were identified. copyright 1997 The American Physical Society