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 travellingwave 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.

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The proton-antiproton total cross section was measured at the center-of-mass energy of p s = 541 GeV at the S ppS at CERN using a luminosity dependent method. The result tot = 6 3 : 0 2 : 1 m b is in agreement with the luminosity independent measurement performed earlier by the UA4 collaboration.

A proton beam with the basic structure defined by the the LHC requirements, was first available for injection into the SPS in 1998. At the end of 2002, following a significant beam-studies and RF hardware upgrade programme, a beam having both the nominal LHC intensity and the correct longitudinal parameters was obtained at top energy for the first time. This beam, characterised by high local density, must satisfy strict requirements on bunch length, longitudinal emittance and bunch to bunch phase modulation for extraction to the LHC, where only very limited particle losses are acceptable. The problems to be solved came mainly from the high beam loading and microwave and coupled bunch instabilities which led both to beam losses and to unacceptably large longitudinal emittance on the flat top. In this paper the steps taken to arrive at these nominal beam parameters are presented. Large Hadron Collider Project NOMINAL LONGITUDINAL PARAMETERS FOR THE LHC BEAM IN THE CERN SPSP. Baudrenghien, T. Bohl, T. Linnecar, E. Shaposhnikova and J. Tuckmantel, CERN, Geneva, Switzerland Abstract A proton beam with the basic structure defined by the the LHC requirements, was first available for injection into the SPS in 1998. At the end of 2002, following a significant beam-studies and RF hardware upgrade programme, a beam having both the nominal LHC intensity and the correct longitudinal parameters was obtained at top energy for the first time. This beam, characterised by high local density, must satisfy strict requirements on bunch length, longitudinal emittance and bunch to bunch phase modulation for extraction to the LHC, where only very limited particle losses are acceptable. The problems to be solved came mainly from the high beam loading and microwave and coupled bunch instabilities which led both to beam losses and to unacceptably large longitudinal emittance on the flat top. In this paper the steps taken to arrive at these nominal beam parameters are presented.

We describe a new method used in the CERN SPS accelerator to measure the longitudinal impedance in the frequency range 100 MHz to 4 GHz. 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. [S0031-9007(97) PACS numbers: 29.27.BdBeam intensity in particle accelerators is usually limited by collective instabilities which are the result of electromagnetic interaction of the beam with its environment. This interaction can be described by a coupling impedance Z͑v͒, the Fourier transform of the wake potential, proportional to the electric field integrated over one turn as seen by a probe particle following a source particle at time interval t. Knowledge of the impedance allows the behavior of intense beams to be explained or predicted [1,2].The main intensity limitation for the proton beam in the SPS, future injector for the Large Hadron Collider (LHC), comes from a longitudinal instability of a single bunch, known as the "microwave instability" since it is associated with the growth of high frequency signals. In this Letter we present results of measurements from which the longitudinal coupling impedance over a wide frequency range has been determined, and the source of the instability identified. Below we consider only longitudinal beam motion (and impedance), which in this case can be treated separately from transverse motion.Many of the vacuum chamber elements through which the beam travels have a cavitylike structure whose impedance can be represented as the sum of several individual resonances of the formcharacterized by a resonant frequency v r 2pf r , shunt impedance R sh , and quality factor Q. Measurements of impedance with a single bunch can give information only about the effective impedancethe actual impedance integrated over the spectrum of the bunch.For a stable bunch the effective impedance is defined by integration over the initial bunch spectrum centered at zero frequency. This effective impedance can be estimated from bunch lengthening (shortening) measurements based on the potential well distortion effect. Since the width of the bunch spectrum is inversely proportional to the bunch length, long bunches "see" only the low frequency part of the coupling impedance. Difficulties in resolving the "fine structure" of the machine impedance led to the creation of the widely used broadband impedance model, which replaces this structure by one resonator with very low Q (usually 1) and resonant frequency defined by the beam pipe cutoff frequency. The parameters of the SPS broadband model deduced from this type of measurement are Q 1, R sh 0.3 MV, and f r 1.3 GHz. This model has been used up to now also to describe...