Providing the Longitudinal Beam Parameters of the LHC Beam

The luminosity foreseen for the Large Hadron Collider (LHC), at present being built at CERN, places stringent requirements on the longitudinal beam parameters of the high intensity proton beam. Critical points in the production of this beam occur both in the injector chain and at transfer and capture in the LHC itself. The consequent design of the superconducting RF system as well as the proposed damping and higher harmonic RF systems is discussed. Emphasis is also placed on some areas in the SPS machine, injector for the LHC, where transient beam loading and both single and multi-bunch instabilities must be carefully controlled to attain the high beam quality required

Impedance Issues in the CERN SPS

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 programme of studies is under way in the SPS to identify, reduce, and remove where possible the sources of these impedances

Impedance Issues in the SPS: summary of session 3

Session 3 of the Chamonix X workshop looked at our present knowledge of the impedance sources in the SPS and the efforts being made to remove, reduce or control these impedances

Preparing the SPS for LHC

The beams that the SPS must accelerate to fill the LHC have stringent requirements both transversely and longitudinally. The single bunch and total intensities have been obtained separately in the past but now have to be produced simultaneously and must be concentrated in a small part of the machine circumference. The transverse emittances will be small as will the allowable blow-up from injection to extraction. Beams with the LHC parameters will not be available in the SPS until the upgrades in the pre-injector chain are finished. Nevertheless a programme of studies is being carried out to examine the different areas where problems might occur. The delicate areas are at injection where single and coupled bunch instabilities have to be controlled in the longitudinal plane and where transverse emittance blow-up is expected, and at top energy where complicated RF gymnastics are required and where longitudinal coupled bunch instabilites can develop on the long flat top

Another Method to Measure the Low-Frequency Machine Impedance

The spectrum of long bunches samples the low-frequency part of the machine impedance which is mostly reactive. The voltage induced by the bunch produces the well known "potential well distortion" when RF is on, but also affects the debunching when RF is off. In this paper we present a method of estimating the reactive impedance by measuring the evolution of bunch parameters, such as the peak line density, during debunching. This method was used to find the inductive impedance of the CERN SPS with a single proton bunch injected above transition at 26GeV

The LHC Superconducting RF System

The European Laboratory for Particle Physics (CERN), the largest high energy physics laboratory worldwide, is constructing the Large Hadron Collider (LHC) in the existing 27 km circumference LEP (Large Electron Positron) collider tunnel. For the LHC, superconducting cavities, operating at 4.5 K, will provide the required acceleration field for ramping the beam energy up to 7 TeV and for keeping the colliding proton beams tightly bunched. Superconducting cavities were chosen, not only because of their high acceleration field leading to a small contribution to the machine impedance, but also because of their high stored energy which minimises the effects of periodic transient beam loading associated with the high beam intensity (0.5 A). There will be eight single-cell cavities per beam, each delivering 2 MV (5.3 MV/m) at 400 MHz. The cavities themselves are now being manufactured by industrial firms, using niobium on copper technology which gives full satisfaction at LEP. A complete cavity prototype assembly including cryostat, tuner and couplers is now being tested at CERN. In addition to a description of the LHC RF superconducting system, results on the prototype cavity assembly will be reported

Microwave Instability and Impedance Measurements in the CERN SPS

The single bunch intensity of protons in the SPS is limited by the microwave instability. The main sources of the SPS impedance responsible for this instability have been determined by measurements of the spectrum of unstable single bunches injected into the machine. New experimental data and the improved impedance model have helped to explain some previously contradictory results concerning the microwave instability threshold - one of the critical parameters for the SPS as LHC injector. Ways of curing the instability are discussed