Thys Risselada

A 3 TeV

A possible design of a multi-TeV e+e- linear collider is presented. The design is based on the CLIC (Compact Linear Collider) two-beam technology proposed and developed at CERN. Though the study has shown that this technology is applicable to a linear collider with centre-of-mass energies from 500 GeV or less up to 5 TeV, the present report focuses on the nominal energy of 3 Te V. First, a short overview is given of the physics that could possibly be done with such a collider. Then, the description of the main-beam complex covers the injection system, the 30 GHz main linac, and the beam delivery system. The presentation of the RF power source includes the beam-generation scheme, the drive-beam decelerator, which consists of several 625 m long units running parallel to the main linac, and the power-extraction system. Finally, brief outlines are given of all the CLIC test facilities. They cover in particular the new CLIC test facility CTF3 which will demonstrate the feasibility of the power production technique, albeit on a reduced scale, and a first full-scale single-drive-beam unit, CLICI, to establish the overall feasibility of the scheme.

e^+ e^-

Injection intensities for the LHC are over an order of magnitude above damage level. The TI 2 and TI 8 transfer lines between the SPS and LHC are each about 2.5 km long and comprise many active elements running in pulsed mode. The collimation system in the transfer lines is designed to dilute the beam energy sufficiently in case of accidental beam loss or mis-steered beam. A system using three collimator families spaced by 60 degrees in phase advance, both in the horizontal and the vertical plane has been chosen. We discuss the reasons for this choice, the layout and, the expected performance of the system in terms of maximum amplitudes and energy deposition.

Linear Collider Based on CLIC Technology

The dynamic aperture (DA) of the LHC is estimated by tracking a thin-lens model of the lattice over a real machine time of 9 seconds. This raw result is then processed to predict the DA over longer times and realistic conditions. We discuss the reliability and limits of the method, the phenomena limiting the DA and scaling laws used for fast estimates. The long-term dynamic aperture at injection is close to the requirements, with prospects of further improvements.

Collimation in the Transfer Lines to the LHC

This year’s dynamic aperture experiments at the SPS concentrated on the study of the diffusion enhancement by tune modulation in presence of strong nonlinear fields. The aim of this experiment is to specify an upper limit for the power supply ripple in view of the LHC, the proposed hadron collider in the LEP tunnel. Due to strong, unavoidable non‐linearities of the superconducting magnets we expect this accelerator to be very sensitive to power supply ripple. An additional tune modulation, large in comparison with the natural one, was therefore introduced in the SPS and a particle diffusion was meaured for several modulation amplitudes. The combined effect of two frequencies with equal ampltiude was also tested. In parallel long‐term tracking studies have been performed, which allow to find the dynamic aperture for a model of the SPS with the added non‐linearities and tune modulations.

Overview of the LHC dynamic aperture studies

The collimation system of the Compact Linear Collider (CLIC) must fulfil a number of conflicting requirements, namely it should (1) remove beam halo to reduce the detector background, (2) provide a minimum distance between collimators and collision point for muon suppression, (3) ensure collimator survival and machine protection against errand beam pulses, (4) not be excessively long, and (5) not amplify incoming trajectory fluctuations via the collimator wake fields. Two optical systems have been designed — the first linear, the second non‐linear —, which promise to meet all these requirements for the design beam energy of 1.5 TeV. We decribe the various design criteria, a preliminary performance assessment, and outstanding questions.

The 1991 dynamic aperture experiment at the CERN SPS

The eight superconducting quadrupoles and their cryogenic equipment for this insertion were installed in the ISR at the end of 1980. The insertion has been used to assess the problems of running a superconducting insertion in a storage ring as well as to provide high luminosity for physics. The luminosity is increased at intersection 8 by a factor of 7. By means of dedicated collimators and orbit corrections, safe working conditions could be established for the superconducting magnets during injection, accumulation, stable beam periods and when dumping the beams. Quenches were mainly caused by large accidental beam losses. Operating parameters for all standard beam energies, including acceleration to 31.4 GeV/c, have been established. At 26.6 GeV/c, with currents of 30.6 A in ring 1 and 30.3 A in ring 2, a luminosity of 1.4 1032 cm-2s-1 was obtained in the insertion. This is the highest luminosity reached so far in storage rings and it was obtained during a physics run. Satisfactory beam conditions could also be provided for antiproton physics at 26.6 GeV/c in ISR with both low-ß insertions on, in I1 and I8, respectively.

Collimation for CLIC

A slow energy loss of unbunched protons and antiprotons in the ISR has been observed which is larger than expected for interactions with the residual gas and for synchrotron radiation., It can be explained as a parasitic mode loss due to the wall currents induced by the longitudinal Schottky noise. This noise as seen at the wall of a circular chamber of radius a extends to a typical frequency of the order ¿ ~ c¿/a limited by the finite opening angle of ~1/¿ of the fields created by a relativisitic particle. Due to the statistical longitudinal distribution of the particles the total energy loss due to the induced wall current is proportional to the total number of particles and the loss per particle becomes independent of current. This loss leads to a steady decrease of the orbit radius which can be observed for long storage times. Using these results obtained at different energies, the frequency dependence of the resistive longitudinal impedance in the range of several tens of GHz is obtained. This impedance is compared with the prediction of the diffraction theory.

Operational Experience with the Superconducting High-Luminosity Insertion in the CERN Intersecting Storage Rings (ISR)

The preservation of the transverse emittance of the proton beam at injection into the LHC is crucial for luminosity performance. The population of the beam tails is also important for beam losses and collimation. The transfer and injection process is particularly critical in this respect, and several effects can contribute to the expected emittance increase and tail repopulation, like optical and geometrical mismatch, injection offsets and coupling, etc. The various effects are described, together with the tolerance limits on the parameters, and the expected contributions evaluated analytically where possible. The emittance growth and tail distributions are also simulated numerically using realistic errors. The implications for the tolerances on the matching of the transfer lines are discussed.

Measuring the ISR Impedance at Very High Frequencies by Observing the Energy Loss of a Coasting Beam

The variation of the transition energy γt m_oc² across the momentum aperture in the ISR has been determined by measuring the non-linear change of the revolution frequency as a function of the radial displacement and the momentum deviation and by measuring the phase oscillation frequency variation across the aperture while operating close to transition energy. The ratio of the relative slopes of γ_t and γ was found to be dγ_ttc ≈ -0.75. This value could be changed to nearly zero by strongly exciting the sextupole magnets. All measurements are in good agreement with each other and with computations carried out with a modified version of the program AGS. The change of γ_t across the aperture causes an area change of the empty buckets while traversing the beam during phase displacement acceleration. This leads to a momentum blow-up.

Expected Emittance Growth and Beam Tail Repopulation-From Errors at Injection into the LHC

Each ring of the ISR is equipped with a collimator system consisting of 10 movable, remotely controlled, blocks inside the vacuum system. One vertical and one horizontal collimator at the same azimuth limit the beam size during all machine operations. These primary collimators are followed at ¿/2, ¿ and 3¿/2 betatron phase advance by secondary collimators, which intercept the high energy protons scattered at small angles from the surface of the primary collimators by Coulomb forces. This system makes it possible to limit the beam dimensions without scattering protons into the intersections downstream of the collimators. Considerable improvement has been achieved in background and induced radioactivity conditions in all physics intersections. The properties of different metals with respect to collimation, temperature rise and ultrahigh vacuum compatibility are discussed.