Rudolf Bossart

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^-

For the Compact Linear Collider Test Facility (CTF) at CERN a new rf gun with a laser driven photocathode is under construction. The new rf gun will replace the present 112 cell gun and will consist of 212 cells accelerating the beam to a momentum of 7.0 MeV/c with an electric field strength of 100 MV/m. The strong space-charge forces at low beam energy caused by the high charge density of the electron bunches are contained by radial and longitudinal rf focusing in the gun. The rf gun under construction has been optimized by MAFIA beam simulations for an injector assembly comprising a second accelerating rf structure and an intermediate solenoid magnet correcting the beam divergence of the 212 cell gun. The beam loading of the rf gun, by a train of 48 bunches with 21 nC charge each, causes a strong energy decay accompanied by an increase of the flight time for the bunches with lower energy. These effects can be corrected by slightly shifting the acceleration frequency of the gun. The experimental results obtained with the 112 cell gun and booster section presently in operation are reported.

Linear Collider Based on CLIC Technology

Abstract Theoretical studies show that owing to the abrupt change of the magnetic field occurring at the magnet edges synchrotron radiation will be emitted in the visible light range, by a high-energy proton beam. Experiments have been carried out at the CERN Super Proton Synchrotron (SPS) in order to check for the validity of the theory and measure the properties of the emitted light. Special attention has been devoted to the energies and intensities of the proton beam, as profile measurement is foreseen as an immediate application.

A 3 GHz photoelectron gun for high beam intensity

Abstract In circular proton accelerators, the synchrotron radiation emitted by high energy beams at the edges of bending magnets contains an appreciable amount of visible light. This effect is used in the CERN SPS to measure by a non interceptive method the transverse proton density distribution above 200 GeV. The device consists of a telescope followed by an image sensor and electronics processing; its performances and some examples of applications are given in this paper.

Observation of visible synchrotron radiation emitted by a high-energy proton beam at the edge of a magnetic field

Abstract In order to increase the beam energy of the electron source for the linear collider test facility at CERN, the existing rf gun is complemented by a second rf structure of four cells, which boosts the beam momentum from 4.5 MeV/c at the exit of the gun to 11 MeV/c. Both the 1- 1 2 cell gun and the four cell section are standing wave structures excited in the π-mode at a frequency of 2998.5 MHz, with an accelerating field strength of up to 115 MV/m in the rf gun and 70 MV/m in the four cell section. Between the rf gun and the four cell section a short solenoid magnet is installed for beam focusing. The solenoid magnet compensates the space charge forces in the drift space, corrects the beam divergence caused by the rf gun and compresses the radial dimensions of the beam at the exit of the four cell section. Beam simulations with the computer programme MAFIA have shown that the beam can be made to converge radially at the exit of the four cell section, so that electron bunches with high charge density can be produced.

Proton beam profile measurements with synchrotron light

For beam intensities above 1012 protons per pulse in the SPS, collective transverse beam instabilities develop with frequencies between 15 kHz and 3 MHz because of the resistive wall effect of the vacuum chamber1). An active feedback system2) with an electrostatic deflector has been installed in the SPS for damping the resistive wall instabilities in both the vertical and horizontal planes. Measurements have been made to determine the threshold and growth rate of these instabilities. As a novel application, the damper can be used also for the excitation of small coherent betatron oscillations. A phase-locked loop tracks the beam oscillations and provides a continuous display of the betatron wavenumber Q during the cycle.

Modular electron gun consisting of two rf sections and an intermediate focusing solenoid

Theoretical studies, followed by experiments, show that owing to the abrupt change of the magnetic field occurring at the magnet edges, synchrotron radiation is emitted in the visible light range by a high energy proton beam. The spatial photon density being proportional to that of the proton beam the analysis of the emitted ‘image’ by a dedicated camera gives an accurate representation of the beam profiles. Based on these properties a non-interceptive detector has been developed and installed at CERN SPS proton synchrotron in order to measure the profile of the circulating beam. The results show that for an energy higher than 250 GeV and a beam intensity of at least 0.7 mA (1011 p) the results are satisfactory. The spatial resolution being 100 μm many beam parameters can be evaluated with good accuracy.

The Damper for the Transverse Instabilities of the SPS

The objectives of the CLIC Test Facility (CTF) are to study the generation of short intense electron bunches using a laser driven photocathode in an RF gun, to generate 30 GHz RF power for high gradient tests of prototype CLIC components, and to test beam position monitors. The performance of the CTF has improved dramatically in the course of the past year and highlights are presented.

Proton Beam Profile Monitor using Synchrotron Light

Two complementary techniques have been developed which allow the betatron tunes to be controlled during energy ramping of the SPS to a precision of better than ¿0.002. By the use of these techniques the setting-up of the SPS as a collider and a fixed target machine is substantially simplified and better physics understanding has been gained. The first technique uses an electrostatic beam deflection every 60 ms. A Fast-Fourier Transformation of the beam response in a dedicated computer yields the betatron tunes with a precision better than 0.01. In the second technique the beam is continuously excited. The frequency of the excitation is measured and fed back to the beam by a Phase-Locked-Loop. This measurement is accurate to better than 0.001 but requires a reasonably well tuned machine. Tune deviations are automatically compensated by acting on the main quadrupoles through a software loop.

CLIC Test Facility developments and results

In view of new modes of operation of the SPS, the old superheterodyne receivers designed for the closed orbit measurements of fixed target beams are being replaced by homodyne 200 MHz receivers with a better time resolution for measuring single bunches. The new receivers have a bandwidth of 4 MHz, a dynamic range of 70dB, and are able to measure the position either of continuous beams with 1012 to 5×1013ppp bunched at 200 MHz CW for fixed target physics, or of single bunches shorter than 5ns with 109 to 2×1011 particles per bunch when the SPS is used as a ppbar collider or as an e+e- injector for LEP. The homodyne receiver has been chosen for its simple layout, linearity, excellent noise inmunity and high accuracy of the beam centre position. The built-in calibration facility using the intensity signal of the beam allows to calibrate all 240 receivers and acquisition systems of the closed orbit measurement for zero offset errors to less than 5×10-3 of the monitor radius and for relative gain errors to less than 4×10-2. The same synchronous receivers, but running at 20 MHz and 50 MHz, are also used for intensity and lifetime measurements of the SPS collider.