Gunther Geschonke

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^-

Power couplers for the 352 MHz LEP2 superconducting RF cavities have been plagued by vacuum and electron outbursts which are attributed to multipacting. Processing of these couplers has been a lengthy operation which was often needed again after high power running even if only for a relatively short time. We report here on recent progress made in improved production methods of coupler parts and special treatment of surfaces, as well as practical tests and simulations of geometrical coupler modifications.

Linear Collider Based on CLIC Technology

The LEP Superconducting RF system reached its maximum configuration of 288 four-cell cavities powered by 36 klystrons in 1999. In 2000, this system, together with 56 cavities of the original copper RF system, routinely provided more than 3630 MV, allowing the beam energy to be raised up to 104.5 GeV. This not only required operating the cavities more than 15% above their design gradient, but has also demanded a very high operational reliability from the entire system. This paper will describe the operation of the LEP RF system during 2000, including new features, operational procedures and limitations.

Improvements to power couplers for the LEP2 superconducting cavities

The Two-Beam Accelerator concept is one of the most promising methods for producing RF power for future linear colliders. In particular it allows upgrades to multiTeV energies. One of its challenges is the production of the high current drive beam, which as it passes through decelerating structures, produces RF power for acceleration of the main beam. These challenges must be studied at a smaller scale test facility.

Ultimate performance of the LEP RF system

The LEP (Large Electron Positron colliding beam accelerator) uses a dual-frequency acceleration system consisting of coupled accelerating and storage cavities. Basic design features of this system with its accessories and tuning system are described. One hundred and thirty cavities have been built by industry; the assembly and conditioning with RF power were done at CERN. All cavities are now installed in the LEP tunnel. The procedures used and first experiences are presented.<<ETX>>

CTF3 Drive-Beam Injector Design

Two options for a linear e+e-collider are presently under development, the ILC and CLIC. The energy reach of the two machines is different, which leads to two different technological choices. ILC is based on superconducting acceleration technology, the CLIC design uses a two-beam acceleration system with normal conducting copper cavities. Nevertheless considerable synergy between the two design groups has been developed. The paper will highlight the major differences a well as the status and the plans of both machines.

Status of the LEP accelerating structure

The state of the art of superconducting radio frequency structures for particle acceleration is shown, with special reference to the suitability of the available technology for high current linear accelerators. It will be demonstrated that the basic requirements for these applications can be met today and the limitations will be discussed.

The next energy-frontier accelerator - A linear e+ e- collider?

Dipole coupled bunch oscillations were observed at an early stage of LEP commissioning for currents above about 150 /spl mu/A per bunch. An improvised feedback system, acting on the phase of some of the accelerating cavities was developed and has been in operation for about three years. However, due to the small bandwidth of the RF cavities this system can only be used with four bunches or less per beam. With plans for eight bunch operation (the Pretzel scheme) the construction of a dedicated longitudinal feedback system was approved in 1991. The system operates at 999.95 MHz with phase modulation of a 200 kW klystron feeding four seven-cell cavities. The necessary bandwidth of 260 kHz is obtained by heavy over-coupling. With a total cavity voltage of 1.9 MV a damping rate of about 450 s/sup -1/ is obtained with phase excursions of one radian. The system has been in routine operation since July 1992 with a feedback cavity voltage of 1.2 MV and a damping rate of about 100 s/sup -1/. Longitudinal feedback eases operation and usually increases the maximum currents which can be accumulated.<<ETX>>

Superconducting structures for high intensity linac applications

The results from experimental and three-dimensional computational studies of a radio-frequency structure for use in the CERN Linear Collider are presented. The structure would perform the function of converting beam energy from a UHF accelerated drive beam to 30-GHz radiation for powering the high gradient cavities of a second parallel beam, which would be accelerated to an energy of 1 TeV. Design considerations of this unusual component are discussed, along with the results of low-power radio-frequency tests on a scaled model which reveal the electrical characteristics of the structure. In addition, a comparison of the experimental findings with 3-D computations, performed with the MAFIA codes, is given, and good agreement is found. The results of tests with the structure excited by beams from the LEP (Large Electron-Positron Collider) preinjector linac are also presented. The transfer structure is found to satisfy many of the requirements needed for the role it would play in a two-beam accelerator.<<ETX>>

Longitudinal feedback in LEP

In electron storage rings where the bunch repetition rate is low the dissipation of RF power in the accelerating cavity can be reduced by modulating the RF. A scheme using the energy oscillation between a storage resonator and the accelerating cavity has been successfully tested. Results of both low-and high-power tests at 500 MHz as well as design considerations for a 352 MHz prototype are presented.