Penelope Brown

Ultimate performance of the LEP RF system

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.

The LEP Radio-Frequency Low Power System

A brief description of the low power and controls electronics for running the 128 cavity assemblies in the initial phase of LEP is given. Virtually identical hardware will be used for the future extension of LEP with superconducting niobium cavities. One complete RF unit consisting of 16 cavities and all associated drive and controls electronics has been successfully tested.1

Status of the LEP accelerating structure

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>>

Longitudinal feedback in LEP

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>>

Development and First Results with a Storage Resonator Enhancing the Shunt Impedance of an Electron Accelerating Cavity

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.