Hans Frischholz

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.

Generation and Distribution of Radio-Frequency Power in LEP

The beam energy in LEP Phase I at maximum luminosity is 55 GeV. With a circulating current of 3 mA per beam, an effective cavity shunt impedance of 40 M..cap omega../m and RF power losses in the waveguide system of 7.5%, a total of 14.9 MW RF generator power is required for achieving this energy, the respective values for injection being 20 GeV and about 0.26 MW. This RF power is generated by 16 klystrons and supplied to the 128 accelerating structures via a WR 2300 waveguide system. Two klystrons, together with 16 accelerating structures, the controls and associated waveguide system, form an RF unit.

Compensation of Beam Loading in the ISR RF Cavities

The RF cavities employed for stacking in the ISR are equipped with a powerful feedback system, which compensates for most of the beam-induced voltage. This system is, however, limited - mainly by the current in the final amplifier tube - to injected beam intensities of about 5 × 1012 protons at most. Higher intensities are now available from the PS. Additional beam-load compensation has, therefore, been added to the existing cavities and feedback amplifiers. This beam-load compensation consists of a pick-up, a one-turn delay cable, a wide-band amplifier chain and a final amplifier, directly connected to the accelerating gap. This system injects into the cavity a current approximately equal and opposite to the beam current.

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>>

The LEP II RF power generation system

By means of the LEP II RF system the maximum circumferential accelerating voltage will be increased from 350 MV at present to about 2000 MV, required for W/sup /spl plusmn pair production at centre-of-mass energies around 180 GeV. The accelerating voltage will be provided by the existing copper RF cavity system (LEP Phase I) and 192 superconducting RF cavities to be installed in LEP by 1995. These sc cavities will be powered by 12 high power klystrons via junction circulators and a WR 2300 waveguide power distribution system. Compared to the existing LEP I installation the output power of the LEP II klystrons has been increased from 1000 to 1300 KW cw by making use of the full capacity of the HV power converters, rated at 100 kV and 40 A. Simultaneously, the power-handling capability of the junction circulators, when operated into a sliding short, could also be enhanced by the same amount. A description of the LEP II RF power generation system is given and, in particular, the improved performance of the high power klystrons, circulators, thyratron crowbars and hard tube modulators is discussed.<<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.