Phase and voltage control in the LEP radio-frequency system

The LEP (Large Electron Positron collider) RF system is of modular construction, the basic module, an RF unit, consisting of 16 coupled accelerating cavity/storage cavity assemblies driven by two 1-MV klystrons. The klystrons operate at slightly different frequencies (f₁ and f₂), with the difference determined by the bunch spacing in LEP. The units are synchronized with the frequencies f₀=352.254024 MHz and f_B=44.982 kHz. From the reference frequencies, the klystron frequencies f₁=f₀-f _B and f₂=f₀+f_B are generated and applied to the klystrons through two identical chains of control electronics. The klystron phase loop compensates for phase variations in the klystron and the circulator. The input power to the klystrons is controlled from the drive level control system, which includes an AGC (automatic gain control) loop. The cavity gap voltage is controlled by a voltage loop acts on the modulation anode of the klystron. The correct waveform is obtained with a differential loop which corrects for differences in output power from the two klystrons. For use in the servo loops 350-MHz control elements have been developed. The most important elements are the voltage-controlled attenuators, which have low phase-shift variations, and the voltage detectors, which have low voltage standing wave ratio. The analog and digital variables can be controlled either locally by a touch screen connected to a data manager or remotely through the LEP control system. A large number of monitoring and status signals are sent to the control system; control voltages for phase shifters, voltage-controlled crystal oscillators and attenuators, detector voltages and loop status signals are some of the signals sent