A RF phase and amplitude control system for helical postaccelerator

A helical postaccelerator for heavy ion acceleration is under construction in Peking University. Either the 4.5 MV Van de Craaff facility or the EN tandem with terminal voltage of 6.5 MV will be used as injectors. The postaccelerator consists of one helix resonator used as a buncher, two helix loaded resonators and one integral split-ring loaded resonator. Because of the low mechanical rigidity of the helix, the amplitude and phase control become primary important. The major effect will be vibrations caused by ponderomotive force at high power level. To compensate the frequency shift of 17.7 KHz/KW, it is necessary to use a slow frequency tunning loop e.g. as with power input from 0-50 KW, the frequency of the helix loaded cavity will change about 147.5 KHz. To ensure good beam quality, fast closed control loops of RF phase and amplitude are also important so as to maintain the phase and amplitude of the field in each cavity within ±1deg and ±1 % respectively. (author)