Control Electronics of the Pep

The opera&on of the major components used for controlling the phase and field level of the PEP RF cavities is described. The control electronics of one RF station is composed of several control loops: each cavity has a tuners' servo loop which maintains the frequency constant and also keeps the fields of each cavity balanced; the total gap voltage developed by a pair of cavities is regulated by a gap voltage controller; finally, the phase variation along the amplification chain, the klystron and the cavities are compensated by a phase lock loop. The design criteria of each loop are set forth and the circuit implementation and test results are presented. 1. Cavity Tuners' Servo Two types of perturbations are dealt with here: one is the detuning of the cavity due to thermal expansion and to reactive beam loading, the other is an instability of the cavity brought about by an uneven temperature distribution which can result in a run away condition, where more and more of the stored energy concentrates in one end of a cavity. The first perturbation is stabilized by a feedback loop controlling the common penetration of the tuners. The second perturbation can be minimized by the differential positioning of the tuners (Fig. 1). The design of the tuning control loop is based on the data of Fig. 2 which represents the cavity detuning as a function of the tuner's travel. Between start-up, with a cooling water temperature of 70°F, and full power, with a cooling water temperature of 95'F, we have measured a total detuning of 300 kHz and a tuner's travel of 70 mm. Let us assume that the phase across the cavity is to be held constant within 0.2' over this range of operation; the phase detector signal which corresponds to 0.2' is 5.4 mV and it must be amplified so as to produce a tuner motion of 70 mm. The phase detector will be described subsequently in this report. The resolution of the tuner's positioning servos is determined in the following way: the cavity 3 dB bandwidth being taken as 17 kHz, with the help of the curve of Fig. 2, one finds that a phase variation of 90' is obtained with a tuner travel of 2 mm. We desire a resolution of 20.2'; this corresponds to a tuner motion Fig. Block diagram of the cavity servo loops. * Work supported by the Department of Energy, contract DE-AC03-76SF00515. 500 -