CLOSED LOOP RF TUNING FOR SUPERCONDUCITNG CYCLOTRON AT VECC

The RF system of Superconducting cyclotron has been operational within 9 - 27 MHz frequency. It has three tunable half-wave coaxial cavities as main resonators and three tunable RF amplifier cavities. A PC-based system takes care of stepper motor driven coarse tuning of cavities with positional accuracy ~20μm and hydraulically driven three couplers and three trimmers. The couplers, in open loop, match the cavity impedance to 50 Ohm in order to feed power from RF amplifier. Trimmers operate in closed loop for fine tuning the cavity, if detuned thermally at high RF power. The control logic has been simulated and finally implemented with Programmable Logic Controller (PLC). Precision control of trimmer (~20μm) is essential to achieve the accelerating (Dee) voltage stability better than 100 ppm. and also minimizing the RF power to maintain it. Phase difference between Dee-in and Dee-pick-off signals and the reflected power signals (from cavity) together act in closed loop for fine tuning of the cavity. The closed loop PID control determines the final positioning of the trimmer in each power level and achieved the required voltage stability.IMP;Chinese Academy of Science