An overview of the rf system of K500 superconducting cyclotron at VECC

The radio-frequency system's role in a cyclotron/accelerator is to provide high rf voltage across the accelerating electrodes, which in turn impart energy to the particles each time they cross the gap. The electrodes along with the tuning elements form rf cavity. Because of geometrical and mechanical constraints, we have vertical coaxial resonator system in the superconducting cyclotron (SCC) which has three dees placed 120 degree apart in the valleys. The rf cavities that handle high rf power are not superconducting. These cavities are cooled by normal LCW. The use of superconducting magnet in SCC results in high bending power available and its effective utilization needs very high energy-gain per turn. This is achieved by having multiple (three) dees. Also the use of fundamental mode (h=l) requites a 3-phase rf system i.e. three dees are fed separately from three chains of amplifiers maintaining 120 degree phase difference between each of them. A wide frequency variation (1:3) is mostly selected to facilitate acceleration of a large cross section of the ions. The handling of high rf power, high dee voltage and high rf current requires very special precision fabrication techniques, high and clean vacuum conditions, special material and components. The challenges involved are in the attainment of high amplitude and phase stability, maintaining 120 degree phase difference between the rf phases of various dees, in spite of cross-talking between them. Each cavity along with its driver amplifier is to be aligned using tens of servo controls and other feedback loops to achieve the required stability. As can be seen, the job requires handling of hundreds of kilowatts of rf power, DC power supplies, sophisticated feedback and control circuits. crowbar circuits and PLC-based protective interlocks for the safe and reliable operation of the system. (author)