Steady-State Operation of High CW Power Circulator: Challenges and Solutions through Simulation and Experiments

The lower hybrid current drive (LHCD) system of the steady-state superconducting tokamak-1 (SST-1) has been augmented with two more klystrons, each rated for 500-kW continuous wave (CW) power at 3.7 GHz to increase its power rating to 2-MW CW. To protect these klystrons at rated power, the existing 250-kW CW circulators, connected to each of the klystrons, are required to be upgraded to 500-kW CW rating. Simulation studies are carried out using COMSOL Multiphysics to ascertain the key parameters which limit its performance to the desired value. In this article, we present the simulation and experimental studies of the high power circulator, highlighting major challenges and its solutions. The effect of ferrite temperature on the RF performance of the circulator is analyzed with the help of multiphysics simulations. Accordingly, the configuration of the circulator is modified. A series of low power tests are carried out to characterize the circulator performance. High power tests, up to a few seconds, have been successfully carried out to ensure that no breakdown is observed at rated power. The results from the simulation studies and experiments have enabled us to conclude that the circulator can handle a forward power of 400-kW CW under matched load conditions.