Effect of Cathode Coil on the Commissioning of 42-GHz Gyrotron for ECRH System in SST-1 and Aditya-U

A new 42 GHz Gyrotron capable to deliver 500 kW power for 500 ms duration has been commissioned successfully to renew the existing ECRH system for SST-1 and Aditya-U. The Gyrotron delivers 500 kW power at ~50 kV beam voltage and draws 20 A beam current. The Gyrotron was tested at factory for full parameters (500 kW-500 ms) with a different cryomagnet and commissioned at IPR on another cryomagnet magnet of existing 42 GHz ECRH system on SST-1 and Aditya-U. The new Gyrotron installed successfully on the existing cryomagnet and tested for the pulse operation. The burn pattern at the exit of matching optic unit ensures good gaussian output of Gyrotron. The Gyrotron started testing on dummy load for high power and long pulse operation, while increasing power beyond 350 kW at 42 kV, it was observed that the beam current increases with beam voltage and power remains constant. It was not safe condition for the Gyrotron as it would increase the thermal loading on the collector. An analysis is carried out to study the distributions of magnetic field with different magnetic configuration. Accordingly, a suitable cathode coil is designed to control the regime of Gyrotron operation. The cathode coil is installed on the Gyrotron below the cryomagnet in between cathode and cavity. The effect of cathode coil is observed gradually by varying the current from 0.5 to 2.0 A. As a results, the effect of uncontrolled growth of beam current was not observed and beam voltage can be increased safely up to 50 kV which is a required value for full power. Finally, the Gyrotron is tested successfully for full power 500 kW at 50 kV beam voltage and around 18 A beam current. The anode voltage was set to +22 kV, cryomagnet current was 28.2 A, and current in cathode coil was 1.0 A for full power operation. The efficiency of Gyrotron is more than 50% as actual input power is ~1 MW (50 kV-20 A), if we include anode voltage also with net accelerating voltage ~70 kV, the net electronic efficiency of Gyrotron is around 37%.