Active heater control and regulation for the Varian VGT-80 11 gyrotron

On past DIII-D experiments involving the use of Varian VA-8060 gyrotrons for ECH (electron cyclotron heating) power, significant gyrotron heater-emission depletion was experienced for pulse widths >300 ms. This decline in heater-emission directly results in gyrotron-cathode current droop. Since RF power from gyrotrons decreases as cathode current decreases, it is necessary to maintain a constant cathode current level during gyrotron pulses for efficient gyrotron operation. Therefore, it was determined that a filament-heater control system should be developed for the Varian VGT-8011 gyrotron which will include cathode-current feedback. The author discusses the mechanisms used to regulate gyrotron filament-heater voltage by using cathode-current feedback. A 110 GHz ECH system has been completed and is operational using the active filament-heater control and regulation network that has been developed