Impact of gyrotron power modulation on the collector of the 2 MW, 170 GHZ gyrotron for ITER

In ITER it is foreseen to control different types of MHD instabilities by means of modulated Electron Cyclotron Current Drive (ECCD) with modulation frequencies up to 5 kHz. This has direct consequences for the EC system of ITER, since both the gyrotons and power supplies have to be capable of modulated operation. Since the electron gun of the 2 MW EU gyrotron is a diode type gun and the gyrotron is equipped with a depressed collector, two modulation scenarios were investigated: Modulation with the body power supply (BPS): In this case the gyrotron output power will be controlled by the voltage of the BPS. The required change of the BPS voltage will reduce both the output power and efficiency of the gyrotron but will also reduce the deceleration voltage of the depressed collector. This fact, together with the reduced efficiency, will increase the power load on the collector wall significantly.; Modulation with the main power supply (MPS): In this case the output power is controlled by a reduction of the voltage provided by the MPS. Since the deceleration voltage of the depressed collector will not be affected, the power load on the collector is only influenced by the reduced interaction efficiency.