DIII-D Electron Cyclotron Heating System Status and Upgrades

The DIII-D electron cyclotron heating (ECH) system consists of six 110-GHz gyrotrons with corrugated coaxial 31.75-mm waveguide transmission lines, and steerable launching mirrors. The system has been gradually updated, leading to an increased experimental flexibility and a high system reliability of 91% in the past year. Operationally, the gyrotrons can generate up to a total of 4.8 MW of RF power for pulses up to 5 seconds. The maximum ECH energy injected into the DIII-D is 16.6 MJ. The HE1,1 mode content is over 85% for all the lines, and the transmission coefficient is better than -1.1 dB for all the transmission lines, close to the theoretical value. A new depressed collector gyrotron was recently installed and was injected up to 640 kW of power into the plasma during 2014-2015 tokamak operations. Four dual waveguide launchers, which can steer the RF beams ±20° poloidally and toroidally, are used for real-time neoclassical tearing mode control and suppression. The launchers now have increased poloidal scanning speed and beam positioning accuracy of approximately ±2 mm at the plasma center. Two more gyrotrons are expected to be installed and operational in 2015-2016. The first is a repaired 110-GHz, 1-MW gyrotron that had a gun failure after more than 11 years of operation at DIII-D. The second is a newly designed depressed collector tube in the 1.5 MW class, operating at 117.5 GHz, manufactured by Communications and Power Industries.