DIII-D Electron Cyclotron Heating System Status and Upgrades

Abstract

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.