Operation of a Six Gyrotron System on the DIII-D Tokamak

The gyrotron installation on the DIII-D tokamak has been completed. The system comprises six 110 GHz 1 MW gyrotrons limited administratively to generating pulses up to 5 s in length. Measured transmission line efficiencies are about 75\% for 100 m line length, and injected power $\leq$3.5 MW has been achieved. The rf beams can be directed anywhere in the tokamak upper half plane for co- and counter current drive with complete control of the elliptical polarization. The gyrotrons are modulated by tetrodes either with pre-programmed time dependence of output power or under control of the DIII-D Plasma Control System (PCS) responding in real time to changing experimental requirements. System reliability has consistently exceeded 80\%. Upgrade plans include installation of two more waveguide lines, bringing the total to eight, gradual replacement of the 1.0 MW diode gyrotrons by \hbox{1.5 MW} depressed collector tubes, construction of additional high voltage power supplies, and modification of the launchers to accommodate real time steering of the rf beams under PCS control.