In the DIII-D electron heating and current drive installation, up to six gyrotron microwave generators in the 1-MW class at pulse lengths up to 5 s have been operated simultaneously. The frequency for all the gyrotrons is 110 GHz, corresponding to the second harmonic of the electron gyrofrequency at 2 T. The peak generated power has been >4 MW with peak injected power slightly greater than 3 MW. The radio frequency (rf) generators are located remotely and are connected to the tokamak by up to 100 m of evacuated circular corrugated waveguide carrying the HE1,1 mode with overall transmission efficiency, including coupling to the waveguide, of up to 75%. Ancillary equipment for polarization control, beam switching, power monitoring, control of launch direction, and system protection has been developed.
The system has been used to support a wide variety of physics experiments, including control of magnetohydrodynamic modes, current density profile modifications, basic plasma heating and current drive, transport studies, and rf-assisted start-up. The gyrotron complex is being upgraded by the acquisition of additional tubes with 5- to 10-s pulse length capability.