Gyrotrons for magnetic fusion applications at 110 GHz and 170 GHz

Two megawatt-class gyrotrons at frequencies of 110 GHz and 170 GHz have recently been fabricated at CPI. The 110 GHz gyrotron is designed to produce 1.2 MW of output power for 10-second pulses, and will be used for electron cyclotron heating and current drive on the DIII-D tokamak at General Atomics. This gyrotron has completed factory testing and has been delivered to General Atomics for installation and additional testing. The 170 GHz gyrotron, though specified as a 500 kW CW system, has been designed with the goal of generating up to 1 MW CW. Oak Ridge National Laboratory will use this gyrotron in ITER ECH transmission line testing. This gyrotron has been fabricated and is awaiting factory testing, Design features of each gyrotron are described, and test data for the 110 GHz gyrotron are presented. 1 Design and testing of a 1.2 MW, 110 GHz, 10-second gyrotron CPI's VGT-8115 gyrotron is specified to generate 1.2 MW of output power at 110 GHz, for pulse lengths up to 10 seconds. The gyrotron design is based on that of an earlier prototype (1), but incorporates several improvements, including a larger collector formed from a strengthened copper alloy, as well as a numerically-optimized dimpled-wall launcher and phase-correcting mirror system. The gyrotron operates in the magnetic field produced by a 43 kG superconducting magnet system (SCM) with three separately controllable axial coils (two for generating the desired field in the gyrotron cavity region, and one for adjusting the field at the cathode location) and two orthogonal sets of transverse field coils to ensure that the electron beam is centered in the cavity. The SCM cryostat maintains the necessary temperatures for the superconducting coils using a sealed refrigeration system that does not consume liquid cryogens. The gyrotron employs a single-anode magnetron injection gun to generate an annular electron beam. The electron beam interacts with the TE22,6,1 mode of the cylindrical interaction cavity, and this mode is converted to a Gaussian output beam using the optimized launcher and mirror system. The Gaussian beam passes through an edge- cooled chemical vapor deposition (CVD) diamond disc window. The depressed collector dissipates the residual power in the electron beam by using a combination of iron shielding to expand the size of the annular beam and active sweeping with a large collector coil to vary the beam strike point and lower the time-averaged power-density. A schematic of the gyrotron layout is shown in Figure 1.