Design and Implementation of a 200kW, 28GHz gyrotron system for the Compact Toroidal Hybrid Experiment

The Compact Toroidal Hybrid (CTH) is an l = 2,m = 5 torsatron/tokamak hybrid (R₀ = 0.75 m, a_p ∼0.2 m, and |B| ≤ 0.7 T). It can generate its highly configurable confining magnetic fields solely with external coils, but typically operates with up to 80 kA of ohmically-generated plasma current for heating. A 28 GHz, 200kW pulsed gyrotron system operating at 2nd harmonic for ECRH is being installed to supplement the existing 10kW klystron system operating at the fundamental frequency; the latter will be used to generate the target plasma. Ray-tracing calculations that guide the selection of launching position, antenna focal length, and beam-steering characteristics of the ECRH have been performed with the TRAVIS code[1]. The calculated absorption is up to 95.7% for vertically propagating rays; however, the absorption is more sensitive to magnetic field variations than for a side launch where the field gradient is tokamak-like. The design of the waveguide path and components for the top-launch scenario will be presented, as well as initial ECRH results.

[1]N.B. Marushchenko, Y. Turkin, H. Maassberg, Comp. Phys. Comm. 185 165 (2014)