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

The Compact Toroidal Hybrid (CTH) is an $\ell=2, m=5$ torsatron/tokamak hybrid ($R_0=0.75$\,m, $a_p\sim 0.2$\,m, and $|B|\leq 0.7$\,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. New studies of edge plasma transport in stellarator geometries will benefit from CTH operating as a pure torsatron with a high temperature edge plasma. Accordingly, a 28\,GHz, 200\,kW gyrotron operating at 2nd harmonic for ECRH is being installed to supplement the existing 15\,kW klystron system operating at the fundamental frequency; the latter will be used to initially generate the 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.\\ $[1]$N.B. Marushchenko, Y. Turkin, H. Maassberg, Comp. Phys. Comm. 185 165 (2014)