MIXED-MODE DISTRIBUTION SYSTEMS FOR HIGH AVERAGE POWER ELECTRON CYCLOTRON HEATING.

The ELMO Bumpy Torus-Scale (EBT-S) experiment consists of 24 simple magnetic mirrors joined end-to-end to form a torus of closed magnetic field lines. In this paper, the authors first describe an 80% efficient mixed-mode unpolarized heating system which couples 28-GHz microwave power to the midplane of the 24 EBT-S cavities. The system consists of two radiused bends feeding a quasi-optical mixed-mode toroidal distribution manifold. Balancing power to the 24 cavities is determined by detailed computer ray tracing. A second 28-GHz electron cyclotron heating (ECH) system using a polarized grid high field launcher is described. This system uses the same mixed-mode output as the first system but polarizes the launched power by using a grid of WR 42 apertures. The efficiency of this system is 32%, but can be improved by feeding multiple launchers from a separate distribution manifold.