Improved system for perpendicular electron‐cyclotron emission measurements on TMX‐U

Perpendicular electron‐cyclotron emission (PECE) is used on Tandem Mirror Experiment‐Upgrade (TMX‐U) to diagnose thermal‐barrier hot electrons (T H ∼100–400 keV, yielding the time history of the temperature of these relativistic electrons. We describe an improved quasioptical viewing system for these measurements that uses high‐sensitivity superheterodyne receivers at fixed frequencies of 60, 98, 130, and 196 GHz. The improved viewing and transport system consists of an off‐axis ellipsoidal mirror that images the plasma onto a V‐band conical collection horn, an overmoded circular waveguide ( 7/8 in. diam) that transports the radiation outside the vacuum vessel where the polarization is selected, and a high‐absorptivity Macor beam dump to prevent internal wall reflections from entering the viewing system. A relativistic code is used to calculate optically thin PECE signals from relativistic electrons for various energy and pitch angle distributions.