Optically thin perpendicular electron‐cyclotron emission from hot electrons in TMX‐U

Perpendicular electron‐cyclotron emission (PECE) from relativistic (T H ∼100–400 keV) hot electrons within the thermal‐barrier region of Tandem Mirror Experiment‐Upgrade (TMX‐U) is detected at 35, 60, 94, and 98 GHz. For the operating regime of TMX‐U, these signals are optically thin (τ≪1) and thus proportional to the radial hot‐electron line density. A relativistic code is used to calculate the theoretical temperature dependence of the perpendicular emission coefficient, j ⊥(ω, T H ), for each of the detected frequencies. This dependence has been verified experimentally by x‐ray measurements of the hot electron temperature, T H . The observed qualitative agreement demonstrates that optically thin PECE signals can be used to determine the temporal evolution of T H . An inability to absolutely calibrate the present PECE waveguide system has prevented quantitative agreement.