Absorption Analysis of Electron Cyclotron Waves in the Magnetospheric Plasma Device RT-1

The absorption efficiency of electron cyclotron heating is investigated theoretically and experimentally to understand the wave heating mechanisms under the overdense state and the density limit. The features of self-organizing mechanisms have been observed in the dipole confinement system [M. Nishiura et al., Nucl. Fusion 55, 053019 (2015)]. The modulated 2.45 GHz electromagnetic (EM) wave is applied to the RT-1 plasmas to evaluate the EM wave's absorption efficiency from the diamagnetic signals' response. The absorption efficiency maintains a constant 100% beyond the O-mode's cutoff density. However, it decreases rapidly near the 1.6 x 10(17) M-3 line-averaged density, which is twice higher than the 2.45 GHz O-mode cutoff. At less than 0.6 x 10(17) M-3, the absorption efficiency simulated by a ray-tracing code in a two-dimensional model explains the experimental absorption efficiency. However, it deviates from the experimental value near the cutoff density and is more significant at the density limit. We discuss the difference between the experimental and numerical results. (C) 2022 The Japan Society of Plasma Science and Nuclear Fusion Research