Direct Measurement of a Mode-Converted Electron Bernstein Wave in the Internal Coil Device Mini-RT

Abstract

An electric field measurement system in the Electron Cyclotron Range of Frequency (ECRF) has been developed for the direct detection of mode-converted Electron Bernstein Waves (EBWs), where a low-power ECRF wave (1-2.1 GHz, 10 W) is injected into overdense plasmas (typical peak density is 7 × 10¹⁶ m^-3 ), and the ECRF electric field is directly measured using a small monopole antenna (an element size of 5 mm) inserted into the confined plasma. A density gradient is an important parameter for mode conversion to EBW. In the internal coil device, Mini-RT, a steep density gradient (e.g., L_n ∼ 5 cm) is available at the plasma surface. We have detected short-wavelength waves with refractive indices of ∼10 or more around the electron cyclotron harmonic resonance layers, and confirmed a phase reversal of an ECRF electric field at the same locations. Since the EBW is a backward wave with a short wavelength, experimental results suggest the direct measurement of the mode-converted EBW in plasma.