Measurement of Radio Frequency Wave Induced Electron Density Fluctuations by a Microwave Reflectometer on LHD

A. Ejiri, T. Tokuzawa, K. Saito, T. Seki, H. Kasahara, T. Mutoh, R. Kumazawa, I. Yamada, Y. Takase
2014 Proceedings of the 12th Asia Pacific Physics Conference (APPC12)   unpublished
Radio frequency (RF) waves are useful tools for heating plasma and for driving a current in plasma. In order to understand RF wave heating and to develop an efficient heating scenario, a qualitative internal RF field measurement should be established. Microwave reflectometry is one such measurement, and it measures electron density fluctuations induced by RF electric fields. An O-mode microwave reflectometer with a probing frequency of 30.5 GHz has been designed and installed on LHD to measure
more » ... on LHD to measure RF induced electron density fluctuations near the ICRF antenna located at port 3.5. The target plasma was heated by the ICRF wave using the hydrogen minority heating scenario. The frequency was 38.47 MHz, and the discharges were sustained by the ICRF power alone. RF field can be estimated from the measured density fluctuation level with the help of dispersion relation for the ICRF wave. The right figure shows the estimated RF field (perpendicular to the magnetic field and the radial direction) as a function of cutoff position. The RF induced density fluctuation level was less than 0.03 % for this data. As shown in the figure, the RF field increases with the cutoff position. It should be noted that the reflectometer measures the cutoff density of 1.1 x 10 19 m -3 , and the density profile changes with the horizontal axis. Fig. RF electric field as a function of cutoff position.
doi:10.7566/jpscp.1.015038 fatcat:ocdntf46unb55kojowpuvjsud4