O-X-B Heating of Overdense Plasmas by 54.5 GHz Electron Cyclotron Waves in CHS

Abstract

An evident increase in the plasma stored energy by applying 54.5 GHz electron cyclotron (EC) waves has been observed in plasmas sustained by neutral beam injection (NBI) with line-averaged electron density of above 3.8 × 10¹⁹ m^-3, that is, the cut-off density of the 54.5 GHz waves, in the Compact Helical System (CHS). The heating effect can be seen even for high-density over 8 × 10¹⁹ m^-3: more than twice the cut-off density. The 54.5 GHz EC-wave beam is obliquely injected to high-density plasmas. The scanning of the EC-wave beam direction gives the evident increase in the stored energy at the beam direction where the leakage EC-wave power from the CHS vacuum vessel becomes almost the minimum. The most possible cause for this heating mechanism is an electron Bernstein wave heating via an Ordinary - eXtraordinary - Bernstein (O-X-B) mode conversion process.