Evident increases in the plasma stored energy by applying 54.5-GHz electron cyclotron (EC) waves have been observed in overdense plasmas sustained by neutral beam injection in the Compact Helical System. The heating effect was seen even for a high density of 8 × 1019 m-3, that is, more than twice the cutoff density of 3.8 × 1019 m-3 of the 54.5-GHz waves. The 54.5-GHz EC wave beams were obliquely injected into high-density plasmas. Dependences of the heating effect on the experimental conditions such as the polarization and the injection power of the EC waves, and the magnetic field were investigated. A higher left-hand circular polarization fraction and higher injection power resulted in a longer plasma duration time and a higher increment of the plasma stored energy. Variation of the electron temperature profile in the magnetic field scan experiment shows the power deposition in the plasma core region inside the plasma cutoff layer. These experimental results show that the main cause for this heating mechanism is electron Bernstein wave heating via an Ordinary-eXtraordinary-Bernstein (O-X-B) mode conversion process.
