Production and heating of overdense plasmas by mode-converted electron Bernstein waves at very low toroidal field in the Compact Helical System

In the Compact Helical System [S. Okamura et al., Nucl. Fusion 39, 1337 (1999)], overdense plasmas whose electron density exceeds the cutoff density were produced and heated for the first time by perpendicularly and/or obliquely launching of 2.45GHz electron cyclotron waves (ECWs) at very low toroidal field in a very wide range of magnetic field of 20%–120% for the resonant field Bres(=0.0875T). In these overdense plasmas, absorbed ECW power density measured by the power modulation technique concentrated inside the high density region beyond the cutoff layer for various magnetic configurations where electron cyclotron and upper hybrid resonance layers are scanned in space widely. This fact clearly demonstrates that the electron Bernstein wave (EBW) excited mainly through “fast extraordinary wave-to-EBW” and/or “ordinary wave-to-slow extraordinary wave-to-EBW” mode conversion processes plays a key role in producing and heating overdense plasmas.