Electron heating experiment using the high harmonic fast wave on LHD

Electron heating experiments using the High Harmonic Fast Wave (HHFW) were performed on the Large Helical Device (LHD). An electron temperature increase from 2.5 keV to 3.6 keV by 1.2 MW of HHFW was observed when both ECH and NBI were used to create a target plasma with high stored energy and electron temperature. When ECH works effectively, the electron density is pumped out, but the rate of decrease of the electron density is reduced when HHFW heating is applied. This result indicates that HHFW is absorbed effectively when the central electron temperature and electron beta are high enough, and suggests that parallel heating of electrons by HHFW reduces electron density pump out caused by perpendicular electron heating by ECH. According to a 1-D calculation, raising the density is more effective for improving single-pass damping than raising the temperature. According to a 2-D full-wave calculation, electron damping occurs in an off-axis region at low density, but wave fields become more concentrated in the core and absorption becomes more centrally localized at high density.