Third harmonic electron cyclotron heating of overdense plasmas in Heliotron DR

Third harmonic electron cyclotron heating has been experimentally studied in Heliotron DR (R = 90 cm, a-bar ≅ 7 cm) by using a 28 GHz, 200 kW gyrotron. When the line averaged electron density n-bare exceeds a certain critical value, ne,crit ∼ 0.5 x 1013/cm3, the absorption power increases rapidly and plasmas are substantially heated as long as the third harmonic resonant layer is located within the last closed magnetic surface. On the other hand, heating of the bulk plasma is not effective at lower density, n-bare < ne,crit, although strong hard X-rays are emitted, which suggests the generation of high energy electrons. In the high density regime, the central plasma region is heated even when the electron density is much higher than the cut-off value of the O-mode (overdense plasma), almost independent of the location of resonant layers. It is found that overdense plasmas produced by third harmonic heating have basically the same characteristics as plasmas produced by second harmonic heating in Heliotron DR. The absorption of the electron Bernstein wave generated via a mode conversion process is considered to be the heating mechanism of overdense plasmas at the third harmonic resonance. (author). 23 refs, 9 figs