Electron Heating and Potential Formation due to Mode Conversion of ICRF Fast Waves

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Effects of an ICRF wave injected into the central cell for formation of the ion confining potential were investigated in the HIEI tandem mirror. Measurements on wave propagation revealed that the fast magnetosonic wave propagating into the plug cell converts into an electrostatic wave with resonance phenomenon when it encounters the ion-ion hybrid resonance layr from the high-field side. In this case, rise of electron temperature in the plug cell was observed in the measurement on intensities of atomic emission lines. Simultaneously, measurements by the end loss energy analyzer showed that the ion confining potential (- 70 V) is formed. The axial particle confinement time was found to increase by a factor of 3-4, which leads to reduction of the end ion loss and density increase in the central cell. The mechanism of potential formation would be the velocity space diffusion of electrons by ICRF waves.In the combination of such an improved axial confinement and the improved radial confinement by the limiter biasing (Physical Review Letters, Vol.70(1993)pp.4071-4074), a plasma with high temperature (ion temperature : 0.1 keV) and high density (10^<13>cm^<-3>) was realized, where the beta valuereaches - 14%. The global energy confinement time increases by a factor of -4, where the axial and radial energy confinement times increase by a factor of 3.8 and 4.0, respectively. The production of such a high-beta plasma results from the enhancement on the global confinement. This high-beta plasma is sustained within the stability criteria by ponderomotive force of propagating ICRF electric fields.We have concluded that electron heating and potential formation due to mode conversion of the ICRF fast wave is effective on the enhancement on confinement of a tandem mirror.