Study on RF Tandem Mirror

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The main purpose of the RF tandem mirror project is to demonstrate plasma confinement in purely axisymmetric magnetic field configuration by using ICRF power only. We have revealed that the plasma production, MHD-stabilization, ion heating, and plug and thermal barrier potential formation can all be achieved by a combination of the central RF and the plug RF.The highest peak density produced by the helicon/fast wave excited by the central RF is 1.8X10^<13>cm^<-3> with the beta value of - 10 % for helium plasma. In a two-ion-species plasma, ion temperatures of Ti_1 - 220 eV have been achieved at densities of 0.4x10^<13>cm^<-3>. This heating is due to the mode conversion of the helicon/fast wave into the slow Alfven wave at the hybrid resonance. The plasma is MHD-stable due to the RF ponderomotive effect. The central RF produces the potential dip at the plug midplane. This dip is formed by the sloshing ion distribution created by the minority ion heating in the plug cell. The plug potential phi c of 60- 80V is observed resulting in reduced end loss flux. The axial confinement time is improved by a factor of 2-3. By the additional application of the plug RF, the thermal barrier potential of phi B - 90 V is created with phi c - 100 V. The value of phi B increases with the plug RF power. it is likely that the barrier pumping is provided by the non-ambipolar ion convection driven by the azimuthal ponderomotive force of the plug cell RF.