Studies on High-Density Helicon Wave Plasmas in Toroidal Magnetic Field

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High-density (n_e> 10^<18> m\<-3>) argon plasmas have been produced with RF power off =13.6 MHz and P<10 kW in a toroidal helical device, Heliotron DR (major and minor radii are 0.9 m and 0.11 m, respectively). An elliptical loop antenna is used to excite helicon waves in the toroidal vacuum vessel. Two types of matching circuits have been examined, the RF antenna is electrically (a) grounded or (2) floated from the vacuum vessel. The toroidal magnetic field is 50G-900G and filling gas pressure is 2x10^<-5> Torr -5x10^<-3> Torr. To investigate plasma performance, two electrostatic and one magnetic probes are installed around the torus. When the antenna is floated from the vacuum vessel, the discharge changes from low-density to high-density mode as the RF power and/or gas pressure are increased. At the present input power level of about 6 kW, the toroidally uniform plasmas with n_e= 2x10^<18>m^<-3> are produced. If the gas pressure is increased further high-density plasmas with n_e > 10^<19> m^<-3> are obtained but they are localized near the antenna and not uniform around the torus. Such a non-uniformity in the toroidal direction will be solved by increasing the RF input power.