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An advanced probe measurement system consisting of high-speed Langmuir probes with a preamplifier and a Copper shield was designed and installed in TU-Heliac. Potential and density fluctuations were measured in the hot-cathode biased plasma, and power spectra were calculated using the complex Fourier transform. There were low frequency fluctuations ( f < 10 kHz), high frequency fluctuations (70 < f < 200 kHz) and sharp spectra applied for the plasma production (10 < f < 50 kHz). In the region above 200 kHz, the power decreased by a factor of 1/ f 2 for the potential fluctuation and 1/ f 2∼3 for the density fluctuation. The phase shift between potential and density fluctuations was almost 0 rad at ρ = 0.54. On the other hand, the phase shift was not 0 rad at ρ = 0.21, especially in the 100 ∼ 200 kHz region. The high frequency fluctuation at ρ = 0.21 grew on the time scale of 10 −5 s, which was obtained from the time dependent signals of the high frequency fluctuation.

The ohmic heated plasma in TU-Heliac was biased by a hot-cathode to drive E × B poloidal rotation. Coincident measurements of the line density and ion saturation currents revealed the existence of density collapse accompanied by fluctuation in the poloidally rotating plasma. The radial density profiles just before and after the collapse were estimated. The steep density gradient vanished with the density collapse. The power spectra of the fluctuation were calculated from the ion saturation current obtained with a high-speed triple probe. The frequency of fluctuation was compared to the E × B poloidal rotation frequency, where the fluctuation appeared with density collapse. The fluctuation frequency was 2 to 3 times the rotation frequency. This suggested that the poloidal mode number of fluctuation was m =2 or 3.

A new method for rotating magnetic islands by external perturbation fields is proposed. In the experiments, perturbation fields were produced using four pairs of cusp field coil, in which alternating currents having a π/2 phase shift flowed. A phase shifter for the coil currents was designed and constructed. The phase difference in the floating potential signals measured using two Langmuir probes confirmed that the magnetic islands rotated in the counterclockwise direction (c/cw). The clockwise (cw) rotation was also observed in the plasma biased by the hot cathode electrode. These experimental results suggest the ability of producing plasma poloidal rotation driven by rotating islands. InroductionA study on the effects of magnetic islands on transport in helical devices is important, because it leads to the advanced control method for a plasma periphery in a helical fusion reactor. The magnetic island effects on transport have been surveyed widely in the Large Helical Device (LHD) [1][2][3][4][5][6]. For the research on the island effects on the confinement modes, the Tohoku University Heliac (TU-Heliac) has advantages that (1) the position of a rational surface is changeable by selecting the ratio of coil currents, (2) the island formation can be controlled by external perturbation field coils, and (3) a radial electric field and particle transport can be controlled by electrode biasing. In the TU-Heliac, the improved mode transition was triggered by electrode biasing using a hot cathode composed of LaB 6 . The driving force J × B for the plasma poloidal rotation was externally controlled, and the poloidal viscosity was successfully estimated from the external driving force [7][8][9]. In recent experiments, the ion viscosity in the biased plasma with islands was roughly estimated. It suggested that the ion viscosity increased with the magnetic island width [10]. Therefore, it is expected that the plasma poloidal rotation will be driven by the poloidal rotation of the island. The purposes of this experiment are to propose the new method of rotating islands by the external perturbation fields, survey the ability of the plasma poloidal rotaauthor's e-mail: sumio.kitajima@qse.tohoku.ac.jp tion driven by rotating islands, and study the rotating island effects on confinement modes in TU-Heliac. Experimental Setup TU-HeliacThe TU-Heliac is a four-period heliac (major radius, 0.48 m; average plasma radius, 0.07 m). The heliac configurations were produced using three sets of magnetic field coils; 32 toroidal field coils, a center conductor coil, and one pair of vertical field coils, as shown in Fig. 1 (a). Three capacitor banks, consisting of two-stage pulse forming networks, separately supplied coil currents of 10 ms flat top [11]. The target plasma for external perturbation fields was He plasma produced by low-frequency joule heating ( f = 18.8 kHz, P out ∼ 35 kW). The joule heating power was supplied to one pair of the poloidal coils wound outside the toroidal coils [12]. The vacuum vessel was filled with fueling n...