Effects of Rotating Magnetic Islands Driven by External Perturbation Fields in the TU-Heliac

Abstract

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...

Effects of Magnetic Islands Produced by External Perturbation Fields in the Tohoku University Heliac

“…2. The perturbation field coil current I ex was three times greater than that in the previous rotating island experiment [5].…”

“…1. We have explored the possibility of the poloidal rotation of islands by changing the phase of each perturbation coil current [5]. In this experiment, to study the island structure located at fixed poloidal positions, we used only two pairs of external perturbation field coils at the toroidal angles φ = 90 • and 270 • .…”

“…The result suggested that the poloidal viscosity increased with author's e-mail: sumio.kitajima@qse.tohoku.ac.jp increasing magnetic island width [4]. Therefore, it is expected that plasma poloidal rotation will be driven by the poloidal rotation of the island [5].…”

Effects of Magnetic Islands on Poloidal Flow in TU-Heliac

“…The vacuum field calculations showed that the perturbation field produced by these coils can produce the m = 3 magnetic island at a rational flux surface. By using alternative currents and phase control in the perturbation field coil current, these coils can produce a rotating magnetic island [10]. In this experiment, to study the relationship between plasma flow and magnetic island at a fixed poloidal position, we used only two pairs of external perturbation field coils at toroidal angles φ = 0 • and 180 • .…”