LH transition by a biased hot cathode in the Tohoku University Heliac

Kitajima, S.; Takahashi, H.; Takeda, Yutaka; Utoh, Hiroyasu; Sasao, M.; Takayama, M.; Nishimura, K.; Inagaki, S.; Yokoyama, M.

doi:10.1088/0029-5515/46/2/002

Cited by 28 publications

(39 citation statements)

References 29 publications

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“…Figure 1 shows the top view of TUHeliac. The flux surface is formed by the toroidal field coils, the center conductor coil, and the vertical field coils [3]. The heliac configuration has the flexibility of a rotational transform and the magnetic well depth.…”

Section: Methodsmentioning

confidence: 99%

“…Electrode biasing can drive E × B rotation in a small device resulting in an improved confinement mode. In Tohoku University Heliac (TU-Heliac), negative biasing experiments have been carried out to study the transition to the improved mode [3][4][5][6][7][8][9][10][11]. A hot cathode made of LaB 6 was used for electron injection into the plasma and a negative potential profile was formed.…”

Section: Introductionmentioning

confidence: 99%

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Density Collapse and Fluctuation Observed in Poloidally Rotating Plasma on TU-Heliac

Takeda

Kitajima

Sasao

et al. 2008

Plasma and Fusion Research

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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.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Density Collapse and Fluctuation Observed in Poloidally Rotating Plasma on TU-Heliac

Takeda

Kitajima

Sasao

et al. 2008

Plasma and Fusion Research

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“…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.…”

Section: Inroductionmentioning

confidence: 99%

“…7. Before the application of the external perturbation fields, we measured the velocity of the plasma poloidal rotation by the phase velocity of high-frequency fluctuations in the floating potential [8,15]. The result was that the plasma rotation vanished when the electrode current was about 2A without the external perturbation fields.…”

Section: Phase Shift In Probe Measurementsmentioning

confidence: 99%

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

Kitajima

Umetsu

Sasao

et al. 2008

Plasma and Fusion Research

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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...

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“…8͒ studies pure electron plasmas confined on magnetic surfaces. 17 The results presented in this article focus primarily on a parameter regime where the neutral pressure, p n Ͻ 1.5 ϫ 10 −7 Torr, plasma potential, ͉ p ͉ Ͻ 400 V, and magnetic field strength, B Ͼ 0.01 T. For these parameters, the plasmas are stable and do not exhibit sudden confinement jumps. CNT is concentrated on understanding the equilibrium, stability, and transport of the pure electron plasmas created by this thermionic emission.…”

Section: Introductionmentioning

confidence: 99%

Confinement of pure electron plasmas in the Columbia Non-neutral Torus

et al. 2007

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The Columbia Non-neutral Torus ͑CNT͒ ͓T. S. Pedersen, J. P. Kremer, R. G. Lefrancois, Q. Marksteiner, N. Pomphrey, W. Reiersen, F. Dahlgreen, and X. Sarasola, Fusion Sci. Technol. 50, 372 ͑2006͔͒ is a stellarator used to study non-neutral plasmas confined on magnetic surfaces. A detailed experimental study of confinement of pure electron plasmas in CNT is described here. Electrons are introduced into the magnetic surfaces by placing a biased thermionic emitter on the magnetic axis. As reported previously, the insulated rods holding this and other emitter filaments contribute to the radial transport by charging up negatively and creating E ϫ B convective transport cells. A model for the rod-driven transport is presented and compared to the measured transport rates under a number of different conditions, finding good agreement. Neutrals also drive transport, and by varying the neutral pressure in the experiment, the effects of rod-driven and neutral-driven transport are separated. The neutral-driven electron loss rate scales linearly with neutral pressure. The neutral driven transport, presumably caused by electron-neutral collisions, is much greater than theoretical estimates for neoclassical diffusion in a classical stellarator with strong radial electric fields. In fact the confinement time is on the order of the electron-neutral collision time. Ion accumulation, electron attachment, and other effects are considered, but do not explain the observed transport rates.

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LH transition by a biased hot cathode in the Tohoku University Heliac

Cited by 28 publications

References 29 publications

Density Collapse and Fluctuation Observed in Poloidally Rotating Plasma on TU-Heliac

Density Collapse and Fluctuation Observed in Poloidally Rotating Plasma on TU-Heliac

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

Confinement of pure electron plasmas in the Columbia Non-neutral Torus

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