Correlation between Low-Frequency Fluctuations and Plasma Stored Energy in GAMMA 10

Abstract

The instabilities observed in many magnetic confinement devices induce radial particle transport. Density and potential fluctuations were measured using a gold neutral beam probe in GAMMA 10. The plasma stored energy repeatedly increases and decreases during ion-cyclotron range of frequency heating. A drift-type fluctuation was observed in both the density and the potential as the plasma stored energy decreased. These results indicated a correlation between the radial particle transport induced by drift-type fluctuations and decreasing plasma stored energy. In magnetically confined plasmas, particle and energy confinement are important because particle transport decreases the density and temperature. The plasma particles escape across the confinement magnetic field as a result of fluctuations due to instabilities in the plasma, which cause fluctuations that lead to the transport [1]. Drift waves, called universal instability, have been observed in many devices because of the plasma density gradient [2-4]. Therefore, it is important to study the mechanisms of particle transport due to fluctuations. In this paper, we present a correlation between low-frequency fluctuations and the plasma stored energy in the GAMMA 10 tandem mirror.Experiments were conducted in the GAMMA 10 tandem mirror. The lengths of the central, anchor, and plug/barrier cells are 6.0, 4.8, and 2.5 m, respectively. The magnetic field strength at the mid-plane of the central cell is 0.41 T in standard operation. The plasma is produced by plasma guns and is heated and sustained using ion cyclotron range of frequency (ICRF) heating systems. The plug/barrier cells are located at both ends of GAMMA 10, where the ion and electron axial confinement potentials are produced by electron cyclotron resonance heating.Density and potential fluctuations are measured by a gold neutral beam probe (GNBP) system [5][6][7]. The probe beam is neutral gold particles. The energy and incident angle of the primary beam passing the plasma center are about 12 keV and 40 • to the horizontal direction, respectively. A typical primary beam current of 2 µA is confirmed from a Faraday cup measurement. A parallelplate-type electrostatic energy analyzer with an incident angle of 45 • to the ground plate is installed. In the analyzer, a micro-channel plate detector with 32 anodes de- After the plasma is initiated with both sides of the plasma gun, ICRF heating systems are used in the central cell. Figure 1 shows the line-integrated electron density and diamagnetism in the central cell. The diamagnetism indicates the plasma stored energy. The line-integrated electron density fluctuated substantially at 122, 130, 138, and 145 ms, and the diamagnetism decreased at the same time. The diamagnetism repeatedly increases and decreases during the ICRF heating period. The density and potential fluctuations were measured at R ∼ 0.01 m using a GNBP. Figures 2 (a) and 2(b) show contour plots of the frequency spectra of the density and potential fluctuation levels, respectively, obtained by ti...

First Results of Simultaneous Multipoint Plasma Potential Measurements Using a Gold Neutral Beam Probe

“…It is quite important to study plasma particle transport across the confinement magnetic field caused by fluctuation. In the GAMMA 10 tandem mirror, the electrostatic and magneto-hydrodynamic fluctuations were measured and were found to be related to radial transport [4,5]. In addition, a decrease in the plasma stored energy due to the radial transport was also observed.…”