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

Abstract

The physical mechanisms of the improvement of plasma confinement due to the formation of electrostatic potential and electric field were studied. A 12-keV gold neutral beam probe (GNBP) with the same capabilities as a heavy ion beam probe was operated on the GAMMA 10 tandem mirror. The simultaneous multipoint system was developed to measure the local electric field. The structure of the novel analyzer and the technique for measuring the local electric field were studied by using the calculation code of three-dimensional beam trajectory. It was possible for the first time to measure the local electric field in a single plasma shot by the simultaneous multipoint plasma potential measurement system using the GNBP in the GAMMA 10 tandem mirror. Fluctuations caused by instabilities in plasmas are observed in some magnetic confinement devices [1][2][3]. 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.The physical mechanisms of the plasma confinement improvement due to the formation of electrostatic potential and electric field were studied [6,7]. A beam probe is a useful tool in terms of the energy conservation of a heavy ion beam for potential and fluctuation measurements. A gold neutral beam probe (GNBP) with same capabilities as heavy ion beam probes is operated on the GAMMA 10 tandem mirror [8][9][10]. Figure 1 shows a schematic view of the GNBP system. The energy and incident angle of the neutral beam that pass the plasma center are about 12 keV and 40 degrees to the horizontal direction, respectively. The typical negative ion beam diameter and current intensity are 5 mm and 2 µA, respectively, as measured by a Faraday Cup detector. The gold positive ion beam produced in collisions with plasma electrons is analyzed by parallel-plate type electrostatic energy analyzer with the incident angle of 45 degree to the ground plate. The width and length of the current slit are 0.7 and 12 mm, respectively. The positive ion beam is detected by a micro-channel plate (MCP) with 32 anodes. The width and length of each anode are 2.4 and 31 mm, respectively.It is difficult to measure the local electric field with author's e-mail: miyata.yoshiaki@jaea.go.jp high precision. Previously, two measuring techniques were used to measure the radial potential profile and electric field in the core plasma. In the first measuring technique, the measuring position is changed by the deflecting beam trajectory for each shot. This measuring technique has high time resolution; however, the calculation of radial electric field has a large error induced by the poor reproducibility of the plasma shot. Another measuring technique involves changing the measuring position by sweeping the applied choppy voltage to the deflecting plat...

Simultaneous evaluation of potential fluctuation from the core plasma to the end region in GAMMA 10

“…GAMMA 10 is an effectively axisymmetrized minimum-B anchored tandem mirror with thermal barriers at both end mirrors (figure 1). It is described in detail elsewhere [6][7][8][9][10][11][12][13][14][21][22][23][24][25]. The x-axis and y-axis are perpendicular to the magnetic field in the vertical and horizontal directions, respectively.…”

“…We used the heavy ion beam probe to measure the potential of the core plasma in the central cell of GAMMA 10 [12,[21][22][23][24][25]]. We measured the potential and its fluctuations using the GNBP in the central cell (z = 1.18 m) (figure 2).…”

“…GAMMA 10 has a good potential measurement system consisting of a gold neutral beam probe (GNBP) system [12,[21][22][23][24][25]. We can observe the density and potential fluctuations when the axial confinement potential is formed during the application of P/B-ECH in a single plasma shot.…”

“…However, the earlier study could obtain only the potential and electric field at different times in the same plasma shot. Thus, we developed the twopoint potential measurement GNBP system by changing the incident slit to a double-slit configuration in the GNBP analyser [22,24]. This enables a more detailed study of the suppression mechanisms of drift-type fluctuations during the formation of a confinement potential produced by P-ECH.…”

Suppression of flute-like fluctuation by potential formation in GAMMA 10/PDX

“…The density fluctuation is observed using microwaves, such as the interferometer, reflectometry and Fraunhofer diffraction method, and electrostatic probes [1][2][3]5,6]. The potential fluctuation is observed by the gold neutral beam probe (GNBP) and the end plates [4,[15][16][17][18][19]. In the former experiments, we studied the drift-type fluctuation suppression by applying the P-ECH [5,6,[15][16][17][18][19].…”