M. Mizuguchi

Use of a Gold Neutral Beam Probe to Study Fluctuation Suppression During Potential Formation in the GAMMA 10 Tandem Mirror

Abstract In the tandem mirror GAMMA 10, plasma confinement is achieved not only by the magnetic mirror configuration but also by the high potentials at both end regions. Before applying plug–electron cyclotron resonance heating (P-ECH), drift-type fluctuations were observed in the potential and density measurements. The suppression potential and density of these fluctuations were clearly determined by using a gold neutral beam probe (GNBP) system during the potential formation by the application of P-ECH. We studied the relationship between the suppression levels of the potential fluctuations and the effects of potential produced by changing the applied electron cyclotron heating power. Moreover, the particle-flux-related values obtained by examining the phase difference between the potential and density fluctuations were measured by using the GNBP. We clearly show that radial anomalous transport induces radial particle transport, which decreases the stored energy of the plasma.

Upgrade of the analyzer design for multipoint measurements by a gold neutral beam probe on the GAMMA 10 tandem mirror

The study of the mechanisms of both transport and fluctuations is one of the main issues of fusion plasma researches. Spatial fluctuation structure measurements are important for studying of cross correlation between the two fluctuations. Gold neutral beam probe systems are used in GAMMA 10 tandem mirror plasma device. Currently, the plasma potential and its fluctuations cannot be measured at different radial positions simultaneously. This paper presents the upgrade design of the 45 degrees electrostatic energy analyzer for the simultaneous multipoint measurements of plasma parameters.

Radial density profile measurement by using the multichannel microwave interferometer in GAMMA 10

Plasma density radial profile measurements are an important study for fusion plasma researches. We reconstructed a multichannel microwave interferometer for radial plasma electron density and density fluctuation measurements with both changing the transmission horn position and using the Teflon lens by only using this system in a single plasma shot. By using this system, we can successfully measure the radial density and density fluctuation spectra in a single plasma shot.

High-Density plasma production in the Gamma 10 central cell with ICRF heating on both anchor cells

Abstract In the GAMMA 10 tandem mirror, divertor simulation experiments that utilize particle flux toward the west end region (called end-loss flux) have been implemented. Since a positive correlation has been reported between the end-loss flux and the central-cell density, an increase of the central-cell density is important for obtaining a higher end-loss flux on the divertor simulation experiments. By arranging the ion cyclotron range of frequency (ICRF) systems so as to excite strong ICRF waves in both anchor cells simultaneously, we have succeeded in producing high-density plasmas (line density of 1.2 × 1014 cm−2) in both anchor cells. As a result, a higher central-cell density of 4.4 × 1012 cm−3 and a higher end-loss flux of more than 1023 m−2s−1 have been obtained. One of the possible mechanisms of the high density production is a formation of positive potentials on both anchor cells. Plasmas in the central cell are confined due to those potentials.

Study of radial potential fluctuations by using a gold neutral beam probe system.

A study of fluctuation is one of the main issues in fusion plasma researches. The potential and its fluctuation are measured by a gold neutral beam probe (GNBP) in the tandem mirror GAMMA10. Since there is no stable isotope ((197)Au approximately 100%), there are little measurement errors. GNBP system can measure the potential with about +/-30 V of error. The time and spatial resolutions are estimated to be about 3 micros and 5 mm, respectively. During the formation of axial confining potentials, the radial profiles of the potential and its fluctuation were successfully measured in GAMMA10. GNBP is a useful tool for the study of radial potential fluctuation directly in plasmas.

Electron Temperature and Density Distributions in a Divertor Simulation Experimental Module with Gas Injection in GAMMA 10/PDX

Abstract In tandem mirror GAMMA 10/PDX, H2 gas has been injected to a divertor simulation experimental module (D-module) for radiation cooling and detachment study. Electron temperature (Te) and density (ne) distributions have been measured by Langmuir probes in the D-module. With increase of H2 gas amount, Te has decreased especially at the V-shaped target plate from 30-40 eV to 3-5 eV. Moreover, ne and ion saturation current Iis have increased at the inlet but decreased at the target and had a steeper distribution toward the corner. These results suggest detachment. The movement of the ionization front to the upstream side with increase of the gas is suggested from the change in the plasma parameters profiles along the magnetic field line.

Status and Plan of GAMMA 10 Tandem Mirror Program

Abstract Recent progress and near future plan of GAMMA 10 efforts are presented. With high power plug electron cyclotron heating (ECH) up to ~400 kW, the ion confining potential of more than 2 kV was confirmed. The drift type low frequency fluctuations were suppressed by the positive radial electric field produced by plug ECH. It is found that the efficient EC heating on mirror devices from a strong B field side requires the minimization of the stray microwave in addition to the 100% X-mode excitation to avoid the enhancement of the ion loss. The development of a gyrotron, the key tool of these ECH experiments, has been made in collaboration with NIFS (National Institute for Fusion Science), More than 1.5 MW for more than 1s has been demonstrated at 77GHz. The plan of the boundary plasma research program with modification of GAMMA 10 is in progress. The new program includes the physics and technology studies of the divertor and SOL plasmas and PWI relevant to torus plasmas like ITER. The high heat flux experiments using the open end mirror throat has been started and we have obtained successful preliminary data, which include the heat flux of 8 MW/m2.

Correlation Analysis of 3–4 Kilohertz Core and Edge Density Fluctuations in the GAMMA 10 Tandem Mirror Device

Abstract Propagation behavior of 3–4 kHz electron density fluctuation in the core and edge regions was investigated in the GAMMA 10 tandem mirror device. Frequency analysis was employed for high spatiotemporal-resolution fluctuations measured by gold neutral beam probe (GNBP) in the central cell and Langmuir probes in the end region. We found same-frequency periodic fluctuations in both signals and applied the correlation analysis technique. By considering the geometry of the measurement system, propagation speeds of the periodic fluctuation along and across the magnetic field lines were estimated.

Relation Between Electron Density and Temperature of Divertor Simulation Plasma and Upstream Plasma in GAMMA 10/PDX

Abstract In GAMMA 10/PDX, the relationship between electron density and temperature of the divertor simulation plasma in the divertor simulation experimental module (D-module) and the density of the upstream plasma has been studied. As the amount of gas (H2 and Ar) injected into the D-module increases, the line-averaged electron density (ne_WP) of the west plug plasma (i.e., upstream plasma) increases due to gas flow from the D-module to the upstream. The electron temperature of the divertor simulation plasma monotonically decreases with increase in ne_WP. The electron density of the divertor simulation plasma becomes saturated against increase in ne_WP when the plasma is sustained only by the ion cyclotron heating. This may suggest a sign of the beginning of the density roll-over. Additional electron cyclotron heating to the upstream plasma increases the density of both the upstream plasma and the divertor simulation plasma, and the saturated density of the divertor simulation plasma recovers to a linear dependence on ne_WP, suggesting an enhanced ionization of neutral gases in the upstream and the D-module.

2-Dimensional Soft X-Ray Behavior of ECR Heated Plasma in GAMMA 10

The 2-Dimensional soft X-ray behavior during ECR heating have been examined in the GAMMA 10 tandem mirror. The peak position shifts several cm during the ECRH period, and it is found the peak position of the soft X-ray is rotating around the center axis of the vessel and its direction is same as that of E×B drift. In high power heating case, the oscillation of the peak position, decrease of the diamagnetism and line density are observed. The radial ion current measured by the ion sensitive probe increases depending on the ECRH power together with increase in the radius of the SX-peak rotation.