A. Terakado

Measurement of neutral gas pressure in the D-module of GAMMA 10/PDX by using ASDEX type fast ionization gauge

In the divertor simulation experiments in the GAMMA 10/PDX tandem mirror, pressure of the neutral gas was investigated by using a fast ionization gauge. The gauge was absolutely calibrated for hydrogen gas by using a capacitance manometer. Change of the gauge sensitivity due to the magnetic field of GAMMA 10/PDX was also evaluated. The typical gas pressure measured in detached plasma experiments was 0.1-10 Pa. The degree of plasma detachment determined from the reduction of heat flux was enhanced as the gas pressure increases. Rapid increase of the gas pressure under the plasma flow was also observed.

Electron temperature and density distributions of detached plasma in divertor simulation experiments in GAMMA 10/PDX

Spatial characterization of plasma detachment attributed to molecular activated recombination (MAR) has been done by measuring distributions of electron temperature, electron density, space potential and floating potential of divertor simulation plasma in GAMMA 10/PDX tandem mirror with Langmuir probes. As the hydrogen pressure in the divertor simulation experimental module was increased by supplying additional hydrogen gas, the electron temperature near the target decreased from 30 eV to about 2 eV and the electron density first increased to an order of nearly 1017 m−3, and then decreased in contrast to the density measured at the upstream of the target which became saturated. The density decrement was higher near the corner of the V-shaped target although temperature distribution was almost flat near the target. In addition, space potential decreased, floating potential increased, and then the potential difference between those became small. The potential difference decreased to almost zero near the tar...

Ion sensitive probe measurement of divertor simulation plasma in GAMMA 10/PDX

An ion sensitive probe (ISP) has been installed in the divertor simulation module of the largest tandem mirror plasma device GAMMA 10/PDX in order to evaluate the property of the high temperature end loss plasmas. The ISP has the capability to evaluate ion temperature (Ti), electron temperature (Te), electron density and plasma space potential, simultaneously. We have tested the ISP measurement for several discharge and neutral gas pressure conditions. Tievaluated by using an ion collector of the ISP shows ∼6 eV which corresponds to the perpendicular component of Ti in principle. Te evaluated by the current-voltage characteristics of an electron guard electrode of the ISP shows ∼4 eV which is lower than Te obtained by Langmuir probes upstream and downstream of the ISP. These results indicate the existence of anisotropic ion and electron temperatures in the D-module.

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.

Blob- and hole-like structures outstanding during the transition from attached to detached divertor states in GAMMA 10/PDX

We have measured multipoint fluctuations inside a divertor simulation experimental module (D-module) during the transient discharge from attached to detached states in the tandem mirror device GAMMA 10/PDX. It was first observed that blob-/hole- like positive/negative spikes of the ion saturation current appeared non-periodically around the radially outer side edge on the V-shaped target plate. This tendency became outstanding in the transient state, and the typical speed and size of the isolated blob-like structures in the azimuthal direction were estimated. Additionally, lower-frequency periodic mode fluctuations and their changes due to plasma detachment were investigated not only in the D-module but also in the central cell with the microwave interferometer, etc. Odd and even modes continuously appeared during the discharge, in the range of several kilohertz.We have measured multipoint fluctuations inside a divertor simulation experimental module (D-module) during the transient discharge from attached to detached states in the tandem mirror device GAMMA 10/PDX. It was first observed that blob-/hole- like positive/negative spikes of the ion saturation current appeared non-periodically around the radially outer side edge on the V-shaped target plate. This tendency became outstanding in the transient state, and the typical speed and size of the isolated blob-like structures in the azimuthal direction were estimated. Additionally, lower-frequency periodic mode fluctuations and their changes due to plasma detachment were investigated not only in the D-module but also in the central cell with the microwave interferometer, etc. Odd and even modes continuously appeared during the discharge, in the range of several kilohertz.

Study of the impurity transport by injecting the gas to D-module in GAMMA 10/PDX

In this study, spectroscopic measurement at the plug/barrier-cell, near the midplane in GAMMA 10/PDX was carried out for the first time. From the comparison of the spectroscopic data measured in the end-cell and plug/barrier-cell, transport of impurities injected to divertor simulation module in the end-cell is investigated. It was found that Ne hardly ionized at both observation points. On the other hand, N2 dissociated and ionized before reaches or at the plug/barrier-cell while emission spectrum derived from N2 was dominated at the end-cell. From the comparison of the emission spectrum at the end-cell and plug/barrier-cell, it suggests that Ne was more likely to transport toward upstream region through the plasma than N2. Time evolution of specific emission spectra, electron temperature and density suggests the reduction of the impurity transport during ECH pulse.

Spatial distribution measurement of molecular activated recombination using hydrogen Balmer line intensities of divertor simulation plasma in GAMMA 10/PDX

In GAMMA 10/PDX, spatial distribution of the occurrence of molecular activated recombination (MAR) has been studied by measuring two dimensional distributions of Hα and Hβ line intensities of the divertor simulation plasma. The V-shaped target in the D-module which is installed in the west end region was exposed to the end loss plasma, and additional hydrogen gas was injected in the D-module. The electron temperature near the corner of the V-shaped target decreased to ∼2 eV with increase in the neutral gas pressure in the D-module and the density roll over was observed, indicating the plasma was detached. The two dimensional distributions of the intensity ratio of Hα and Hβ line intensities indicates that MAR was enhanced with increase in the neutral gas pressure and the region of the MAR occurrence spread and moved to the upstream side.In GAMMA 10/PDX, spatial distribution of the occurrence of molecular activated recombination (MAR) has been studied by measuring two dimensional distributions of Hα and Hβ line intensities of the divertor simulation plasma. The V-shaped target in the D-module which is installed in the west end region was exposed to the end loss plasma, and additional hydrogen gas was injected in the D-module. The electron temperature near the corner of the V-shaped target decreased to ∼2 eV with increase in the neutral gas pressure in the D-module and the density roll over was observed, indicating the plasma was detached. The two dimensional distributions of the intensity ratio of Hα and Hβ line intensities indicates that MAR was enhanced with increase in the neutral gas pressure and the region of the MAR occurrence spread and moved to the upstream side.

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.