N. Ezumi

Influence of Plasma Resistance and Fluctuation on Probe Characteristics in Detached Recombining Plasmas

In order to find the causes of the strong anomaly of current-voltage characteristics of Langmuir probe observed in detached recombining plasmas in a linear divertor plasma simulator, NAGDIS-II, we have investigated plasma resistance along a magnetic field and potential fluctuations in the detached recombining plasmas. Simple calculation on the ratio between the plasma length, at which plasma resistance and resistance of ion sheath formed around a probe tip become equal, and an electron collection length indicates that the evaluation of electron temperature T e becomes inaccurate at T e of less than 0.6 eV when plasma density and neutral pressure are 1.0 x 10 18 m -3 and 10 mtorr, respectively. The potential fluctuation in detached recombining plasmas was found to be so large compared to T e /e, which can also modify the probe characteristics.

Ion temperature measurement using an ion sensitive probe in the LHD divertor plasma

Abstract The first reliable measurement of ion temperature in the divertor plasma of the Large Helical Device has been done by using an ion sensitive probe. The satisfactory current–voltage characteristics of the ion collector for evaluating the ion temperature were obtained at the outer part of the divertor leg. Furthermore, simultaneous ion and electron temperature measurements were successfully done in this part. The results show that the ion temperature is higher than the electron temperature in the part. There is a possibility that the profiles of the evaluated ion temperature which shows relatively higher than the electron temperature at the outside of divertor leg are qualitatively explained by particle’s orbits around the edge and divertor region.

Transport Characteristics in the Stochastic Magnetic Boundary of LHD: Magnetic Field Topology and its Impact on Divertor Physics and Impurity Transport

Abstract Transport characteristics of the stochastic magnetic boundary of the Large Helical Device (LHD) are investigated, based on three-dimensional Monte-Carlo Braginskii-type fluid model code, EMC3, coupled with the kinetic neutral transport code EIRENE, in direct comparison with experimental observations for aspects of the relation between the magnetic topology and the resulting transport in terms of counter acting flux tube flows and impurity screening/transport. Divertor probe measurements show a rather weak divertor parameter dependence on upstream density in contrast to those of tokamaks at high-recycling regime. This is found to be due to the loss of parallel momentum via cross-field interaction between the stochastic flux tubes, where strong flow shear exists. The three-dimensional modeling predicts an impurity screening potential of the stochastic scrape-off layer (SOL) at high densities. The remnant island geometry affects the energy transport, which leads to suppression of the thermal forces by increasing cross-field energy flux across islands at high collisionality. The screening effect is most pronounced at the edge surface layers with a strong friction force exerted by the background plasma flow, where the flow toward divertor is enhanced due to the rich ionization source. Modeling results are compared to the edge carbon emission obtained in experiments, where a reasonable agreement on the density dependence is found, indicating the existence of the impurity screening mechanism in the stochastic SOL of LHD.

Measurement of ion and electron temperatures in plasma blobs by using an improved ion sensitive probe system and statistical analysis methods

We have measured ion temperature as well as electron temperature in plasma blobs observed in a linear plasma device by using an improved ion sensitive probe. Current-voltage characteristics of the ion sensitive probe inside and outside plasma blobs were re-constructed with a conditional sampling method. It is clearly found that both ion and electron temperatures in plasma blobs decrease more slowly in a cross-field direction than those in a bulk plasma without plasma blobs.

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.

Overview of recent progress and future in GAMMA 10/PDX project

This paper presents an overview of the recent progress of GAMMA 10/PDX project. In the GAMMA 10/PDX project, development of fusion reactor relevant research related to magnetic mirror devices has been conducted. One of the main objectives is divertor simulation studies under the similar condition of actual fusion devices, which contributes to comprehensive development strategy towards the divertor plasma physics and plasma wall interaction (PWI) studies. The recent progress of this research is that an additional plasma heating (ICRF and ECH) significantly increased the ion flux up to 3.3 × 1023 /m2 s at the end-mirror exit, which proved an effectiveness of additional plasma heating for generating high ion flux from the mirror end. Superimposing a short pulse of ECH produces the maximum heat flux of 21 MW/m2 by improving the west plug ECH antenna system, which exceeds the heat-load of ITER divertor plates. In detached plasma formation experiments using the divertor simulation experimental module (D-module)...

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 plasma behavior during impurity injection in the end-cell of GAMMA 10/PDX by fluid code

The effect of neutral Ar injection on the plasma parameters is investigated numerically by using a multi-fluid code in this research. Reduction of electron and ion temperature has been observed according to the increment in injected neutral Ar density. The heat flux on the target plate has been also reduced with the increasing of injected neutral Ar density. For Ar: 2.0 × 1018 m−3 injection, electron temperature on the target plate reduced to about 1.5 eV. Increase in electron and ion density has been observed at lower injected Ar density. However, at higher Ar injection, electron density and ion density goes to be saturated. These outcomes indicate that the plasma in the end region approaches towards the plasma detachment state.