Akira Tsushima

Observation of spatial Alfven resonance

Abstract By exciting axisymmetric Alfen waves with Stix coils, the spatial properties of shear Alfven resonance are investigated. The maximum azimuthal magnetic field of the excited waves is located at this resonance layer, which is a function of both frequency and wavelength. The polarization also changes continuously through the resonance layer.

Non-Axisymmetric Alfvén Wave Excited by a Helical Coupler in an Inhomogeneous Plasma

Non-axisymmetric Alfven waves are excited with a helical coupler in a finite beta and a cylindrical inhomogeneous plasma surrounded by a conducting wall. The helical coupler, which consists of two symmetrical helical windings, exhibits a strong propagation directionality; slow waves of the m=-1 mode and fast waves of the m=+1 mode can be launched simultaneously from the coupler, but propagate in opposite directions along the static magnetic field. Dispersion relations including the attenuation length for both modes are compared with a magnetohydrodynamic theory given by Woods on the assumption of appropriate boundary conditions. As a nonlinear phenomenon, subharmonic slow Alfven waves of the m=-1 mode have been observed when the pump frequency is close to or above the ion cyclotron frequency.

Direct Measurement of Ion Behavior using Modified Ion Sensitive Probe in Tokamak Boundary Plasma

Simultaneous measurement of ion temperature and electron temperature as well as correlation with a H α line and the ion behavior are performed by using a modified ion sensitive probe in the boundary plasma in the JFT-2M tokamak. The modified ion sensitive probe for the ion temperature measurement mounted in the JFT-2M can be used to vary the height of the electron barrier for suppressing the electron current by remote control without breaking of the vacuum. The electron temperature is simultaneously measured using the electron guard as a Langmuir probe while the ion temperature is measured by the ion collector of the probe. It has been confirmed that the ion temperature is higher than the electron temperature in the tokamak boundary plasma. It has also been found that the decrease of the ion current at the ion collector coincides with that of the H α line intensity when the plasma confinement is improved during the divertor biasing.

Formation of spatial Alfven resonance and cutoff fields in an inhomogeneous plasma

By measuring the time evolution of the perturbed magnetic field vectors, the formation of the spatial Alfven resonance layer and cutoff fields has been first observed in an inhomogeneous plasma. A simple MHD theory including the effect of the ion-neutral collision is described as a background to understand the experimental results.

MHD Instabilities Developing to Alfven Waves Observed during Plasma Production by MPD Arcjet

We have observed two kinds of magnetohydrodynamic (MHD) instabilities excited in a current-carrying cylindrical plasma that was produced by a magneto-plasma-dynamic (MPD) arcjet. The “net current” in the plasma excites the instability with the azimuthal mode number m =1, which is deduced to be a kink mode from MHD theory. On the other hand, the m =0 instability is excited by the “total current” flowing to the cathode, and this mode was found to be responsible for the modulation of electron temperature. They are converted respectively to m =1 compressional Alfven wave and m =0 global Alfven eigenmode, propagating along the axial magnetic field in the current-free region.

Control of radial potential profile by biased segmented endplates in an ECR plasma

A radial potential profile is successfully changed flom hill- to well-shaped by biasing segmented endplates in a strongly magnetized plasma produced by the electron-cyclotron-resonance. The potential profile is settled down to hold the condition where there is no net electric current to the wall surrounding the plasma. There appears a radial electric current due to the non-ambipolar radial ion loss, balanced with an axial electron current determined by the sheath potential drop in front of the endplates. The radial ion loss is observed to be remarkably enhanced by increasing the outward radial electric field.

Electron Energy Distribution Function Measured by Electrostatic Probes at Divertor Plasma in JFT-2M Tokamak

The electron energy distribution function (EEDF) as well as the plasma parameters were measured by the electrostatic probe for divertor plasmas in the JFT-2M tokamak. The necessary condition for obtaining the precise EEDF including a wide range of electron energy is studied, in which the electrostatic probes should be placed in the region where the ratio of the electron saturation current to ion saturation current is sufficiently large. The behaviors of the EEDF estimated from the first derivative of the probe current were discussed for the ohmic heated (OH) and the electron cyclotron resonance heated (ECRH) plasmas.

Sheath-Plasma Criterion of Fluid Theory with Finite Ion Temperature

with a polytropic index . The value of is important in relation to probe measurements, and one may set 1⁄4 3 for an adiabatic case (one-dimension), or 1⁄4 1 for an isothermal case. In fact, varies from three to unity owing to limited heat flow, since there is no heat flow in the adiabatic case and unhindered heat flow in the isothermal case. The value of for the sheath-plasma criterion has remained a problem. To derive the sheath-plasma criterion, equations applicable in a sheath region with space charge are used in general. However, there is another method of deriving the sheath-plasma criterion, in which equations applicable in a plasma or presheath region are used. Since quasi-neutrality is assumed in the equations in the second method, the equations become invalid in the sheath region owing to space charge, and a singularity gives a sheath-plasma boundary, where the ion drift speed is given by eq. (1) for Ti 1⁄4 0. The extension of the second method for Ti 61⁄4 0 is achieved by considering ion pressure, and shows the sheathplasma criterion indicating that the ion drift speed at the sheath-plasma boundary is given by eq. (2). As for the first method for Ti 61⁄4 0, the generalized sheath-plasma criterion of the ion velocity distribution function is obtained, although the details of the ion velocity distribution function are not determined. The purpose of this report is to find a solution of the equations in the second method for a finite ion temperature, which may vary in accordance with the polytropic approximation, because only the solution under the condition of Ti 1⁄4 0 or Ti 1⁄4 const: is known. Basic equations of the fluid theory treating a steady-state one-dimensional plasma with a uniform ion generation G (1⁄4 const:) are written as

Evaluation of Ion Temperature Using Ion Sensitive Probe in the Boundary Plasma of the JFT-2M Tokamak

In order to extend the capability of the ion sensitive probe (ISP) for the precise measurement of ion temperature, the dependence of the ion current on the height of the electron guard has been evaluated numerically by taking a finite width of the ion collector and the guard into account. It is shown that the ion temperature for strongly magnetized plasma could be experimentally deduced from the current–height characteristics for the ISP more accurately. The ISP technically improved by remote controllable heights has been applied to measure the ion temperature in the ohmically heated and L/H transition plasmas in JFT-2M tokamak.

An Ion Drift Velocity at a Sheath Edge of a Plasma with a Finite Ion Temperature

An ion drift velocity at a sheath edge v D in a bounded plasma with a finite ion temperature is studied using a one-dimensional electrostatic particle simulation code. This code includes spatially uniform particle generation and electron heating and determines v D at the sheath edge, defined by ( n i - n e ) / n i = 0.1 with an ion density n i and an electron density n e . In comparison with the ion acoustic velocity from the fluid equations, \(v_{\rm D} = \sqrt{(T_{\rm e}{}^{\rm center} + \gamma T_{\rm i}{}^{\rm edge}) / m_{\rm i}}\) is obtained, where T e center is an electron temperature at the plasma center, T i edge is an ion temperature at the sheath edge, m i is an ion mass, and γ= (3.5 + T i edge / T e center ) / (1 + T i edge / T e center ). It is also found that a sheath potential change little but an ion acceleration potential in a presheath decreases with the increasing T i edge / T e center .