Hiroshi Oshiyama

Asymmetric Flux Generation and Its Relaxation in Reversed Field Pinch

The toroidally asymmetric flux enhancement (“dynamo effect”) and the axisymmetrization of the enhanced fluxes that follows in the setting up phase of Reversed Field Pinch are investigated on the STP-3(M) device. A rapid increase in the toroidal flux generated by the dynamo effect is first observed near the poloidal and toroidal current feeders. Then, this inhomogeneity of the flux propagates toroidally towards the plasma current. The axisymmetrization of the flux is attained just after the maximum of plasma current. The MHD activities decrease significantly after this axisymmetrization and the quiescent period is obtained.

Self-organization and plasma dynamo in a reversed field pinch

The equilibrium of the STP-3(M) reversed field pinch has been studied under various discharge conditions. With increasing the magnetic Reynolds number, the values of field reversal ratio F and pinch parameter θ tend to be kept constant by the self-organization which is closely related to the plasma dynamo generating the toroidal flux ψ. When we vary the poloidal input energy to control the volt second for the fixed condition of a toroidal circuit, this self-organization keeps the value of K /ψ 2 constant, where K is the magnetic helicity, and prohibits the plasma from attaining the helical equilibrium state.

Experiment on the Spatial Confinement of Plasma in the Heliotron B Magnetic Field

A magnetic field is produced by an electric current in a series of pair coils wound around the discharge tube at regular intervals. The electric current in each coil of the pair differs both in intensity and direction. The lines of force near the tube axis undulate without crossing the wall, while those near the tube wall cross the wall. Thus the high‐temperature plasma can be produced by Ohmic heating in the central region of the field. Experiments were carried out in order to verify the spatial confinement of the plasma in this field. The plasma current flows only within the central region whose mean‐cross‐section diameter is about 3 cm, and the plasma has a sharp boundary. Outside the region there is no current. The spectra from the hydrogen discharge are mainly composed of the pure Balmer series, and the impurity spectral lines are very weak or hardly perceptible. From these experimental results, it can be concluded that the hot plasma is almost perfectly isolated from the stainless‐steel‐tube wall by the Heliotron magnetic field and has very low impurity level.

Reversed Field Configuration in Force-Free State

Two reversed field configurations in force-free state of rot B =α B have been considered; one is a straight cylindrical configuration with α( r ) dependent on radial coordinate r and the other is a toroidal one. The cylindrical configuration is obtained by choosing functional form of α( r ) in such a way that it closely match experimental results. The toroidal configuration is analyzed by using an expansion procedure. A new description of F-\( \varTheta \) diagram has been introduced in order to explain experiments.

Fast Electron Behavior in Comparison with Dynamo Models in a Reversed-Field Pinch.

On the basis of soft-X ray diagnostics, it is shown that the role of the edge fast electrons in the reversed-field pinch (RFP) dynamo mechanism is insignificant in the early phase of a discharge in the STE-2 RFP. In the high-fill-pressure region, this is also the case even with the current maximum phase. The pressure dependence of the edge magnetic fluctuations is possibly different from the expected dominance of the MHD dynamo activity in the high-pressure regime, indicating the importance of direct measurement of the phase relation between velocity and magnetic field fluctuations.

Non-Thermal Radiation at Runaway Electron Instability

Non-thermal microwave radiation from a tokamak NOVA \(\varPi\) operated in a runaway mode exhibited two distinct time behaviors at X-band synchronized with the positive spikes in loop voltage, depending on the vertical magnetic field. One can be explained by synchrotron radiation through the pitch angle scattering of the runaways. The other is closely related with the ratio ω c e /ω p e .

Experiment on the Ohmic Heating of Plasma in the Heliotron C Magnetic Field. I. Equilibrium

The equilibrium of the current-carrying toroidal plasma in the Heliotron C field is studied experimentally. The equilibrium state is very sensitive to a change in the transverse field of order 10 -3 of the main confining field. The effect of vertical magnetic field on the equilibrium is precisely examined by measuring the electrostatic equipotential surface of the plasma and the apparent electric conductivity of the plasma current. The observed values of the optimum vertical field B v , which is necessary for the equilibrium of a current-carrying toroidal plasma, are about 1/2∼1/3 of the theoretical values.

Magnetic Fluctuations in an Asymmetric Reversed-Field Pinch

Internal magnetic fluctuations have been studied by pick-up coils in an asymmetric reversed-field pinch (RFP) device STE-RFP. Measured profiles of fluctuations were compared with theoretical ones of m =1 ( m is the poloidal mode number) internal resistive kink mode which was calculated using the measured equilibrium profiles assuming toroidal symmetry. Radial and poloidal components of the measured profiles were in good agreement with theoretical ones, while toroidal asymmetry seems to cause a slight deviation of toroidal component from calculated results. The local nature of fluctuation is also discussed.

MHD Diffusion in the Reversed Field Pinch Plasma

Transverse diffusion coefficient in a toroidal reversed field pinch (RFP) is investigated for the MHD region. The configuration of the RFP is characterized by both a poloidal magnetic field B ω and a longitudinal field B φ which reverses on the outside of the plasma with respect to its value on the axis. Owing to the value of B ω being comparable to B φ and the short connection length of the magnetic lines of force, the diffusion coefficient in the toroidal RFP does not considerably exceed the so-called classical diffusion of binary collision, D ⊥ = n T e /σ ⊥ B 0 2 where T e is the electron temperature, n is the plasma density and σ ⊥ is the transverse conductivity of the plasma. Near the characteristic magnetic surface in the RFP, where B φ =0, the field B ω makes a dominant role for determining the diffusion coefficient which results in the order of D ⊥ = n T e /σ ⊥ B ω 3 .

Studies on Setting Up the Reversed Field Pinch Configuration by Using the Heliotron C Magnetic Field

The setting up the reversed field pinch configuration with the aid of separatrix of the Heliotron C magnetic field has been tested in the small scale linear pinch device. In the experiment, important results have been obtained; first the reversed field configuration can be easily set up without any programming and secondly the separatrix has the magnetic limiter action which is expected to keep the plasma without striking the wall in the phase of setting up.