Katsunori Saito

Neoclassical reversed-field pinch equilibrium with dominant self-induced plasma current

The neoclassical magnetohydrodynamic equilibrium of reversed-field pinch (RFP) is self-consistently solved considering the self-induced plasma current IϕSI in a reactor relevant parameter regime. The current IϕSI consists of bootstrap currents due to α particles as well as the bulk plasma, where the conventional neoclassical transport theory is applied without considering the finite banana-width effect, Pfirsch–Schluter current, and diamagnetic current. The neoclassical effect enhances IϕSI in a low-aspect-ratio RFP (aspect ratio A=2), when β is high. An IϕSI as high as 94.4% of the plasma current in the equilibrium with ellipticity κ=1.4 and triangularity δ=0.4 is obtained with a flat pressure profile, which provides a toroidal β of βt=63% (volume-averaged β, ⟨β⟩=66%) stable against both the ideal kink mode and Mercier mode. This dominant self-induced current makes the steady-state approach feasible. Its hollow current profile gives a strong magnetic shear and a high-stability β.

Pitch-Modulated l=1 Torsatron

The pitch modulation on an l =1 torsatron field leads to the improvement of the confinement properties due to the formation of a local magnetic well, the reduction of the field ripple and the increase of the rotational transform. The pitch-modulated l =1 torsatron has the simplest coil structure and generates a compact helical magnetic axis configuration with a much smaller excursion amplitude than the coil radius. Here, the vacuum configuration and the finite beta equilibrium of this system are described.

Helical Magnetic Axis Configuration Combined with l=1 and Weak l=-1 Torsatron Fields

The superposition of a relatively weak l =-1 torsatron field on a main l =1 torsatron field leads to the improvement of the confinement properties due to the formation of a local magnetic well, which results from the local curvature of the helical magnetic axis with a larger excursion in the major radius direction. This l ±1 helical magnetic axis system has a comparatively simple, compact coil structure. Here the vacuum configuration properties of l =±1 system are described.

Spheromak Formation by Theta Pinch

Production of a spheromak configuration is attempted by using a theta pinch combined with a z-discharge. A loop current which is formed by a z-discharge current flowing in the plasma and reversing on its surface generates a toroidal field in the spheromak. Mechanism of the current reversal is explained from toroidal flux conservation in the plasma during implosion phase of the theta pinch. It is found that the reverse ratio is controlled in actual experiments by an external inductance of the z-discharge circuit, a length and a compression ratio of the plasma. Experiments show that 70% reversal of the z-current is easily realized using a conventional theta pinch.

Equilibrium of high beta plasma in closed magnetic line system (MBT)

The beta effects on the plasma equilibrium in Modified Bumby Torus (MBT) sector, which is an asymmetric closed line system with l=0 and fairly large l=+or-1 field distortions, are studied. For this purpose, the equilibrium of high beta plasma produced by theta-pinch is compared with that of betaless plasma numerically calculated from the measured magnetic field profiles in device. The equilibrium condition depends weakly on beta value, but the plasma cross-section is vertically elongated as the beta value increases. The m=1 long wavelength MHD instability is not observed during the observation time of approximately 15 mu s. These experimental results are compared with MHD theory based on the new ordering taking the finiteness of l=+or-1 field distortion ( delta +or-1>or approximately=1) into account, which suggest significant stabilizing effects due to self formation of magnetic well and also due to the conducting wall.

EQUILIBRIUM AND STABILITY OF THETA-PINCH PLASMA IN MODIFIED TOROIDAL MULTIPLE MIRROR FIELD

To confine a high-beta plasma a new toroidal magnetic configuration with closed lines of force has been proposed [1]. The configuration is an appropriate superposition of l = 0, l = ± 1, l = ± 2,…, helical fields. In this experiment, it is generated by modifying the multiple mirror field by enclosing the discharge tube in a copper shell which has a longitudinal gap. This configuration is preferred for the wall stabilizing effect to that with the separated helical windings. The characteristics of the equilibrium conditions are examined based on the near-axis approximation theory and compared with the experimental results. The stability of plasma in the configurations with l = 0 field and with superposition of l = 0, l = ± 2 fields is investigated in linear geometry.