Shoichi Shiina

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.

Diffusive Transport Analysis in Low Aspect Ratio Reversed Field Pinch

For the evaluation of confinement property of a low aspect ratio reversed field pinch (RFP) plasma, diffusive transport analysis is studied. Numerical analysis based on the neoclassical transport model in a low aspect ratio RFP show that a considerable bootstrap current contibution can be expected in this configuration, when incorporating the effects of non-circular cross-section and low aspect ratio. So-called neclassical RFP equilibrium are also found to attain both high stablity beta limit and high bootstrap current fraction. However, of transport phenomena in an RFP, the energy confinement time τE might be determined by anomalous transport due to micro-instabilities. To analyze such transport phenomena in a low aspect ratio RFP, the applicability of the TASK (Transport Analysing System for TokamaK) simulation code are considered.