Search

Low-aspect-ratio reversed field pinch (RFP) plasmas are expected to produce equilibria in which the resonant surfaces for tearing modes are well separated compared with high-aspect-ratio RFP plasmas. The profile of the safety factor, q, determines the mode separation of the resonant surfaces. The q profiles, the ratio of plasma pressure to magnetic energy, β, and the parallel current density to the magnetic field lines, μ, are studied with the varying aspect ratio, A. A new Mercier stable RFP plasma is proposed by introducing a finite surface toroidal current density at the plasma edge. The aim of this study is to investigate the effect of a finite surface toroidal current with a low-aspect-ratio (low A) on the profiles of q, μ, β and the ratio of plasma pressure to poloidal magnetic energy, β p . The investigation is performed using an equilibrium model with a finite pressure, in which unknown functions in the Grad-Shafranov (GS) equation are specified. Specified functions are composed of some numerical parameters that allow systematic variation of equilibria. The GS equation is solved in the toroidal coordinate for various A values, where the β limit of Mercier stable equilibria is determined. First we confirmed that q at the centre, q 0 , increases as A decreases. The result shows that the profiles of q, β and β p , strongly depend on A. The β and β p obtained decrease with decreasing A. In particular, the decrease in β differs from the case of a tokamak. The decrease in β is recovered, in spite of a low A, by setting the toroidal current to be finite at the plasma edge as a boundary condition.

Numerical simulations using magnetohydrodynamic (MHD) equations are performed for the pulsed poloidal current drive (PPCD) in reversed field pinch (RFP) plasmas. The PPCD is simulated by reducing the toroidal magnetic field at the wall, B z ( a ), in the short period. The simulations take into account the values of reduced B z ( a ), the period of changing B z ( a ) and the start time as the characteristic parameters of the PPCD. The effects of the parameters on m =1 resistive modes, which play the main role in the sustainment of RFP configurations, are calculated. The PPCD has stabilizing effect for the m =1 modes over a wide range of the parameters. The dependence on the parameters for the formation of quasi-single helicity after the PPCD and for the growth of the m =0 mode is calculated. The stabilizing factor due to the enhanced magnetic shear for the resistive modes is also analyzed by the linear growth rate in the equilibrium model of the transient configuration during the period of the PPCD.

According to a simplified costing algorithm for a steady-state fusion reactor power plant, the relative attractiveness of advanced physics modes mainly depends on the stability and noninductive current drive of the equilibrium. The high-stability beta and the good alignment of the equilibrium current profile with a self-induced plasma current profile are compatible with the low-aspect-ratio neoclassical Reversed-Field-Pinch equilibrium solved self-consistently considering the self-induced plasma current. The high-stability beta is due to the hollow current profile making the magnetic shear increase and the force-free field dominant. The good alignment of the current profile significantly reduces the power required for noninductive current drive to generate the steady-state magnetic field configuration. As a result, the fusion power plant based on the neoclassical RFP equilibrium with low aspect ratio enables electricity to be generated at relatively low cost.

Poloidal and toroidal rotations of the reversed field pinch plasmas induced by the magnetohydrodynamic resistive tearing modes are numerically studied by using the ohm's law with the ion inertia term. The modes with the poloidal mode number m =1, which mainly grow around the center in the cylindrical plasma are taken into account. In the case of the non-resonant mode, the direction of the induced toroidal rotation velocity of the plasma around the center is same as that of the toroidal magnetic field. The toroidal velocity at the center is decelerated about the time when the magnetic energy of the unstable mode reaches the nonlinear saturation level. The direction of the toroidal velocity of the mode at the boundary is opposite to that of the plasma around the center before the time the instability reaches the nonlinear saturation level. In the case of the resonant modes the reversal of the rotations appears around the singular point. In the multi-mode case the behavior of the plasma rotations shows such ...

An averaged transverse velocity of stochastic magnetic field lines between two tearing modes is presented by using the renormalized weak turbulence theory. The averaged velocity gives the friction coefficient of moving magnetic field lines which is expressed in terms of a renormalized propagator. A correlation length (time) of magnetic field lines, which is the time integrated renormalized propagator, is presented with respect to various parameters. The diffusion coefficient given in a previous paper (Sugimoto et al 1994 Plasma Phys. Control. Fusion 36 383) and the friction coefficient obtained here are related to each other by the so-called Einstein relation. Although the renormalized propagator given here involves more effects such as collision and friction types than that in the previous paper, which has the term of the diffusion type effect alone, the former effects do not greatly modify the renormalized propagator. Effects of collision and friction types do not play an important role in the propagator and the diffusion type effect alone is essential.

The instability of the axisymmetric spherical force free plasma with the parallel plasma flow to the magnetic filed is studied by using the MHD equations under the condition of incompressibility. The linear growth rate of the unstable mode with the fixed toroidal mode number n =1 is obtained and it decreases with the increase of Alfven Mach number C A in the rage of C A

A renormalized diffusion coefficient for magnetic field lines is presented by using the renormalized weak turbulence theory. It is defined on locally destroyed magnetic surfaces caused by two tearing modes. The renormalized diffusion coefficient is given in terms of a renormalized propagator expressed by the integral of the Bessel function of the first kind, J1. It is free from delta -function-type divergence. An averaged renormalized propagator is introduced whose imaginary part represents the behaviour of the memory of the initial condition. Numerical calculation shows that the averaged propagator decreases within a correlation damping time.

A new RFP device with a very small aspect ratio (A = 2.1) is designed and constructed. The aspect ratio of A = 2.1 is much smaller than those of existing devices (A > 3). At this small A, no rational magnetic surface appears from the axis to ψ = 0.4 in the normalized flux surface coordinate, which may reduce the chaotic properties of the plasma in the core region. The proposed examination of bootstrap current generation could also be done. For this new device, a compact and high voltage center coil and an aluminum vacuum vessel (R/rw = 0.53 m/0.25 m) are developed. The maximum plasma current is designed to attain 100 kA. The construction of the device has been completed successfully. In the initial series of discharges, a stable region with a very high Θ value up to ˜2.5 is newly found.

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.