Takasi Kurasawa

Stochastic diffusion of magnetic field lines

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.

Reversed field pinch plasma equilibria with shear flow

The Grad–Shafranov equation with flow, which is derived by a variational method, involves unknown functions such as the dynamic pressure, P(Ψ). These functions are specified by minimizations of free energies under the constraints of constant P′(Ψ), magnetic helicity, flow helicity, and cross helicity in reversed field pinch (RFP) plasmas. New flow and cross helicities are introduced based on the analogy of the magnetic helicity, which are different from those used in fluid mechanics. The constraint of constant flow helicity provides flow with profiles from high- to low-shear flow. The Suydam-stable RFP equilibria obtained with flows are extremely different in β and βp from RFP equilibria without flow. The high-shear flow can extend the Suydam limit, allowing higher β and βp.

Electromagnetic fields as functionals acting on observation systems

Abstract A proposal is made that electromagnetic fields should be understood to be linear functionals operating on advanced fields, in order to justify their jump conditions. Our advanced field is literally a test function, as in the theory of generalized functions, and is interpreted to give a measure of contribution of a unit strength of electromagnetic field to the meter reading.

Friction coefficient and correlation length of stochastic magnetic field lines

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.

Magnetohydrodynamic Instability in Axisymmetric Spherical Plasma with Parallel Flow to Magnetic Field.

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 <1 in the case of the free boundary. The nonliner numerical analysis in the free boundary case shows that the tilting mode plays a main role, and the damping rate of the original helicity and the toroidal flux decreases with the increase of C A .