Numerical Evaluation of Magnetic Coordinates for Particle Transport Studies in Asymmetric Plasmas

Particle transport in thermonuclear plasmas depends basically on the number of directions of symmetry in the plasma configuration. The search for toroidal confinement devices alternative to the tokamak has led to systems like the stellarator, heliac, etc., all with less symmetry and consequently enhanced diffusion, especially at low collisionality regimes. Even for the tokamak, asymmetry effects are important for it is well known that a tokamak ripple of 1% or less is able to significantly enhance the ion loss. Therefore, any numerical technique enabling us to understand asymmetric plasmas would be most welcome. Deficiencies in our tools have long been recognized. Not only are the equilibrium and stability properties difftcult to simulate, but even the single particle confinement characteristics are largely unknown. In these asymmetric configurations coordinates based on the magnetic field lines become the natural coordinates to use as it is intrinsically capable of higher accuracy and speed. In this system, the rapid motion of particles parallel to the field line is well-separated from the slow drift motion across the field lines, and hence large errors arising from numerical diffusion is avoided. It has been realized both computationally [ 11 and, more recently, experimentally [2] that the presence of an electric field is essential to good particle confinement in stellarators. Since electric potential is uniform over a flux surface, the magnetic coordiantes provide an extremely easy method of incorporating electric field effects. The usefulness of magnetic coordinates has long been 261 002 l-999 l/83 $3.00