Plasma Equilibria with Stochastic Regions and Magnetic Islands

Abstract

Here we present the principle of operation of fast algorithm based on the selection of few toroidal harmonics which visibly affect the plasma boundary and discuss the influence of the plasma induced magnetic fields on the shape and sizes of plasma boundary. The existing moderate pressure profile dependence increases as one passes from the shearless system to the system with large shear. Traditional methods for combating plasma induced changes in magnetic configurations by external axisymmetric magnetic fields are analyzed. It is shown that the quadrupole field can not only restore the rotational transform profile but usually visibly destroys external magnetic surfaces. An ill-posed problem of MHD stellarator equilibria identification from the external measurements is investigated and the possibility to restore the plasma profiles in the frame of a two-parametric set of functions using the system of local probes is discussed. In the second part of the work we generalize the well-known method of flux coordinates for the investigation of plasmas with nested magnetic surfaces to compute large-scale magnetic islands. The method for computing structures with several magnetic axes using non-single valued flux coordinates is discussed. 5 refs., 3 figs.