Chaos at edges of plasma in tokamaks and stellarators

Plasma in tokamaks and stellerators (two major types of facilities used to conduct research on controlled thermonuclear fusion) is confined by magnetic fields. Toroidal and poloidal components of the field form a system of infinitely many nested surfaces of magnetic field force lines. Transport of particles and energy along the lines is at least six orders of magnitude larger than perpendicular to the surfaces. In result the so-called plasma confinement coefficient is relatively high. Inside the plasma the effects are desirable, producing sufficient temperature and density to run the thermonuclear reaction. However, a too large energy/particle density at the plasma edge is a reason for an accelerated erosion of plasma chamber walls and in some unfavourable circumstances may even destroy the chamber. One of the methods to decrease amount of energy dissipated at plasma edges is to introduce there the so-called stochastic region. Stochastic force lines of the field do not form any concentric magnetic surfaces, but -- in line with the ergodic theorem -- fill up the cavity. Such field topology helps to disperse flux of heat and particles interacting with chamber wall on a larger interaction area. In tokamaks, the stochastic region is produced by means of some additional small resonance magnetic fields; in stellarators such regions are inherent parts of their 3D configurations.

Professor Ludwik Dobrzyński