Stochastization of magnetic field lines in magnetized fusion plasma

An important goal of fusion research with magnetically confined plasmas is to maximize the achievable plasma pressure. In a tokamak, neoclassical tearing modes (NTMs), i.e. magnetic islands with poloidal (m) and toroidal (n) mode numbers driven unstable by the loss of bootstrap current inside the island, are of major concern as they are considered to be the most severe limitation to the maximum achievable plasma pressure. Developing of such magnetic islands requires resistive reconnection of the magnetic field lines. The resistive reconnection assumes a resistive diffusion of the magnetic field through the plasma which is a rather slow process. It was found in the ASDEX Upgrade that some MHD processes involving these tearing modes have much faster time scales compared with the resistive diffusion time. These processes involve the interaction of several modes and can be explained by stochastization of the magnetic field lines. Stochastization means that for the trajectory of an object time averaging and spatial averaging can be exchanged. This means also that the trajectory of an object starting from any point in a stochastic area comes infinitely close to any other point in this volume.

 

Poincare plot for a single (3,2) tearing mode (A) and for the interaction of (3,2) with (4,3) and (1,1) ideal modes (B). Stochastic region is clearly seen in figure (B).

References:

  1. O. Dumbrajs, V. Igochine, D. Constantinescu, H. Zohm, and ASDEX Upgrade Team, Stochastization as a possible cause of fast reconnection in the frequently interrupted regime of neoclassical tearing modes - Phys. of Plasmas, 2005, 12, 110704
  2. O. Dumbrajs, V. Igochine, H. Zohm, and the ASDEX Upgrade Team, Diffusion in a stochastic magnetic field in ASDEX Upgrade - Nucl. Fusion, 2008, 48, 024011
  3. O. Dumbrajs, V. Igochine, A. Gude, M. Maraschek, H. Zohm, and ASDEX Upgrade Team, Temporal evolution of neoclassical tearing modes in the frequently interrupted regime - Phys. of Plasmas, 2010, 17, 042118