The ratio between the heat diffusion coefficients parallel and perpendicular to the magnetic field lines, χ||/χ⊥, influences the flattening of the temperature profile inside magnetic islands and the driving term of neoclassical tearing modes (Fitzpatrick 1995 Phys. Plasmas2 825). The value of this anisotropy is, however, not easily accessible experimentally. This paper presents a method to determine it from a systematic comparison of temperature measurements at magnetic islands with numerical heat diffusion simulations. The application of the method is demonstrated for a 2/1 magnetic island in the TEXTOR tokamak, where a heat diffusion anisotropy of 108 is observed. This is lower by a factor of 40 than predicted by Spitzer and Härm (Spitzer and Härm 1953 Phys. Rev.89 997) and a strong indication that the heat flux limit determines the flattening of the electron temperature across magnetic islands.
General kinetic solution for the Biermann battery with an associated pressure anisotropy generation