On the electrostatic field of a tokamak in the limit of large aspect   ratio and concentric circular flux surfaces
  From a common expression for the poloidal electrostatic field of a tokamak, in the limit of large aspect ratio and concentric circular flux surfaces, one may determine the associated potential. This potential satisfies Poisson's equation, which reduces to Laplace's equation when the medium has vanishing charge density, in axial geometry but not toroidal geometry. A simple transformation takes the potential over to the correct harmonic form for tokamak coordinates, and the resulting electrostatic field is calculated. From the radial field one may estimate the supporting charge density on the boundary, and from the poloidal field one may determine a prediction for the radial dependence of the electron temperature, which does not compare well with a rough estimate of the profile often seen in a tokamak. 
Robert W. Johnson