Characteristics of runaway electrons in a tokamak

The characteristics of the high-energy tail (∼1 MeV) on the electron distribution in a tokamak have been determined with a new technique from measurements of the heat flux to a movable target on the outboard midplane. The measurements give the absolute value of f(v) and a direct estimate of the diffusion coefficient for energetic electrons at the edge. From this information, average confinement time, diffusion coefficient, and source position can be inferred. The diffusion coefficients are in the range 1-4 m2/s, comparable with those of the bulk particles, which are much larger than the values expected for energetic electrons subject only to electrostatic turbulence that would account for the bulk diffusivity