Fokker-Planck Velocity Diffusion Coefficient in Plasma Turbulence

A direct method of measuring the Fokker-Planck velocity diffusion coefficient in plasma turbulence is developed. The method is consistent with, but more general than, the quasilinear approximation. The velocity variance $〈\Delta v^2〉$ is measured by integrating the space-time potential correlation function along unperturbed particle orbits. Correlation times and velocity diffusion coefficients are measured as a function of velocity in ion-beam-plasma turbulence and the experimental results are compared with the Fokker-Planck theory requirements.