Direct measurement of velocity-space transport in a fully ionized plasma

Direct measurements of velocity-space transport coefficients, which may be interpreted as the Fokker-Planck coefficients, in the direction of the confining magnetic field are reported. The measurements were made using the techniques of laser-induced fluorescence and optical tagging, which allow ions to be followed in phase space. The results show that in a plasma with nearly only thermal fluctuations the data agree well with test particle calculations made by N. Rostoker for classical collisional processes. In the presence of larger-amplitude drift-wave fluctuations there is a pronounced enhancement of the velocity diffusion coefficient at a velocity less than the thermal speed.