Direct measurement of velocity space transport in a plasma

The direct measurement of the velocity space transport of ions in a plasma is reported. Measured diffusion and convection coefficients are compared to the calculated Fokker-Planck coefficients for a fully ionized plasma. The measurements were made in a Q-machine barium plasma (Ti= T e = 0.17 eV, 5 × 108 ≤n≤ 8 × 10 9 cm-3) with both high and low fluctuation levels. At low fluctuation levels the measured coefficients agree with classical collision theory. Coefficients measured in the presence of large amplitude fluctuations generally are larger and have different velocity dependences. Test particle distributions are created and interrogated using the technique of optical tagging. The short-time (≪90° collision time) relaxation of the test particle distribution function was measured as a function of density and temperature of the background plasma and as a function of the velocity of the test particle distribution. The values of the convection and diffusion coefficients were extracted from these measurements. Longer time relaxations (∼90° time) also were measured. © 1994 American Institute of Physics