Measurements of Diffusion in Velocity Space from Ion-Ion Collisions

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Abstract
Diffusion of particles in velocity space due to ion-ion collisions is measured experimentally using the spatial ion-wave echo as a diagnostic technique. The experiment was performed in a highly ionized plasma. The ion-ion collisions are described by a Fokker-Planck term in the ion kinetic equation. The good agreement between the measurement of the echo position, phase velocity, and amplitude with the theoretical predictions verifies that the diffusion term of the Fokker-Planck equation is appropriate for describing the effect of small-angle Coulomb collisions. The magnitude of the second-order coupling coefficient relating the amplitudes of the excited waves to the echo amplitude was also measured. The agreement with theory indicates the validity of the second-order expansion of the linearized Vlasov equation.

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