Drift wave ion fluid velocity field measured by planar laser-induced fluorescence

The first plasma planar laser induced fluorescence (PLIF) diagnostic has been developed and used to study Ar plasma discharges in Caltech's Encore tokamak. The first two-dimensional time resolved measurements of the ion fluid velocity have been made. PLIF excited in a poloidal cross section by a narrow linewidth laser sheet is imaged onto a 10 x 10 anode microchannel plate photomultiplier. Both the ion temperature and one component of the fluid velocity of metastable ArII ions are measured by scanning the laser wavelength through the Doppler broadened and shifted PLIF absorption line. The maximum measured wavelength shifts correspond to velocities ∼ (1.5 ± 0.08) x 105 cm/s. A periodic spatial structure in the fluid velocity field is observed to oscillate in phase with a coherent, large amplitude, mostly electrostatic drift-Alfven wave with poloidal and toroidal mode numbers m = 2, n = 1. Using PLIF, oscillations in the temperature (Ti ∼ 3eV) out of phase with the drift wave potential have been observed. Langmuir probe measurements were made in a nearby poloidal cross section. The calculated ion fluid flow pattern in the drift approximation agreed qualitatively with the measured velocity field, but the calculations predict much larger velocities than are measured. The general agreement, despite the stochastic dynamics emphasizes the robust nature of the two-fluid description of the plasma. An explicit connection is made between the ion distribution and Poincare maps of the single particle dynamics in prescribed mean fields by considering the characteristics of the collisionless Vlasov equation. A self-consistent distribution function is restricted to being constant in stochastic regions of phase space where one particle orbit comes arbitrarily close to any point in the region. The implications of this viewpoint are explored for a simplified model of the drift wave