Wave-Modified Ion Distributions and Cross-Field Transport Measurements by Laser-Induced Fluorescence

We present results of studies of cross-field transport and ion heating produced by electrostatic ion-cyclotron waves (EICW). Both coherent and turbulent wave spectra were generated, and the modification of ion velocities and phase-space trajectories by fluctuations of each type were measured using two optical diagnostic techniques recently developed at U. C. Irvine. The optical diagnostics employed in these experiments are based upon the laser-induced fluorescence of barium plasma ions. Ion velocity distributions are obtained from the Doppler broadening of the plasma absorption lines. In order to provide increased resolution and sensitivity, a scanning-single-frequency CW dye laser is used to provide simultaneous excitation and velocity selection. The limits of applicability due to optical thickness, collisionality, and especially optical-pumping effects, are also discussed. Optical pumping (the change in quantum-level populations induced by intense optical fields) forms the basis of a new plasma diagnostic technique which we term "optical tagging." We show that it is possible to directly observe the phase-space transport of ions by using either one or two lasers to provide the Tag and Detection beams. In the simplest application of optical tagging, the axial drift of plasma ions parallel to a steady-state magnetic field was measured using broadband tagging lasers. Velocity -selective tagging, using single-frequency laser beams, was also demonstrated. The application of these optical diagnostic techniques to an investigation of current-driven ion-cyclotron waves forms the core of this work. These waves, destabilized by an electron current drawn to a 6mm diameter disk, are characterized by a coherent, large-amplitude saturated state. The principal ion response to these wave fields is also coherent: as many as 70% of the ions initially in the unstable region are accelerated in a self-consistent manner to mean energies of the order of leV, which is more than three times the amplitude of the oscillating potential. These energetic ions form an aximuthal ion-ring-beam surrounding the unstable current channel. Optical tagging experiments reveal that the trajectories of these ring ions close back upon the unstable region within one wave period, so that the ring formation does not correspond to significant irreversible radial transport. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.