Fast Ion and Thermal Plasma Transport in Turbulent Waves in the Large Plasma Device (LAPD)

The transport of fast ions and thermal plasmas in electrostatic microturbulence is studied. Strong density and potential fluctuations ($\delta n/n\sim \delta \varphi /kT_e \sim 0.5$, f$\sim $5-50 kHz) are observed in the LAPD in density gradient regions produced by obstacles with slab or cylindrical geometry. Wave characteristics and the associated plasma transport are modified by driving sheared E$\times $B drift through biasing the obstacle, and by modification of the axial magnetic fields (B$_{z})$ and the plasma species. Cross-field plasma transport is suppressed with small bias and large B$_{z}$, and is enhanced with large bias and small B$_{z}$. Suppressed cross-field thermal transport coincides with a 180\r{ } phase shift between the density and potential fluctuations in the radial direction, while the enhanced thermal transport is associated with modes having low mode number (m=1) and long radial correlation length. Large gyroradius lithium ions ($\rho _{fast} /\rho _s \sim 10)$ orbit through the turbulent region. Scans with a collimated analyzer and with Langmuir probes give detailed profiles of the fast ion spatial-temporal distribution and of the fluctuating fields. Fast-ion transport decreases rapidly with increasing fast-ion gyroradius. Background waves with different scale lengths also alter the fast ion transport: Beam diffusion is smaller in waves with smaller structures (higher mode number); also, coherent waves with long correlation length cause less beam diffusion than turbulent waves. Experimental results agree well with gyro-averaging theory. When the fast ion interacts with the wave for most of a wave period, a transition from super-diffusive to sub-diffusive transport is observed, as predicted by diffusion theory. A Monte Carlo trajectory-following code simulates the interaction of the fast ions with the measured turbulent fields. Good agreement between observation and modeling is observed.