A multiple magnetic mirror array is formed at the Large Plasma Device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)] to study axial periodicity-influenced Alfvén spectra. Shear Alfvén waves (SAW) are launched by antennas inserted in the LAPD plasma and diagnosed by B-dot probes at many axial locations. Alfvén wave spectral gaps and continua are formed similar to wave propagation in other periodic media due to the Bragg effect. The measured width of the propagation gap increases with the modulation amplitude as predicted by the solutions to Mathieu’s equation. A two-dimensional finite-difference code modeling SAW in a mirror array configuration shows similar spectral features. Machine end-reflection conditions and damping mechanisms including electron-ion Coulomb collision and electron Landau damping are important for simulation.

In a first step toward generating an electron–positron plasma, a proof-of-principle experiment is reported in which externally injected slow positrons are trapped in a magnetic mirror configuration by electron cyclotron resonance heating. With a primary flux of only 530 slow positrons/s from a 600 μCi Na-22 positron source/moderator system, an estimated equilibrium density of 5×102 cm−3 is obtained in a 20 cm3 volume. With an appropriate increase of the injected positron flux, densities in the 107 cm−3 range can be expected.

Strong drift wave turbulence is observed in the Large Plasma Device ͓H. Gekelman et al., Rev. Sci. Instrum. 62, 2875 ͑1991͔͒ on density gradients produced by a plate limiter. Energetic lithium ions orbit through the turbulent region. Scans with a collimated ion analyzer and with Langmuir probes give detailed profiles of the fast ion spatial distribution and the fluctuating fields. The fast ion transport decreases rapidly with increasing fast ion gyroradius. Unlike the diffusive transport caused by Coulomb collisions, in this case the turbulent transport is nondiffusive. Analysis and simulation suggest that such nondiffusive transport is due to the interaction of the fast ions with stationary two-dimensional electrostatic turbulence.

In order to study the interaction of ions of intermediate energies with plasma fluctuations, two plasma immersible lithium ion sources, based on solid-state thermionic emitters ͑Li aluminosilicate͒ were developed. Compared to discharge based ion sources, they are compact, have zero gas load, small energy dispersion, and can be operated at any angle with respect to an ambient magnetic field of up to 4.0 kG. Beam energies range from 400 eV to 2.0 keV with typical beam current densities in the 1 mA/ cm 2 range. Because of the low ion mass, beam velocities of 100-300 km/ s are in the range of Alfvén speeds in typical helium plasmas in the large plasma device.

To study the fast-ion transport in a well controlled background plasma, a 3-cm diameter rf ion gun launches a pulsed, ϳ300 eV ribbon shaped argon ion beam parallel to or at 15°to the magnetic field in the Large Plasma Device ͑LAPD͒ ͓W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 ͑1991͔͒ at UCLA. The parallel energy of the beam is measured by a two-grid energy analyzer at two axial locations ͑z = 0.32 m and z = 6.4 m͒ from the ion gun in LAPD. The calculated ion beam slowing-down time is consistent to within 10% with the prediction of classical Coulomb collision theory using the LAPD plasma parameters measured by a Langmuir probe. To measure cross-field transport, the beam is launched at 15°to the magnetic field. The beam then is focused periodically by the magnetic field to avoid geometrical spreading. The radial beam profile measurements are performed at different axial locations where the ion beam is periodically focused. The measured cross-field transport is in agreement to within 15% with the analytical classical collision theory and the solution to the Fokker-Planck kinetic equation. Collisions with neutrals have a negligible effect on the beam transport measurement but do attenuate the beam current.