Design of the Lanthanum hexaboride based plasma source for the large plasma device at UCLA.

The Large Plasma Device (LAPD) at UCLA (University of California, Los Angeles) produces an 18 m long, magnetized, quiescent, and uniform plasma at a high repetition rate to enable studies of fundamental plasma physics. Here, we report on a major upgrade to the LAPD plasma source that allows for more robust operation and significant expansion of achievable plasma parameters. The original plasma source made use of a heated barium oxide (BaO) coated nickel sheet as an electron emitter. This source had a number of drawbacks, including a limited range of plasma density (≲4.0 × 1012 cm-3), a limited discharge duration (∼10 ms), and susceptibility to poisoning following oxygen exposure. The new plasma source utilizes a 38 cm diameter lanthanum hexaboride (LaB6) cathode, which has a significantly higher emissivity, allowing for a much larger discharge power density, and is robust to exposure to air. Peak plasma density of up to 3.0 × 1013 cm-33 in helium gas has been achieved. The typical operating pressure is ∼10-5 Torr, while dynamic pressure can be achieved through the gas-puffing technique. Discharges as long as 70 ms have been produced, enabling a variety of long-time-scale studies of processes, such as turbulent particle transport. The new source has been in continuous operation for 14 months, having survived air leaks, power outages that led to rapid temperature changes on the cathode and heater, and planned machine openings. We describe the design, construction, and initial operation of this novel new large-area LaB6 plasma source.

Laboratory Observations of Ultra-Low Frequency Analogue Waves Driven by the Right-Hand Resonant Ion Beam Instability.

The Right-Hand Resonant Instability (RHI) is one of several electromagnetic ion/ion beam instabilities responsible for the formation of parallel magnetized collisionless shocks and the generation of ultra-low frequency (ULF) waves in their foreshocks. This instability has been observed for the first time under foreshock-relevant conditions in the laboratory through the repeatable interaction of a preformed magnetized background plasma and a super-Alfvénic laser-produced plasma. This platform has enabled unprecedented volumetric measurements of waves generated by the RHI, revealing filamentary current structures in the transverse plane. These measurements are made in the plasma rest frame with both high spatial and temporal resolution, providing a perspective that is complementary to spacecraft observations. Direct comparison of data from both the experiment and the Wind spacecraft to 2D hybrid simulations demonstrates that the waves produced are analogous to the ULF waves observed upstream of the terrestrial bow shock.

Electrostatic structure of a magnetized laser-produced plasma.

Measurements of the structure of the electrostatic fields produced by the expansion of a laser-produced plasma into a background magnetized plasma are presented. The three-dimensional measurements of the electrostatic field are made using an emissive probe that measures the time-varying plasma potential on two orthogonal planes, one across and one containing the background magnetic field. The inductive electric field is also calculated from probe measurements of the time-varying magnetic fields. Deviations from local charge neutrality at the level of 10(-4) generate a radial electrostatic field with peak strength an order of magnitude larger than the corresponding inductive field. The electrostatic energy density near full expansion is over an order of magnitude larger than that of the induced azimuthal electric field. These measurements show that electrostatic fields must be included in theoretical and computational models of collisionless coupling in magnetized point explosions of laser-produced plasmas and their relation to similar phenomena such as magnetospheric chemical releases.

Laboratory measurements of electrostatic solitary structures generated by beam injection.

Electrostatic solitary structures are generated by injection of a suprathermal electron beam parallel to the magnetic field in a laboratory plasma. Electric microprobes with tips smaller than the Debye length (λDe) enabled the measurement of positive potential pulses with half-widths 4 to 25λDe and velocities 1 to 3 times the background electron thermal speed. Nonlinear wave packets of similar velocities and scales are also observed, indicating that the two descend from the same mode which is consistent with the electrostatic whistler mode and result from an instability likely to be driven by field-aligned currents.

Shear Alfvén wave perpendicular propagation from the kinetic to the inertial regime.

We report on observations of shear Alfvén waves radiated from a source of small transverse size, and the subsequent radial confinement of wave magnetic field energy within a cylindrical plasma. The radius of confinement lies between the kinetic regime of the bulk plasma and the inertial regime at the plasma edge; this radius is found to be a function of wave frequency. Numerical calculations using kinetic theory predict a zero in the perpendicular group velocity at a radius which varies in accord with the observations. An analytic expression for the perpendicular group velocity (valid for small perpendicular wave numbers) is given in the vicinity of the zero crossing.