W. Gekelman

The plasma source of the Large Plasma Device at University of California, Los Angeles

The Large Plasma Device at the University of California, Los Angeles has recently been upgraded. The plasma is now 18m long (the device is 22m long) and is designed to produce a 0.36T axial magnetic field. Its plasma source has also been upgraded, incorporating a 1m square heater, a 72cm diameter cathode and anode, and associated heat shields and reflectors. The barium oxide coated cathode is heated to 750°C and can produce plasmas of diameters up to 0.9m diameter (depending on the magnetic field configuration), and densities up to 7×1012cm−3 with a spatial uniformity of ±10%.

The many faces of shear Alfvén wavesa)

One of the fundamental waves in magnetized plasmas is the shear Alfven wave. This wave is responsible for rearranging current systems and, in fact all low frequency currents in magnetized plasmas are shear waves. It has become apparent that Alfven waves are important in a wide variety of physical environments. Shear waves of various forms have been a topic of experimental research for more than fifteen years in the large plasma device (LAPD) at UCLA. The waves were first studied in both the kinetic and inertial regimes when excited by fluctuating currents with transverse dimension on the order of the collisionless skin depth. Theory and experiment on wave propagation in these regimes is presented, and the morphology of the wave is illustrated to be dependent on the generation mechanism. Three-dimensional currents associated with the waves have been mapped. The ion motion, which closes the current across the magnetic field, has been studied using laser induced fluorescence. The wave propagation in inhomogeneous magnetic fields and density gradients is presented as well as effects of collisions and reflections from boundaries. Reflections may result in Alfvenic field line resonances and in the right conditions maser action. The waves occur spontaneously on temperature and density gradients as hybrids with drift waves. These have been seen to affect cross-field heat and plasma transport. Although the waves are easily launched with antennas, they may also be generated by secondary processes, such as Cherenkov radiation. This is the case when intense shear Alfven waves in a background magnetoplasma are produced by an exploding laser-produced plasma. Time varying magnetic flux ropes can be considered to be low frequency shear waves. Studies of the interaction of multiple ropes and the link between magnetic field line reconnection and rope dynamics are revealed. This manuscript gives us an overview of the major results from these experiments and provides a modern prospective for the earlier studies of shear Alfven waves.

The upgraded Large Plasma Device, a machine for studying frontier basic plasma physics

In 1991 a manuscript describing an instrument for studying magnetized plasmas was published in this journal. The Large Plasma Device (LAPD) was upgraded in 2001 and has become a national user facility for the study of basic plasma physics. The upgrade as well as diagnostics introduced since then has significantly changed the capabilities of the device. All references to the machine still quote the original RSI paper, which at this time is not appropriate. In this work, the properties of the updated LAPD are presented. The strategy of the machine construction, the available diagnostics, the parameters available for experiments, as well as illustrations of several experiments are presented here.

Laboratory experiments on Alfvén waves caused by rapidly expanding plasmas and their relationship to space phenomena

(1) There are many situations which naturally occur in space (coronal mass ejections, supernovas) or are man-made (upper atmospheric detonations) in which a dense plasma expands into a background magnetized plasma that can support Alfven waves. The Large Plasma Device (LAPD) at UCLA is a machine in which Alfven wave propagation in homogeneous and inhomogeneous plasmas has been studied. A new class of experiments which involve the expansion of a dense (initially nlaser-produced/nbackground � 1) laser- produced plasma into an ambient highly magnetized plasma capable of supporting Alfven waves will be presented. The 150 MW laser is pulsed at the same 1 Hz repetition rate as the plasma in a highly reproducible experiment. The laser beam impacts a solid target such that the initial plasma burst is directed across the ambient magnetic field. The interaction results in the production of intense shear and compressional Alfven waves, as well as large density perturbations. The waves propagate away from the target and are observed to become plasma column resonances. The magnetic fields of the waves are measured with a 3-axis inductive probe. Spatial patterns of the magnetic fields associated with the waves and density perturbations are acquired at over 10,000 spatial locations and as a function of time. Measurements are used to estimate the coupling efficiency of the laser energy and kinetic energy of the dense plasma into wave energy. The shear wave generation mechanism is due to field-aligned return currents, which replace fast electrons escaping the initial blast. INDEX TERMS: 7831 Space Plasma Physics: Laboratory studies; 7513 Solar Physics, Astrophysics, and Astronomy: Coronal mass ejections; 7871 Space Plasma Physics: Waves and instabilities; 2111 Interplanetary Physics: Ejecta, driver gases, and magnetic clouds; 7524 Solar Physics, Astrophysics, and Astronomy: Magnetic fields; KEYWORDS: Alfven waves, diamagnetic cavities, supersonic expansion, Alfven wings, high beta plasmas, cross-field expansion, current systems Citation: Gekelman, W., M. Van Zeeland, S. Vincena, and P. Pribyl, Laboratory experiments on Alfven waves caused by rapidly expanding plasmas and their relationship to space phenomena, J. Geophys. Res., 108(A7), 1281, doi:10.1029/2002JA009741, 2003.

Laser-plasma diamagnetism in the presence of an ambient magnetized plasma

This work is an experimental study of the diamagnetic cavity created by a dense laser-produced plasma (initially, nlpp/n0≫1) expanding into an ambient magnetized background plasma (n0=2×1012 cm−3) capable of supporting Alfven waves. The experiments are carried out on the upgraded Large Plasma Device [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)] at UCLA. Two-dimensional data of both the diamagnetic cavity as well as visible light emission are presented and found to be rich in structure with spatially similar characteristics. Laser-plasma diamagnetism has been observed to be relatively unaffected by the presence of a background plasma for nlpp/n0≈10 at time of peak diamagnetism.

Dynamics of exploding plasmas in a large magnetized plasma

The dynamics of an exploding laser-produced plasma in a large ambient magneto-plasma was investigated with magnetic flux probes and Langmuir probes. Debris-ions expanding at super-Alfvenic velocity (up to MA=1.5) expel the ambient magnetic field, creating a large (>20 cm) diamagnetic cavity. We observe a field compression of up to B/B0=1.5 as well as localized electron heating at the edge of the bubble. Two-dimensional hybrid simulations reproduce these measurements well and show that the majority of the ambient ions are energized by the magnetic piston and swept outside the bubble volume. Nonlinear shear-Alfven waves (δB/B0>25%) are radiated from the cavity with a coupling efficiency of 70% from magnetic energy in the bubble to the wave.

High-energy Nd:glass laser facility for collisionless laboratory astrophysics

A kilojoule-class laser (Raptor) has recently been activated at the Phoenix-laser-facility at the University of California Los Angeles (UCLA) for an experimental program on laboratory astrophysics in conjunction with the Large Plasma Device (LAPD). The unique combination of a high-energy laser system and the 18 meter long, highly-magnetized but current-free plasma will support a new class of plasma physics experiments, including the first laboratory simulations of quasi-parallel collisionless shocks, experiments on magnetic reconnection, or advanced laser-based diagnostics of basic plasmas. Here we present the parameter space accessible with this new instrument, results from a laser-driven magnetic piston experiment at reduced power, and a detailed description of the laser system and its performance.

Observation of collisionless shocks in a large current‐free laboratory plasma

We report the first measurements of the formation and structure of a magnetized collisionless shock by a laser-driven magnetic piston in a current-free laboratory plasma. This new class of experiments combines a high-energy laser system and a large magnetized plasma to transfer energy from a laser plasma plume to the ambient ions through collisionless coupling, until a self-sustained MA∼ 2 magnetosonic shock separates from the piston. The ambient plasma is highly magnetized, current free, and large enough (17 m × 0.6 m) to support Alfven waves. Magnetic field measurements of the structure and evolution of the shock are consistent with two-dimensional hybrid simulations, which show Larmor coupling between the debris and ambient ions and the presence of reflected ions, which provide the dissipation. The measured shock formation time confirms predictions from computational work.

Electric field measurements of directly converted lower hybrid waves at a density striation

Previous experiments at the Large Plasma Device (LAPD) at UCLA provide evidence that whistler waves incident on a field-aligned density striation will produce lower hybrid waves via a linear mode-coupling mechanism [Bamber, et. al, 1994; Bamber, et. al, 1995]. These experiments were limited in that only the magnetic fields of the waves were measured. Recent experiments at the LAPD directly measure the whistler and lower hybrid wave electric fields. The lower hybrid wave electric field amplitude is 0.8 times the incident electric field. These measurements are well within theory and allow us to make some estimates about the conversion efficiency of the linear conversion mechanism.

Spectral gap of shear Alfvén waves in a periodic array of magnetic mirrors

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 Alfven spectra. Shear Alfven waves (SAW) are launched by antennas inserted in the LAPD plasma and diagnosed by B-dot probes at many axial locations. Alfven 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.