S. Vincena

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.

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.

Experimental measurements of the propagation of large-amplitude shear Alfvén waves

Experiments on the edge plasma of tokamaks have discovered magnetic fluctuations which are highly correlated along the magnetic field, and are correlated with scale size of the electron inertial length (δ = c/ωpe) across the field. They are, in all probability, shear Alfven waves. The FREJA, FAST, and Interball satellites have frequently encountered density striations in the auroral ionosphere. These can be narrow, also of the order of δ. Intense wave activity has been measured within these structures and tentatively identified as shear inertial (VA>Vthe) Alfven waves. These waves have been studied in great detail in the Large Plasma Device at UCLA. The plasma, which is 10 m in length and 500 ion Larmor radii in diameter (He (λ∥≈2 m), Ar (λ∥≈10 m, 1.5 kG, 40 cm plasma diameter, n = 1.0-4.0×1012 cm-3, fully ionized) supports Alfven waves. Our initial investigations, which will be briefly reviewed, involved low-amplitude (δBwave/B0≈10-4) shear waves launched by modulating a skin depth size current channel, and have examined the wave characteristics in the kinetic (VA<Vthe) and inertial regimes and in magnetic field gradients. Launching higher-power waves (δBwave/B0≥10-3) waves with the use of a helical antenna has extended these studies. Both shear Alfven waves (ω<ωci) and compressional Alfven waves have been investigated. Below fci the wave fields slowly spread across the background magnetic field and the current associated with it forms a rotating spiral. The higher-power wave causes a localized density perturbation when δBwave/B0 exceeds 10-3 even when the wave propagates below the cyclotron frequency. The perturbation is measured using Langmuir probes as well as laser-induced fluorescence (LIF) signal from Ar II ions. We present data of the wave propagation in which the temporal history of the vector magnetic field was acquired at 20 000 spatial locations. The data is used to calculate 3D wave currents, wave phase fronts and energy propagation. In helium the wave pattern is more complex than in argon. We also present the space and time evolution of the density perturbations associated with the wave in an Ar plasma. LIF data was used to directly measure the ion motion in the electric field of the wave, ion polarization currents and the motion of the ions as they form the density non-uniformities.

Three-dimensional current systems generated by plasmas colliding in a background magnetoplasma

Results are presented from an experiment in which two plasmas, initially far denser than a background magnetoplasma, collide as they move across the magnetic field. The dense plasmas are formed when laser beams, nearly orthogonal to the background magnetic field, strike two targets. The merging plasmas are observed to carry large diamagnetic currents. A reconnection event is triggered by the collision and the electric field induced in this event generates a field-aligned current, which is the first step in the development of a fully three-dimensional current system. After several ion gyroperiods, the current systems become those of shear Alfven waves. As local currents move, small reconnection “flares” occur at many locations throughout the volume, but they do not seem to affect the overall system dynamics. The data clearly show that the induced electric field is carried though the system by shear Alfven waves. The wave electric fields as well as local magnetic helicity are discussed.

Laboratory observation of Alfvén resonance

We present laboratory observations of shear Alfven wave resonances in a cylindrical plasma column. Fourier data are presented which show the existence of many standing eigenmodes in the shear Alfven frequency range. Harmonic frequency dependence on ambient is presented for three resonances. A wavelet analysis is used to show the lifetimes of the identified harmonic modes. Experimental Q values for the first five eigenfrequencies are measured and compared to theoretical values.

Generation of polarized shear Alfvén waves by a rotating magnetic field source

Experiments are performed in the Large Plasma Device at the University of California, Los Angeles to study the propagation of field-aligned, polarized kinetic shear Alfven waves radiated from a rotating magnetic field source created via a novel phased orthogonal loop antenna. Both right and left hand circular polarizations are generated at a wide range of frequencies from 0.21≤ω/Ωci<0.93. Propagation parallel to the background magnetic field near the Alfven velocity is observed along with a small parallel wave magnetic field component implying a shear mode. The peak-to-peak magnitude of the wave magnetic field, 33 cm away from the antenna, is on the order of 0.8% of the background field and drops off in the far field. The full width at half maximum of the wave energy changes little over a distance of 2.5 parallel wavelengths while the exponential decrease in wave energy as a function of distance can be attributed to collisional damping. Evidence of electron heating and ionization is observed during the pulse.

Currents and shear Alfvén wave radiation generated by an exploding laser-produced plasma: Perpendicular incidence

Examples of one plasma expanding into another and the consequent radiation of wave energy are abundant in both nature and the laboratory. This work is an experimental study of the expansion of a dense laser-produced plasma (initially, nlpp/n0≫1) into a magnetized background plasma (n0=2×1012 cm−3) capable of supporting Alfven waves. The experiments are carried out on the upgraded Large Plasma Device (LAPD) at UCLA [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)]. It has been observed that the presence of a background plasma allows laser-plasma charge separation to occur that would otherwise be limited by large ambipolar fields. This charge separation results in the creation of current structures which radiate shear Alfven waves. The waves propagate away from the target and are observed to become plasma column resonances. Conditions for increased current amplitude and wave coupling are investigated.

Measurements of classical transport of fast ions

To study the fast-ion transport in a well controlled background plasma, a 3-cm diameter rf ion gun launches a pulsed, ∼300eV ribbon shaped argon ion beam parallel to or at 15° to the magnetic field in the Large Plasma Device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)] at UCLA. The parallel energy of the beam is measured by a two-grid energy analyzer at two axial locations (z=0.32m and z=6.4m) 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...