J. E. Maggs

Whistler wave interaction with a density striation: A laboratory investigation of an auroral process

Whistler waves are launched toward a field-aligned density striation in a laboratory plasma. Characteristic scale length and frequency ratios are scaled to closely reproduce situations found in the auroral ionosphere. Detailed measurements show that at the striation edge nearest the wave-launching antenna, besides a reflected and a transmitted whistler wave, lower hybrid waves are also stimulated on both sides of the striation boundary in a manner consistent with the linear mode-conversion model. We find that the energy density of the mode-converted lower hybrid waves is typically 10% of the incident whistler wave energy density, reaching a maximum of 30% in one region. Lower hybrid waves are confined to within 2–3 perpendicular wavelengths in the interaction zone. Our results show that the interaction of electromagnetic whistler mode waves with density striations can cause significant amounts of energy to be deposited in the largely electrostatic lower hybrid mode and that it may therefore be a significant generation mechanism for these waves in certain regions of the ionosphere.

Modifications of turbulence and turbulent transport associated with a bias-induced confinement transition in the Large Plasma Device

Azimuthal flow is driven in the edge of the Large Plasma Device (LAPD) [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] through biasing a section of the vacuum vessel relative to the plasma source cathode. As the applied bias exceeds a threshold, a transition in radial particle confinement is observed, evidenced by a dramatic steepening in the density profile, similar to the L- to H-mode transition in toroidal confinement devices. The threshold behavior and dynamic behavior of radial transport is related to flow penetration and the degree of spatial overlap between the flow shear and density gradient profiles. An investigation of the changes in turbulence and turbulent particle transport associated with the confinement transition is presented. Two-dimensional cross-correlation measurements show that the spatial coherence of edge turbulence in LAPD changes significantly with biasing. The azimuthal correlation in the turbulence increases dramatically, while the radial correlation length is little al...

Structure of kinetic Alfven waves with small transverse scale length

This analytical study illustrates the spatial pattern of kinetic Alfven waves excited by a current-modulating disk whose dimension a, transverse to the confining magnetic field, is comparable to the ion sound gyroradius cs/Ωi, where cs is the sound speed and Ωi the ion cyclotron frequency. The radial structure of the wave azimuthal magnetic field is found to consist of four regions: a Bessel function behavior for r<a, a near null at r≅a, and a driven Airy pattern for r≫a which merges onto the 1/r asymptotic region. The pattern spreads at an angle given by tan θ=(ω/Ωi)(cs/vA)/2.6, where ω is the modulation frequency and vA is the Alfven speed. This behavior arises because there is a maximum value at finite k⊥ for the ratio of the perpendicular to parallel group velocity, which differs from the cone spreading [G. J. Morales et al., Phys. Plasmas 1, 3765 (1994)] associated with inertial Alfven waves.

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%.

Transition from Bohm to classical diffusion due to edge rotation of a cylindrical plasma

The outer region of the plasma column of a large, linear plasma device is rotated in a controlled fashion by biasing a section of the vacuum chamber wall positive with respect to the cathode (Er<0). The magnitude and temporal dependence of the observed cross-field current, produced when the bias voltage is applied, is consistent with ion current arising from ion-neutral collisions. Flow speeds in the outer regions of the plasma column exceed the local sound speed. In the nonrotating plasma column, cross-field, radial particle transport proceeds at the Bohm diffusion rate. Rotation, above a threshold bias voltage, reduces cross-field transport from Bohm to classical rates, leading to steeper radial density profiles. Reduction of particle transport is global and not isolated to the region of flow shear. The transition from the nonrotating to the rotating plasma edge in the linear plasma column is similar to the low confinement to high confinement mode transition observed in tokamaks when Er<0.

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.

Exponential frequency spectrum and Lorentzian pulses in magnetized plasmas

Two different experiments involving pressure gradients across the confinement magnetic field in a large plasma column are found to exhibit a broadband turbulence that displays an exponential frequency spectrum for frequencies below the ion cyclotron frequency. The exponential feature has been traced to the presence of solitary pulses having a Lorentzian temporal signature. These pulses arise from nonlinear interactions of drift-Alfven waves driven by the pressure gradients. In both experiments the width of the pulses is narrowly distributed resulting in exponential spectra with a single characteristic time scale. The temporal width of the pulses is measured to be a fraction of a period of the drift-Alfven waves. The experiments are performed in the Large Plasma Device (LAPD-U) [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] operated by the Basic Plasma Science Facility at the University of California, Los Angeles. One experiment involves a controlled, pure electron temperature gradient associated...

Modification of Turbulent Transport with Continuous Variation of Flow Shear in the Large Plasma Device

Continuous control over azimuthal flow and shear in the edge of the Large Plasma Device (LAPD) has been achieved using a biasable limiter which has allowed a careful study of the effect of flow shear on pressure-gradient-driven turbulence and transport in the LAPD. The LAPD rotates spontaneously in the ion diamagnetic direction; positive limiter bias first reduces, then minimizes (producing a near-zero shear state), and finally reverses the flow into the electron diamagnetic direction. Degradation of particle confinement is observed in the minimum shearing state and a reduction in the turbulent particle flux is observed with increasing shearing in both flow directions. Near-complete suppression of the turbulent particle flux is observed for shearing rates comparable to the turbulent autocorrelation rate measured in the minimum shear state. Turbulent flux suppression is dominated by amplitude reduction in low-frequency (<10 kHz) density fluctuations. An increase in fluctuations for the highest shearing states is observed with the emergence of a coherent mode which does not lead to net particle transport. The variations of density fluctuations are fit well with power laws and compare favorably to simple models of shear suppression of transport.

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.