Ami DuBois

Electron-ion hybrid instability experiment upgrades to the Auburn Linear Experiment for Instability Studies

The Auburn Linear EXperiment for Instability Studies (ALEXIS) is a laboratory plasma physics experiment used to study spatially inhomogeneous flows in a magnetized cylindrical plasma column that are driven by crossed electric (E) and magnetic (B) fields. ALEXIS was recently upgraded to include a small, secondary plasma source for a new dual source, interpenetrating plasma experiment. Using two plasma sources allows for highly localized electric fields to be made at the boundary of the two plasmas, inducing strong E × B velocity shear in the plasma, which can give rise to a regime of instabilities that have not previously been studied in ALEXIS. The dual plasma configuration makes it possible to have independent control over the velocity shear and the density gradient. This paper discusses the recent addition of the secondary plasma source to ALEXIS, as well as the plasma diagnostics used to measure electric fields and electron densities.

Density gradient effects on transverse shear driven lower hybrid waves

Shear driven instabilities are commonly observed in the near-Earth space, particularly in boundary layer plasmas. When the shear scale length (LE ) is much less than the ion gyro-radius (ρi ) but greater than the electron gyro-radius (ρe ), the electrons are magnetized in the shear layer, but the ions are effectively un-magnetized. The resulting shear driven instability, the electron-ion hybrid (EIH) instability, is investigated in a new interpenetrating plasma configuration in the Auburn Linear EXperiment for Instability Studies. In order to understand the dynamics of magnetospheric boundary layers, the EIH instability is studied in the presence of a density gradient located at the boundary layer between two plasmas. This paper reports on a recent experiment in which electrostatic lower hybrid waves are identified as the EIH instability, and the effect of a density gradient on the instability properties are investigated.

Preliminary characteristics of magnetic field and plasma performance in the Magnetized Dusty Plasma Experiment (MDPX)

The Magnetized Dusty Plasma Experiment (MDPX) device is a newly constructed research instrument for the study of dusty (complex) plasmas. The MDPX device is envisioned as an experimental platform in which the dynamical behavior of all three charged plasma components, the electrons, ions, and charged microparticles (i.e., the ‘dust’) will be significantly influenced by the magnetic force. This brief paper will provide a short overview of the design, magnetic performance, and initial plasma measurements in the MDPX device.

Experimental characterization of broadband electrostatic noise due to plasma compression

For a wide variety of laboratory and space plasma environments, theoretical predictions state that plasmas are unstable to inhomogeneous flows over a very broad frequency range. Such sheared flows are generated in the Earths magnetosphere and intensify during active periods. Specifically, for a velocity shear oriented perpendicular to a uniform background magnetic field, the shear scale length (LE) compared to the ion gyroradius (ρi) determines the character of the shear-driven instability that may prevail. An interpenetrating plasma configuration is used to create a transverse velocity shear profile in a magnetized plasma column, a condition similar to that found in the natural boundary layers. The continuous variation of ρi/LE and the associated transition of the instability regimes driven by the shear flow mechanism are demonstrated in a single laboratory experiment. Broadband wave emission correlated to increasing/decreasing stress (i.e., ρi/LE), a characteristic signature of a boundary layer crossing, is found under controlled and repeatable conditions. This result holds out the promise for understanding the cause and effect of the in situ observation of broadband electrostatic noise.

Suppression of drift waves in a linear magnetized plasma column

In magnetically confined fusion plasmas, drift wave driven turbulence can lead to enhanced particle transport from the plasma. Because of this, a significant research emphasis has been placed on the suppression of drift waves in the plasma edge. However, the combination of the toroidal geometry and short plasma lifetimes can make it difficult to fully characterize the properties of these instabilities. Because linear magnetized plasma devices offer a combination of simpler geometry and steady state plasma generation, it is possible to perform detailed studies of many types of plasma instabilities—including drift waves. This paper reports on a recent experiment in which low frequency instabilities (ω ≤ ωci) in the Auburn Linear EXperiment for Instability Studies plasma device were characterized as drift waves and through changes in the parallel current, it is shown that it is possible to suppress these instabilities.

A laboratory investigation of the dynamics of shear flows in a plasma boundary layer

Summary form only given. At naturally occurring plasma boundaries in the near-Earth space environment, such as the magnetopause and the plasma sheet boundary layer, strong sheared plasma flows are often observed. As plasma boundary layers begin to relax from a compressed state, the ratio of the ion gyro-radius to the shear scale length decreases. At these layers, broadband electrostatic noise has been observed where the frequency ranges from well below the ion cyclotron frequency to the electron plasma frequency. Simulations have confirmed that the free energy in the sheared plasma flows can excite Kelvin Helmholtz instabilities, ion cyclotron-like instabilities, and lower hybrid modes. Kinetic theory described by G. Ganguli (G. Ganguli, M. Keskinen et. al., J. Geophys. Res., A5, 8873, 1994) discusses the three distinct instability regimes in the context of the ion gyro-radius and the shear scale length. The ratio of the ion gyro-radius to the shear scale length acts as a surrogate for the magnitude of stress that a plasma layer is subjected to and determines which mode is dominant. In a recent laboratory experiment, an interpenetrating plasma configuration is used to create a transverse velocity shear profile in a magnetized plasma column. For the first time, the continuous variation of the ratio of the ion gyro-radius to the shear scale length, and the associated transition of the instability regimes driven by the shear flow mechanism, is demonstrated in a single laboratory experiment. This is the first time that a laboratory experiment has reproduced the actual space observation of broadband emission, which is a characteristic signature of boundary layer crossings by a satellite. These results confirm the basic theory that plasma is unstable to transverse velocity shear in a broad frequency range, and provide evidence for theory predicted two decades ago by G. Ganguli (G. Ganguli, M. Keskinen et. al., J. Geophys. Res., A5, 8873, 1994), which proposes that the relaxation of a compressed boundary layer in a collisionless plasma leads to broadband electrostatic noise that is commonly observed by satellites.

Anisotropic Electron Tail Generation during Tearing Mode Magnetic Reconnection

The first experimental evidence of anisotropic electron energization during magnetic reconnection that favors a direction perpendicular to the guide magnetic field in a toroidal, magnetically confined plasma is reported in this Letter. Magnetic reconnection plays an important role in particle heating, energization, and transport in space and laboratory plasmas. In toroidal devices like the Madison Symmetric Torus, discrete magnetic reconnection events release large amounts of energy from the equilibrium magnetic field. Fast x-ray measurements imply a non-Maxwellian, anisotropic energetic electron tail is formed at the time of reconnection. The tail is well described by a power-law energy dependence. The expected bremsstrahlung from an electron distribution with an anisotropic energetic tail (v_{⊥}>v_{∥}) spatially localized in the core region is consistent with x-ray emission measurements. A turbulent process related to tearing fluctuations is the most likely cause for the energetic electron tail formation.

X-ray analysis of electron Bernstein wave heating in MST

A pulse height analyzing x-ray tomography system has been developed to detect x-rays from electron Bernstein wave heated electrons in the Madison symmetric torus reversed field pinch (RFP). Cadmium zinc telluride detectors are arranged in a parallel beam array with two orthogonal multi-chord detectors that may be used for tomography. In addition a repositionable 16 channel fan beam camera with a 55° field of view is used to augment data collected with the Hard X-ray array. The chord integrated signals identify target emission from RF heated electrons striking a limiter located 12° toroidally away from the RF injection port. This provides information on heated electron spectrum, transport, and diffusion. RF induced x-ray emission from absorption on harmonic electron cyclotron resonances in low current (<250 kA) RFP discharges has been observed.