MMS Observations of a Compressed Current Sheet: Importance of the Ambipolar Electric Field.

Spacecraft data reveal a nonuniform ambipolar electric field transverse to the magnetic field in a thin current sheet in Earth's magnetotail that leads to intense E×B velocity shear and nongyrotropic particle distributions. The E×B drift far exceeds the diamagnetic drift and thus drives observed lower hybrid waves. The shear-driven waves are localized to the magnetic field reversal region and are therefore ideally suited for the anomalous dissipation necessary for reconnection.

Development of a high-time/spatial resolution self-impedance probe for measurements in laboratory and space plasmas.

Plasma impedance probes are often used in laboratory experiments as well as in space to make measurements of important plasma parameters such as the electron density. Conventional impedance probe methods involve sweeping the frequency applied to the probe through a range containing the plasma frequency, which can take on the order of a second to complete. This acquisition time leads to very low spatial resolution when making measurements from sounding rockets in the ionosphere.

Using a direct current (DC) glow discharge electrode as a non-invasive impedance probe for measuring electron density.

A small RF signal is applied to the anode of a low pressure ( ≤ 200 mTorr), low temperature ( ≤ 3 eV) direct current (DC) glow discharge operating at an electron density of ∼ 10 cm. The discharge is modeled as a collection of capacitive, resistive, and inductive circuit elements that have resonances at particular frequencies, much like an RLC circuit. The location of these resonances in frequency space provides information about the plasma parameters.

Nonlinear Generation of Electromagnetic Waves through Induced Scattering by Thermal Plasma.

We demonstrate the conversion of electrostatic pump waves into electromagnetic waves through nonlinear induced scattering by thermal particles in a laboratory plasma. Electrostatic waves in the whistler branch are launched that propagate near the resonance cone. When the amplitude exceeds a threshold ~5 × 10(-6) times the background magnetic field, wave power is scattered below the pump frequency with wave normal angles (~59°), where the scattered wavelength reaches the limits of the plasma column.

Screens versus microarrays for ruggedized retarding potential analyzers.

An array of highly miniaturized electrostatic lenses is shown to be a viable replacement for meshes or screens in a retarding potential analyzer (RPA) where mechanical ruggedness or the ability to intercept large currents of energetic particles is desirable. Data from a prototype device are presented cross-calibrated with a traditional planar RPA indicating how the so-called microarray configuration avoids energy-dependent transparency (either reduced or enhanced) associated with meshes or screens while providing accurate energy analysis with reasonable energy resolution. In contrast, another ruggedized configuration employing a screen is presented, showing the severity of energy-dependent enhanced transparency, verified by numerical simulation.

Plasma response to a varying degree of stress.

We report experimental evidence of a seamless transition between three distinct modes in a magnetized plasma with a transverse sheared flow as the ratio of the ion gyroradius to the shear scale length (a measure of shear magnitude) is varied. This was achieved using a dual plasma configuration in a laboratory experiment, where a sheared flow oriented perpendicular to a background magnetic field is localized at the boundary of the plasmas. This confirms the basic theory that plasma is unstable to transverse velocity shear in a broad frequency and wavelength range.

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.

Spontaneous electromagnetic emission from a strongly localized plasma flow.

Laboratory observations of electromagnetic ion-cyclotron waves generated by a localized transverse dc electric field are reported. Experiments indicate that these waves result from a strong E×B flow inhomogeneity in a mildly collisional plasma with subcritical magnetic field-aligned current. The wave amplitude scales with the magnitude of the applied radial dc electric field.