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. It also reveals substructures embedded in the current density, indicating a compressed current sheet.

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. A high-time resolution impedance probe is under development at the U.S. Naval Research Laboratory with the goal of increasing the spatial resolution of measurements in space. To achieve this, a short-time Gaussian monopulse with a center frequency of 40 MHz and containing a full spectrum of frequencies is applied to an electrically short dipole antenna. Laboratory experiments were performed with the Gaussian monopulse triggered once every 10 µs and averaged over ten shots, equating to a spatial resolution of 13 cm for a typical sounding rocket speed. This paper discusses the development of the new high-time/spatial resolution self-impedance probe and illustrates that the short-time pulse method yields results that match well with data taken using conventional methods. It is shown that plasma parameters such as the electron density, sheath frequency, and electron-neutral collision frequency can also be derived from the data. In addition, data from the high-time/spatial resolution impedance probe are shown to compare well with those from theoretical impedance models.

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.

A high time resolution x-ray diagnostic on the Madison Symmetric Torus.

A new high time resolution x-ray detector has been installed on the Madison Symmetric Torus (MST) to make measurements around sawtooth events. The detector system is comprised of a silicon avalanche photodiode, a 20 ns Gaussian shaping amplifier, and a 500 MHz digitizer with 14-bit sampling resolution. The fast shaping time diminishes the need to restrict the amount of x-ray flux reaching the detector, limiting the system dead-time. With a much higher time resolution than systems currently in use in high temperature plasma physics experiments, this new detector has the versatility to be used in a variety of discharges with varying flux and the ability to study dynamics on both slow and fast time scales. This paper discusses the new fast x-ray detector recently installed on MST and the improved time resolution capabilities compared to the existing soft and hard x-ray diagnostics. In addition to the detector hardware, improvements to the detector calibration and x-ray pulse identification software, such as additional fitting parameters and a more sophisticated fitting routine are discussed. Finally, initial data taken in both high confinement and standard reversed-field pinch plasma discharges are compared.

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. The experiment characterizes the compression or relaxation of boundary layers often generated in a variety of laboratory and space plasma processes.