Magnetic reconnection from a multiscale instability cascade

Magnetic reconnection, the process whereby magnetic field lines break and then reconnect to form a different topology, underlies critical dynamics of magnetically confined plasmas in both nature and the laboratory. Magnetic reconnection involves localized diffusion of the magnetic field across plasma, yet observed reconnection rates are typically much higher than can be accounted for using classical electrical resistivity. It is generally proposed that the field diffusion underlying fast reconnection results instead from some combination of non-magnetohydrodynamic processes that become important on the ‘microscopic’ scale of the ion Larmor radius or the ion skin depth. A recent laboratory experiment demonstrated a transition from slow to fast magnetic reconnection when a current channel narrowed to a microscopic scale, but did not address how a macroscopic magnetohydrodynamic system accesses the microscale. Recent theoretical models and numerical simulations suggest that a macroscopic, two-dimensional magnetohydrodynamic current sheet might do this through a sequence of repetitive tearing and thinning into two-dimensional magnetized plasma structures having successively finer scales. Here we report observations demonstrating a cascade of instabilities from a distinct, macroscopic-scale magnetohydrodynamic instability to a distinct, microscopic-scale (ion skin depth) instability associated with fast magnetic reconnection. These observations resolve the full three-dimensional dynamics and give insight into the frequently impulsive nature of reconnection in space and laboratory plasmas

Sintering characteristics and properties of PuS and PuP are determined

Report on the preparation of plutonium monosulphide and plutonium monophosphide includes a description of the sintering characteristics and properties of these high-temperature compounds. data on weight loss, microstructure, density, melting point, thermal expansion, microhardness, Seebeck coefficient, and thermal diffusion are included

Energy Efficiency Analysis of the Discharge Circuit of Caltech Spheromak Experiment

The Caltech spheromak experiment uses a size A ignitron in switching a 59-μF capacitor bank (charged up to 8 kV) across an inductive plasma load. Typical power levels in the discharge circuit are ~200 MW for a duration of ~10 μs. This paper describes the setup of the circuit and the measurements of various impedances in the circuit. The combined impedance of the size A ignitron and the cables was found to be significantly larger than the plasma impedance. This causes the circuit to behave like a current source with low energy transfer efficiency. This behavior is expected to be common with other pulsed plasma experiments of similar size that employ an ignitron switch

Orbiter structural design and verification

The space shuttle development program provided the opportunity to challenge many of the established practices and approaches used in prior manned space flight programs. The most significant accomplishments and resulting precedents which emerged during the structural development of the space shuttle and the space shuttle orbiter are reviewed. Innovations in criteria, design solutions, and certification are highlighted, and brief comments on the lessons learned are included. Thermal stress, graphite epoxy moisture, window structure, and structural inspection are discussed under lessons learned

Opportunity cost and prudentiality : a representative-agent model of futures clearinghouse behavior

Includes bibliographic references (p. 31-38)

Speech perception in MRI scanner noise by persons with aphasia

Purpose. To examine reductions in performance on auditory tasks by aphasic and neurologically-intact individuals as a result of concomitant MRI scanner noise. Methods. Four tasks together forming a continuum of linguistic complexity were developed. They included complex-tone pitch discrimination, same/different discrimination of minimal pair syllables, lexical decision, and sentence plausibility. Each task was performed by persons with aphasia (PWA) and by controls. The stimuli were presented in silence and also in the noise recorded from within the bore of a 3T MRI scanner at three signal-to-noise ratios (S/Ns). Results. Across the four tasks, the PWA scored lower than the controls and performance fell as a function of decreased S/N. However, the rate at which performance fell was not different across the two listener groups in any task. Conclusions. Depending upon the relative levels of the signals and noise, the intense noise accompanying MRI scanning has the potential to severely disrupt performance. However, PWA are no more susceptible to the disruptive influence of this noise than are unimpaired individuals usually employed as controls. Thus, fMRI data from aphasic and control individuals may be interpreted without complications associated with large interactions between scanner noise and performance reduction