Increasingly stringent environmental regulation imposed on both the military and civilian sectors has created a growing demand for alternative abatement methods for a variety of hazardous compounds. One alternative, the nonthermal plasma, shows promise of providing an efficient means for the destruction of dilute concentrations of hazardous air pollutants. The Dahlgren Laboratory of the Naval Surface Warfare Center has extensively investigated one type of nonthermal plasma discharge, the pulsed corona reactor, for the destruction of volatile organic compounds and chemical warfare agents. In this reactor, a fast rise time (∼10 ns), short duration (<100 ns), high-voltage pulse is repetitively delivered to a wire-cylinder electrode geometry, thereby producing a multitude of streamer discharges along its length. The resulting nonthermal plasma contains highly reactive chemical radicals which can interact with and destroy the hazardous molecules entrained in the ambient atmosphere flowing through the reactor volume. Increased electrical efficiency was obtained using a combination of high efficiency constant-current capacitor-charging, high repetition-rate spark gap switching, and resonant energy transfer to the reactor. Promising results have been obtained for toluene, methylene chloride, and dichlorodifluoromethane in air at concentrations of a few hundred parts per million. The device has been operated at voltages up to 30 kV, pulse repetition rates up to 1.4 kHz, and flow rates up to 60 ℓ/min. Detailed electrical measurements have been made to properly characterize the electrical properties of the pulsed corona reactor and to validate subsequent improvements in the reactor energy efficiency.

Order of magnitude estimates suggest that optically controlled bulk semiconductor switches should be able to withstand voltages up to the product of their thickness and the dielectric strength of their material. In reality, however, the devices fail -i.e., exhibit a behavior that resembles dielectric breakdown -already at voltages which are much lower. This deficiency threatens to limit the prospects of the device concept quite seriously and has so far not completely been understood. In our paper, we discuss several mechanisms which may underlie the observed phenomenon, and focus in particular on the dynamical aspects of it, namely on the sudden transition ("sudden breakdown" ) which takes the switch within a few ns from from the resistive off-state to a highly conductive on-state. We investigate a scenario that relates this transition to a second effect also seen during breakdown, namely to the spontaneous onset of current filamentation, and speculate that the magnetic self-contraction of the current (known as the "pinch effect") may play an essential role in the process. On the basis of a mathematical device model which incorporates the effects of particle transport and magnetic interaction, we obtain quantitative results for the speed and the threshold of magnetically driven filamentation, and find those numbers to lie in the A and the ps region, respectively. We conclude that the magnetic pinch may play a essential role in the dynamics of current filamentation and fast breakdown, but cannot explain the fast observed current rise in the ns-range by itself.

Tandem mass spectrometry (ms/ms) is reviewed with illustrations of its applications drawn from the natural products area. The various types of ms/ms scans-daughter, parent, and neutral loss-are described together with their favorable effects on ms detection limits and speed of analysis. The applications of ms/ms in conjunction with desorption ionization for the identification of quaternary alkaloids are illustrated. Isomer identification including energy resolved forms of ms/ms, the use of negative ions, and aspects of fragmentation patterns in ms/ms are all presented. Applications of tandem mass spectrometry to mapping alkaloid distribution, detection of new trace alkaloids, and quantitative analysis are covered.The analysis of natural products has been facilitated by the emergence of multiple stage mass spectrometry (1). A development from metastable ions (2), tandem mass spectrometry, or ms/ms, separates the ionization process from fragmentation and increases the information obtainable from a sample. Early experiments (3) utilized a reversed geometry mass spectrometer, also known as a mass-analyzed ion kinetic energy 'Presented as a plenary lecture at the "Recent Developments in NMR and Mass Spectral Analyses of Natural Substances" Symposium of the 24th Annual Meeting of the American Society of Pharmacognosy at

An analysis of observations of nonohmic current conduction in a copper-compensated GaAs (GaAs:Cu) photoconductive switch is presented. It is demonstrated that conduction during illumination and at modest current densities can be attributed to an optically injected plasma influenced by single-injection contact effects. However, at higher current densities, a double-injection model is more suitable. We provide further evidence that the transition from a single-injection process to a double-injection process is accompanied by the formation of at least one current filament with cross-sectional area of 5×10−4 cm2, and a peak current density greater than 103 A/cm2. We finish by suggesting avalanche breakdown at the n+−i anode junction as a possible mechanism for the onset of significant hole injection at the anode, a condition necessary to justify the use of a double-injection-dominated transport model in an essentially n-i-n device.

Electrical compensation in n-type, silicon-doped, GaAs (GaAs:Si) has been achieved for several different silicon doping densities. The introduction of deep copper acceptors into GaAs:Si through a thermal diffusion process has produced semi-insulating GaAs:Si:Cu. The density of diffused copper is shown to be predicted, to a good approximation, by knowledge of both the annealing temperature at which compensation is observed, and the initial free-electron density. Also, a model based on Fermi–Dirac statistics has demonstrated the same qualitative behavior as the data.