[ skip to content ]

Spring 2003

Schedule Spring 2003

January 14, 2003
3:30 pm (Tuesday)

Prof. Lepsha Vuskovic
Old Dominion University

Excited Atoms and Molecules in High Pressure Gas Discharges

There is an intensive effort to understand various types of high-pressure non-thermal discharges for the reason that wide range of applications could follow. These, partially ionized media in non-equilibrium state, tend to generate complex effects that are difficult to interpret without a detailed knowledge of elementary collision processes. Electronically excited atoms and molecules play an important role as intermediate states in a wide range of atomic collisions, many of which are crucial in high-pressure discharges. They can serve also as reservoirs of energy or as sources of high-energy electrons either through the energy pooling or through superelastic collisions. The analysis of current situation on the processes involving excited atoms of interest for high-pressure gas discharges are presented and pointed out the insufficiency of available cross section data. The new measurements of photon emission from the discharge are also presented and shown how to use these data to develop accurate models and interpretations of the observed phenomena.

March 18, 2003
3:30 pm (Tuesday)

Dr. Doyle Knight
Department of Mechanical and Aerospace Engineering, Rutgers - The State University of New Jersey

Energy Deposition for Control of Sypersonic Flows

High speed air vehicles can experience flow phenomena at off-design conditions which may cause serious performance penalties including severe local aero thermodynamic loads and abrupt degradation in propulsion efficiency. The Edney IV shock-shock interaction is an example of the former, while the transition from regular reflection to Mach reflection in the dual solution domain for intersecting shocks in a high speed inlet is an example of the latter. Localized pulsed electro magnetic energy deposition (e.g., by laser) offers the capability for mitigation of the deleterious effects of such flow phenomena. Experimental results at Mach 3.5 will be presented for pulsed laser energy deposition upstream of an Edney IV interaction and intersecting symmetric shock waves. Simulation results will be presented for the latter case. Both experiment and simulation indicate a beneficial effect of energy deposition, and imply significant opportunities for future research.

March 25, 2003
3:30 pm (Tuesday)

Dr. Frank A. Narducci
Naval Air Systems Command

Polarization Rotation in Hot and Cold Gases for High Precision Magnetometry

In this talk, I will review some of our recent work at developing techniques to measure small magnetic fields using Electro-magnetically induced transparency in both hot Rubidium vapor and in an ultra-cold Sodium gas.

April 1, 2003
3:30 pm (Tuesday)

Dr. Lia Merminga
Center for Advanced Studies of Accelerators (CASA), Jefferson Lab

Jefferson Lab Accelerators: Present Activities, Future Directions and Technology Development

Jefferson Lab's nuclear physics accelerator, the Continuous Electron Beam Accelerator Facility (CEBAF), is the world's largest superconducting radiofrequency (SRF) linac. Also on site is the groundbreaking energy-recovering Free Electron Laser (FEL) with world-record average power in the infrared wavelengths. These accelerators have demonstrated that recirculating SRF linacs can deliver high average power beams of superior quality, with high efficiency, when energy recovery is implemented. The resulting world-wide interest in recirculating and energy-recovering linacs has led to novel accelerator designs for a variety of applications, such as electron-ion colliders for nuclear and particle physics, light sources for the generation of X-ray radiation, high energy electron cooling devices, and high power free electron lasers. To demonstrate feasibility of these next generation accelerators, a number of physics and technology challenges must be met, requiring a broad, interdisciplinary research and development program. I will discuss some of these challenges, which are topics of vigorous research at Jefferson Lab, in connection to the design of future facilities both at Jefferson Lab and elsewhere.

April 8, 2003
3:30 pm (Tuesday)

Dr. Mohammad Zubair
Department of Computer Science, Old Dominion University

Archon: An OAI-Compliant Federated Physics Digital Library for the NSDL

Archon is a federation of physics collections with varying degrees of metadata richness. We provide services on these metadata that make the federation a learning environment useful not only for the researcher but for students as well. Archon uses the Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH) to harvest metadata from several Physics collections that are geographically distributed. The architecture of Archon is largely based on Arc, the first OAI-PMH compliant service provider to provide end-user search services across OAI-PMH repositories. However, Archon provides some new services that are specifically tailored for research and education in the physics community. We present our techniques for the services that are currently implemented, as well as our plans for future services. Archon currently supports searching and browsing of equations and formulae and a citation linking service for arXiv and American Physical Society (APS) archives. In the future, Archon will support features including personalization and creation of new objects in the digital library based on aggregation of content. The Archon project is a NSF funded project.

April 15, 2003
3:30 pm (Tuesday)

Prof. Charles H. Kruger
Mechanical Engineering, Stanford University

Non-Equilibrium Discharges in Air Plasmas at Atmospheric Pressure

There are interesting applications of non-equilibrium diffuse discharges in molecular gases, particularly air, at atmospheric pressure. Desirable conditions are electron densities greater than 1012 cm-3 at gas temperatures less than 2000K. To assess the possibilities, we have constructed two-temperature (Te>Tg) kinetic models for nitrogen and air discharges, accounting for ionizational, chemical, vibrational and electronic non-equilibrium, and incorporating a collisional-radiative model with over 11,000 transitions. These models predict that (even) at atmospheric pressure energetic electrons driven by the discharge and promoting chemical and ionizing reactions can establish and maintain electron-density non-equilibrium of over six orders of magnitude. An unexpected result is an "S-shaped" dependence of ne on Te at steady-state for a given gas temperature. To assess the feasibility of such non-equilibrium discharges, experiments have been conducted in atmospheric-pressure nitrogen and air at both room temperature and around 2000K with electrode spacings of cm scale. Emission and laser-based spectroscopy are used to probe the rotational, vibrational, electronic and ion distributions of the resulting non-equilibrium plasmas. Stable, diffuse DC discharges have been achieved at atmospheric pressure for a range of gas flow and temperature conditions including those which produce ne of 1012 to 1013 cm-3 without significant gas heating. For comparison with the kinetic model, the "S-shaped" curves of electron density vs Te have been converted more readily measured current density vs. electric field by use of Ohm's law and the electron energy equation. Good agreement between this theory and the discharge experiments has been obtained for both air and nitrogen non-equilibrium discharges over a wide range of conditions at atmospheric pressure, including electron densities greater than 1012 in air and 1013 in nitrogen. To reduce the power required to maintain such non-equilibrium, the finite electron recombination time (~10µs) has been exploited by means of pulsed discharges of 10 ns duration. Both single-shot and repetivitively pulsed diffuse discharges at 100 kHz have been demonstrated, with time-average power reductions of over two orders of magnitude for average electron densities greater than 1012.

April 22, 2003
3:30 pm (Tuesday)

Dr. Nora Berrah
Western Michigan University

Studies of Complex Systems: From Atoms to Clusters

We have investigated with unprecedented levels of detail, processes and phenomena involving photodetachment of negative ions and photoionization of molecules using the brightness, spectral resolution, tunability and polarization of the Advanced Light Source at Lawrence Berkeley National Laboratory. In particular, we will report on investigations carried out in K-shell photodetachment of fundamental atomic targets such as Li- and He-. These systems exhibit structure differing substantially from corresponding processes in neutral atoms and positive ions, owing to the dominance of correlation in both the initial and final states. We will also report on spin-resolved photoionization investigation in molecules as an alternative method to probe the molecular environment in triatomic molecules.