Bulletin of the American Physical Society
66th Annual Gaseous Electronics Conference
Volume 58, Number 8
Monday–Friday, September 30–October 4 2013; Princeton, New Jersey
Session UF1: Scientific Legacy of Arthur Phelps |
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Chair: Bruce Wedding, University of South Australia Room: Ballroom I |
Friday, October 4, 2013 1:30PM - 2:00PM |
UF1.00001: Important Research by Art Phelps in the 1950s J.E. Lawler Art Phelps made major contributions to the field of Gaseous Electronics. This talk is a review of some of Art's most important papers from the 1950s while he was a young scientist at Bell Labs and then at Westinghouse. The earliest theories of a simple discharge plasma, a positive column, incorporated the assumption of single-step electron-impact ionization. Both the ionization and power balance of most discharges are dominated by multi-step processes. Art's early studies of helium metastable atoms [1,2] were tremendously clever applications of the experimental technology available in the 1950s. His studies laid the foundation for our modern understanding of many discharge plasmas in which multi-step processes are dominant. Under most circumstance the excitation rates for metastable atoms and/or molecules are much larger than rates for single-step ionization. The relatively long effective lifetimes of metastables leads to high densities of these species. Electron impact and other collisionsal processes can easily ionize or excite the metastables. In the 1950s Art also recognized that radiation trapping could dramatically extend the effective lifetime of atoms in resonance levels and effectively make those levels metastable [3]. He applied radiation trapping theory first to the measurement of excitation coefficients, but the implications were clear. Resonance levels can play a major role in multi-step ionization and excitation. \\[4pt] [1] A. V. Phelps {\&} J. P. Molnar, Phys. Rev. 89, 1202 (1953).\\[0pt] [2] A. V. Phelps, Phys. Rev. 99, 1307 (1955).\\[0pt] [3] A. V. Phelps, Phys. Rev. 110, 1362 (1958). [Preview Abstract] |
Friday, October 4, 2013 2:00PM - 2:30PM |
UF1.00002: Personal Landmarks from the Legacy of Arthur Phelps John Lowke I have been influenced for my whole life by Art Phelps, more than by anyone else -- other than my wife! I first heard of Art Phelps in 1960 when, in the middle of doing my PhD in Adelaide, South Australia, Frost and Phelps published their land-mark paper, not only on drift velocities, the subject of my PhD, but on Boltzmann analyses, which were to deliver detailed cross sections for all common gases. Later I dared to suggest to my university that one of my two external PhD examiners be Phelps, a move that led to me being accepted for a position at Westinghouse Research Laboratories in Pittsburgh for 6 years, with Phelps as my direct supervisor. Throughout this period, Phelps refused to be a co-author of any of my papers, leaving me with severe doubts as to what he thought of their quality! I list areas where insights from Phelps inspired the growth of new fruit. (1) That transverse and longitudinal electron diffusion coefficients differ, typically by a factor of two. (2) That averaging radiation absorption coefficients in electric arcs, using common weightings involving Black Body radiation, can and usually do lead to errors of orders of magnitude. (3) That CO$_{\mathrm{2}}$ laser discharges are largely controlled by electron attachment rather than by diffusion or recombination. (4) That boundary conditions for electrons at metal electrodes in arc welding, are not zero, but from an astrophysical analogy, are zero when extrapolated to one mean free path beyond the surface. (5) That the metastable vibrational states of nitrogen become an energy gain rather than a loss process for low energy electrons as occur in electrical breakdown in air, resulting in increases of the ionisation coefficient by orders of magnitude. Coupled with the detachment of electrons from negative ions by singlet delta states of metastable oxygen molecules, sustaining discharge electric fields are reduced a factor of five. Phelps worked on this problem with me until a few months before he died. [Preview Abstract] |
Friday, October 4, 2013 2:30PM - 3:00PM |
UF1.00003: Gas Breakdown, Low Current diffuse discharges, Townsend's theory: A Friday afternoon experiment Zoran Petrovic Numerous aspects of the ``standard model'' of gas breakdown have been addressed in the past 20 years by Art Phelps and his coworkers. First, his studies of excitation coefficients were carried out in the Townsend regime where electric field is quasi uniform so swarm like conditions prevail. These studies have been extended to very high E/N where non-hydrodynamic effects were to be observed but were overshadowed in most cases by fast neutral excitation. Absolute calibration of emission provided a basis to obtain fast neutral cross section sets. This work necessarily overlapped with the left hand side of the Paschen curve and in extension of an ill fated data gathering experiment a review was made of all the processes that contribute to the secondary electron emission. It was shown that, if one includes all the processes, it is possible to fit the available breakdown data, Paschen curves and effective electron yields by binary collision data obtained in separate experiments. While performing measurements in the low current diffuse (Townsend) regime one can find negative differential resistance and oscillations. Both were explained by taking detailed information on properties of particles close to the cathode and small perturbations to the local field by the growing space charge. Last but not the least Phelps managed, with his coworkers to provide a phenomenology and predictions of the anomalously broadened profiles often observed in various discharges. In all those cases deep knowledge of atomic and molecular physics and of gas discharges were combined with best available data to produce quantitative (quantitative, quantitative) agreement with experiments. Coworkers: Dragana Maric. Supported by MPNTR project ON171037 and SANU project 155. [Preview Abstract] |
Friday, October 4, 2013 3:00PM - 3:30PM |
UF1.00004: WORKSHOP BREAK
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Friday, October 4, 2013 3:30PM - 4:00PM |
UF1.00005: Expansion of Swarm Experiments at JILA to Microplasma Research Kunihide Tachibana Strongly attracted by the Art's remarkable work on metastable atoms in the 50s, I joined JILA as a posdoc at his laboratory in 1978. The assigned machine for me was a drift tube, and the first work was to check the validity of previous results. In a sense, I was tested of my skills as an experimentalist, but soon later I was able to start an original work on the measurement of excitation coefficient of rare gas atoms using the machine. We applied the laser absorption spectroscopy for the measurement of the excited atoms. The argon-ion-laser excited dye-laser at the lab for the light source was awfully unstable, but I was so lucky to have a wonderful support from John Hall, a Novel prize winner in 2005, to stabilize the laser system. After I came back to Japan in 1980, I extended the work to the measurement of Xe metastable (1s5) and resonant (1s4) atoms in a micro discharge cell of a plasma display panel. Then, I have deeply got into the world of microplasmas, exploring the new world with sophisticated arrays of microplasmas to find unusual properties as metamaterials for electromagnetic waves. Throughout my whole research life, I would like to sincerely thank Art for the wonderful experiences with him at JILA. [Preview Abstract] |
Friday, October 4, 2013 4:00PM - 4:30PM |
UF1.00006: Boltzmann analyses of swarm experiments over the years Leanne Pitchford Art Phelps was one of the ``grand old men'' in field of gaseous electronics. He was a graduate student when the GEC got started and he attended almost all of the meetings over the years. During his remarkably long career, he produced a number of the classic papers in our field as a glance at Web of Science will show. Art was my mentor and friend, and I had the privilege of working with him for many years on various topics related mainly to electron scattering and transport in weakly ionized gases. In this talk, I will discuss the originality of some of his early work on these subjects in the context of their times, focusing in particular on his publications from the mid-1960's with his colleagues from Westinghouse Research Laboratories. These report the first numerical solutions of the Boltzmann equation for electrons, to my knowledge, and they inspired much subsequent work related to the extraction of quantitative information about low-energy electron scatting with simple gases from measurements of macroscopic parameters (mobility, diffusion,..). I will outline some of the work he and I did together in this topical area using more sophisticated numerical techniques. This and other work in the field eventually led to the establishment of the ongoing GEC Plasma Data Exchange Project which now involves a number of people (the LXCat team), as discussed in Tuesday's workshop. The LXCat team had completed work on noble gases and had just started working on evaluations of cross sections for simple molecules when Art died. We are fortunate to have had his involvement on these projects. Art had ideas for future work in these areas, and some are included in a long e-mail message from Art a couple of years ago that I will share because it includes some suggestions [to the community] for future work. [Preview Abstract] |
Friday, October 4, 2013 4:30PM - 5:00PM |
UF1.00007: Electron Interactions with Excited Atoms and Molecules Stephen Buckman Excited species, particularly those in long-lived metastable states, can have a profound effect on the behaviour of low temperature gas discharges. They often present a considerably different atomic or molecular structure to their ground state ``parent'' atom or molecule. In the case of rare gas atoms, several of their lowest lying excited states have structures resembling loosely bound, one-electron systems, similar to their nearest alkali neighbor in the periodic table. They have large dipole polarizabilities and, as a consequence, extremely large scattering cross sections for low energy electrons. Combined with their long lifetimes, large internal energy and reasonably high excitation probability, they become an important component of a discharge environment. This talk will review some of the work in studying these important excited states -- their role in low temperature discharges was always a fascination for Art Phelps and he was a strong advocate for their detailed study. [Preview Abstract] |
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