Steven F. Adams

Polarizability calculations on water, hydrogen, oxygen, and carbon dioxide

Based on the semiclassical model of damped oscillators, a procedure is developed for the calculation of the dispersion of refractive index, polarizability, and dielectric permeability, which are treated as complex quantities. Using experimental values of refractive indices and employing the frequencies of strong absorption bands, a least squares fit is used to determine oscillator strengths and damping factors. The absorption coefficient and the imaginary part of the refractive index are also calculated at the corresponding wavelengths. The procedure is applied for the cases of water, hydrogen, and oxygen in both the liquid and gaseous states and for gaseous carbon dioxide, and a good agreement is obtained between calculated and observed values of refractive indices. Results are also compared with those of previous calculations. Calculated absorption coefficients agree well with experimental values in the region of absorption bands. The calculated values of oscillator strengths and damping factors are dis...

Surface and volume loss of atomic nitrogen in a parallel plate rf discharge reactor

The loss of atomic nitrogen due to surface and volume reactions in a parallel plate rf reactor was investigated using a pulsed N2 discharge and two-photon laser induced fluorescence detection of ground-state atomic nitrogen. Stainless-steel and aluminium electrode surfaces as well as silicon and boron nitride substrates were investigated for their reactivity with atomic nitrogen within the pulsed discharge environment at 1-5 Torr N2. Aluminium was found to have a surface loss rate of three to five times less than that of stainless-steel, while boron nitride had the lowest N atom recombination rate of the materials studied. The N atom recombination probability coefficient was found to have an inverse pressure dependence for each of the materials, with values ranging from 0.5 to 0.02%. The volume loss rate of N atoms was also quantified due to minute O2 impurities introduced into the pulsed rf N2 discharge.

Influence of the transverse dimension on the structure and properties of dc glow discharges

Two–dimensional (2D) simulations of a dc glow discharge with a cold cathode in argon have been performed for various radii of the discharge tube. It is shown that the loss of the charged particles to the walls can significantly affect plasma parameters as well as properties of the cathode sheath. The longitude dimensions of the negative glow and Faraday dark space depend on the transverse loss of the charge particles and are not consistently predicted with a 1D model. The common assumption that the cathode sheath can be analyzed independently of the plasma also may not be valid. The transverse inhomogeneity of the plasma leads to a change in the current density distribution over the cathode surface. The thickness of the cathode sheath can vary with radial distance from the discharge axis, even for the case of negligible radial loss of the charge particles. The 2D model results provide an analysis of the conditions of applicability of the 1D model.

Solar Thermionic Space Power Technology Testing: A Historical Perspective

This paper provides a brief overview of both the past and recent efforts aimed at the development and testing of solar thermionic space power systems. Recently, the Air Force has been investigating the feasibility of developing a thermionic generator, heated with a large inflatable solar concentrator, for orbital space power missions with electrical power requirements that exceed 50 kWe. This concept analysis follows a similar study by the NASA Jet Propulsion Laboratory in the 1960’s where the objective was a 500 We power generator for interplanetary probes. Details of the potential missions, system designs, and power specifications, as well as results of ground tests and demonstrations are detailed and compared for each era.

Formation and electron-ion recombination of N4+ following photoionization in near-atmospheric pressure N2

The time dependent behavior of molecular nitrogen ions has been investigated following pulsed photoionization of near atmospheric pressure N(2) using multiphoton laser techniques and kinetic modeling. Multiple fluorescence bands, some unreported previously, with various temporal behaviors were observed after ultraviolet laser photoionization of N(2)(X (1)Sigma(g)). The initial N(2) ionization was generated via resonance-enhanced multiphoton ionization with focused radiation in the 275-290 nm range, where several resonant transitions are accessible. The observed optical fluorescence bands appeared to be unique to the near-atmospheric pressure N(2) condition and were shown by the evidence in this work to be the result of collisional formation and recombination of N(4)(+). Measured time dependent fluorescence spectra during and after pulsed laser photoionization of N(2), together with a coupled rate equation model, allowed for the determination of the absolute densities of N(2)(+) and N(4)(+) as these species evolved.

O2 rotational temperature measurements in an atmospheric air microdischarge by radar resonance-enhanced multiphoton ionization

Nonintrusive spatially resolved rotational temperature measurements in an atmospheric air microdischarge are presented. The measurements were based on coherent microwave Rayleigh scattering (Radar) from resonance-enhanced multiphoton ionization of molecular oxygen. The open air DC microdischarge source operated in a stable “normal-glow” mode and pin-to-pin electrodes spaced 1.3 mm apart. The second harmonic of a tunable dye laser beam was focused between the two electrodes and scanned between 286 and 288 nm. Coherent microwave Rayleigh scattering was used to collect the two-photon rotational spectra of O2 at C3Π(v = 2)←X3Σ(v′ = 0) transitions. The Boltzmann plots from analyses of the O2 rotational lines determined local rotational temperatures at various axial locations between the electrodes. The molecular oxygen rotational temperature varied from ∼1150 K to ∼1350 K within the discharge area. The measurements had an accuracy of ∼±50 K.

Non-local collisionless and collisional electron transport in low-temperature plasma

This paper reviews recent advances in non-local electron kinetics in low-pressure discharges. Non-local electron kinetics, non-local electrodynamics with collisionless electron heating and non-linear processes in the sheaths are typical for such discharges. Progress in understanding the non-local interaction of electric fields with real, bounded plasma created by the fields has been one of the major achievements of the past few decades.

Metastable atom and electron density diagnostic in the initial stage of a pulsed discharge in Ar and other rare gases by emission spectroscopy

Temporal measurements of the emission intensities of the Ar 419.8 and 420.1 nm spectral lines combined with Ar plasma modeling were used to examine the metastable atom and electron density behavior in the initial stage of a pulsed dc discharge. The emission intensity measurements of these spectral lines near the start of a pulsed dc discharge in Ar demonstrated a sharp growth of metastable atom and electron densities which was dependent on the applied reduced electric fields. For lower electric fields, the sharp growth of metastable atom density started earlier than the sharp electron density growth. The reverse situation was observed for larger electric fields. This presents the possibility for controlling plasma properties which may be useful for technological applications. Similar measurements with spectral lines of corresponding transitions in other rare gases are examined.

Rotational temperature analysis of N2 by resonant enhanced multi-photon ionization with fluorescence detection

A non-invasive, optical technique to determine the rotational temperature of molecular nitrogen at atmospheric pressure by direct probing of the N2(X1Σg+,v=0) ground state with subsequent analysis of the rotational state distribution is presented. A tunable probe laser was scanned over resonant-enhanced, multi-photon ionization transitions initiating from various N2(X1Σg+,v=0,J″) states. At atmospheric pressure, the laser photo-ionization also induced N2+ fluorescence bands. Analysis of the relative fluorescence as a function of laser wavelength produced a calculated N2(X1Σg+,v=0,J″) rotational state distribution and determined the rotational temperature. The analysis also resulted in the assignment and tabulation of 11 previously unreported term energies for N2(b1Πu+,v=6) and N2(b1Πu-,v=6) for J′ > 22, based on the experimental data. The method resulted in temperature determinations for two experimental trials in atmospheric N2 gas flows at room temperature and 600 K that were in good agreement with ther...

Measurements of low-energy electron reflection at a plasma boundary

It is demonstrated that low-energy (<3 eV) electron reflection from a solid surface in contact with a low-temperature plasma can have significant variation with time. An uncontaminated, i.e., “clean,” metallic surface (just after heating up to glow) in a plasma environment may have practically no reflection of low-energy incident electrons. However, a contaminated, i.e., “dirty,” surface (in some time after cleaning by heating) that has a few monolayers of absorbent can reflect low-energy incident electrons and therefore significantly affect the net electron current collected by the surface. This effect may significantly change plasma properties and should be taken into account in plasma experiments and models. A diagnostic method is demonstrated for measurements of low-energy electron absorption coefficient in plasmas with a mono-energetic electron group.