Ion Beams in Multi-Species Plasma

Argon and xenon ion velocity distribution functions are measured in Ar-He, Ar-Xe, and Xe-He expanding helicon plasmas to determine if ion beam velocity is enhanced by the presence of lighter ions. Contrary to observations in mixed gas sheath experiments, we find that adding a lighter ion does not increase the ion beam speed. The predominant effect is a reduction of ion beam velocity consistent with increased drag arising from increased gas pressure under all conditions: constant total gas pressure, equal plasma densities of different ions, and very different plasma densities of different ions. These results suggest that the physics responsible for the acceleration of multiple ion species in simple sheaths is not responsible for the ion acceleration observed in expanding helicon plasmas

Confocal Laser Induced Fluorescence with Comparable Spatial Localization to the Conventional Method

We present measurements of ion velocity distributions obtained by laser induced fluorescence (LIF) using a single viewport in an argon plasma. A patent pending design, which we refer to as the confocal fluorescence telescope, combines large objective lenses with a large central obscuration and a spatial filter to achieve high spatial localization along the laser injection direction. Models of the injection and collection optics of the two assemblies are used to provide a theoretical estimate of the spatial localization of the confocal arrangement, which is taken to be the full width at half maximum of the spatial optical response. The new design achieves approximately 1.4 mm localization at a focal length of 148.7 mm, improving on previously published designs by an order of magnitude and approaching the localization achieved by the conventional method. The confocal method, however, does so without requiring a pair of separated, perpendicular optical paths. The confocal technique therefore eases the two window access requirement of the conventional method, extending the application of LIF to experiments where conventional LIF measurements have been impossible or difficult, or where multiple viewports are scarce

Spatial Structure of Ion Beams in an Expanding Plasma

We report spatially resolved perpendicular and parallel, to the magnetic field, ion velocity distribution function (IVDF) measurements in an expanding argon helicon plasma. The parallel IVDFs, obtained through laser induced fluorescence (LIF), show an ion beam with v ≈ 8000 m/s flowing downstream and confined to the center of the discharge. The ion beam is measurable for tens of centimeters along the expansion axis before the LIF signal fades, likely a result of metastable quenching of the beam ions. The parallel ion beam velocity slows in agreement with expectations for the measured parallel electric field. The perpendicular IVDFs show an ion population with a radially outward flow that increases with distance from the plasma axis. Structures aligned to the expanding magnetic field appear in the DC electric field, the electron temperature, and the plasma density in the plasma plume. These measurements demonstrate that at least two-dimensional and perhaps fully three-dimensional models are needed to accurately describe the spontaneous acceleration of ion beams in expanding plasmas

Spontaneous Emission in Microcavity Lasers

An understanding of spontaneous emission processes within microcavities is crucial in addressing the need to make tomorrow\u27s microlasers more efficient. One approach to improving the device efficiency is to reduce the threshold input energy at which lasing begins to occur. It has been suggested that the threshold in a microcavity laser can be decreased by increasing the fraction of spontaneous emission into the lasing mode, this can be accomplished by preferentially coupling the gain medium of the laser to the electromagnetic cavity mode of interest. It therefore becomes necessary to understand the mechanism by which this coupling takes place. This research develops a fully quantum mechanical description of the interaction between a gain medium modeled as a two level atom and a multimode electromagnetic field in a microcavity. Atomic transition probabilities are computed for systems in which the atom couples through a single photon process to electromagnetic cavity modes which range in number from two to 2000. Calculations performed for cavities with widely spaced modes demonstrate that atoms exhibit Jaynes Cummings behavior when closely tuned to one mode. Detuning of the atom from the mode inhibits the exchange of energy, while increasing the strength of the coupling to the mode amplifies this exchange. Two level systems strongly coupled to many closely spaced modes exhibit spontaneous emission rates characteristic of an atom in free space

Spectroscopic and Kinetic Studies of Bismuth Dimers

The spectroscopy of high rotational levels (J ≤ 211) in Bi2 X(Og+) and A(Ou+) was investigated for 2≤v ≤5 and 0≤v\u27≤4 by observing total fluorescence from laser excitation. Dunham coefficients were derived that fit all observed rotational lines to within 0.01 cm-1. Franck-Condon factors were calculated and experimentally verified for transitions originating from the initially populated levels 0≤v\u27≤5. Vibrational energy transfer upon collision with rare gas collision partners was investigated for the low-lying vibrational levels of the A-state using spectrally resolved, continuous wave laser induced fluorescence. Vibrational transfer was adequately modeled by Landau-Teller scaling of the Δv=-1 vibrational transfer rates with fundamental rate coefficients ranging from kv=5.29x10-12 cm3/molec-sec for collisions with helium to 2.38x10-12 cm3/molec-sec for krypton. Electronic quenching and multi-quantum transfer rates were approximately an order of magnitude slower than single quantum transfer rates

Performance of Imaging Laser Radar in Rain and Fog

The Air Force is currently developing imaging laser radar systems (ladar) for use on precision guided munitions and other imaging systems. Scientists at Eglin Air Force Base, in conjunction with Wright Laboratories, are testing a 1.06-um wavelength ladar system and need to understand the weather effects on the ladar images. As the laser beam propagates through the atmosphere, fog droplets and raindrops can cause image degradation, and these image degradations are manifested as either dropouts or false returns. An analysis of the dropouts and false returns helped to quantify the performance of the system in adverse weather conditions. Statistical analysis of the images showed non-linear relationships existed between variables, plus graphical analysis demonstrated the behavior of the dropouts and false returns with changing weather conditions. Statistical control charts identified the weather as a significant influence on the quality of the ladar images. By focusing on the false return data, a study of mean free path and the survival equation was accomplished. The mean free path was derived from the rainfall rate, and this mean free path was used in the survival equation to calculate an expected number of false returns for an image. This work led to the hypothesis that raindrops with a diameter of 3.0 mm and larger were causing the false returns seen in the images. However, further analysis revealed that a 3.0-mm raindrop was not capable of scattering enough energy to be detected by the system. It was then hypothesized that the system detector was also picking up solar spectrum energy scattered by raindrops, and that this detector was unable to distinguish between solar energy and laser energy scattered by raindrops

Shock Waves in Nonequilibrium Gases and Plasmas

An analysis and assessment of three mechanisms describing plasma/shock wave interactions was conducted under conditions typically encountered in a weakly ionized glow discharge. The mechanisms of ion-acoustic wave damping, post-shock energy addition and thermal inhomogeneities were examined by numerically solving the Euler equations with appropriate source terms adapted for each mechanism. Ion-acoustic wave damping was examined by modeling the partially ionized plasma as two fluids in one spatial dimension using the Riemann problem as a basis. Post-shock energy addition in the form of nonequilibrium vibrational energy relaxation was also examined in one spatial dimension using the Riemann problem as a basis. The influence of thermal inhomogeneities on shock wave propagation was examined in two spatial dimensions for both a Riemann shock and a shock generated by a spark discharge. Shocks were propagated through realistic thermal profiles with the resulting shock structure examined through the numerical application of various optical diagnostic techniques. Results from shock simulations indicate that ion-acoustic wave damping has an insignificant effect on the neutral flow at fractional ionization levels typical of glow discharges. Post-shock vibrational energy relaxation is also unable to effect the shock structure on the time scales of interest. An analysis of the effects of thermal inhomogeneities reveals that many of the observed plasma/shock anomalies can be explained based solely on this mechanism