Walter H. Christiansen

Coherent structure effects on the optical performance of plane shearlayers

Althrought there has been extensive research on the optical properties of shear layers, there have been no reported studies of the optical effects on large scale coherent structures existing in turbulent shear layers. The research reported here investigated the effects of such coherent structures existing and of external perturbation of the shear layer on the optical quality of a propagating laser beam.

Mixed Mode Contributions to Absorption in CO 2 at 10.6μ

A rise of temperature in a molecular system causes the higher energy levels to become more populated. For the case of CO(2) absorption (or, in a lasing medium, gain) calculations based only on single line considerations are sometimes inappropriate as the contributions from some higher energy levels can be appreciable. This paper summarizes this effect for 10.4micro band of CO(2) . The calculations are compared with the experimental measurements of others.

Gain Kinetics of CO2 Gasdynamic Laser Mixtures at High Pressure

A combined analytical and experimental investigation of rapidly expanded CO2 laser mixtures is described. Study of the vibrational kinetics indicates that population inversions with high vibrational energy density can be produced at high pressures by utilizing low (starved) concentrations of CO2. Scaling laws for predicting the gain of starved systems are developed. A description is given of gain measurements carried out using a CO2 laser to probe the flow in a nozzle at the point where the Mach number is approximately four. Population inversions at static pressures as high as about 1 atm have been observed and found to be in reasonable agreement with predictions.

Gasdynamic lasers and photon machines

The basic operational highlights of CO 2 -N 2 gas-dynamic lasers (GDLs) are described. Features common to powerful gas lasers are indicated. A simplified model of the vibrational kinetics of the system is presented, and the importance of rapid expansion nozzles is shown from analytic solutions of the equations. A high-power pulsed GDL is described, along with estimations of power extraction. A closed-cycle laser is suggested, leading to a description of a photon generator/engine. Thermodynamic analysis of the closed-cycle laser illustrates in principle the possibility of direct conversion of laser energy to work.

ABSORPTION IN CO2 AT 10.6 μ WITH ROTATIONAL LINE OVERLAP

The results of a calculation and measurement of the absorption of 10.6‐μ laser radiation by CO2 at high pressures are described. It is shown that the effect of rotational line overlap becomes significant at pressures slightly greater than 1 atm. At 8 atm, for example, the calculated absorption coefficient is twice that without including line overlap. Measurements, however, indicate a larger effect than is predicted.

Experimental investigation of transitional free shear layer optics

Measurements of Strehl ratio vs downstream distance, when a beam propagates through a shear layer (Reynolds number=810/length), reveal minimal phase distortion in the pretransition region and a sudden decrease in the Strehl ratio when transition occurs. The Strehl ratio levels off to a constant asymptotic value (which is a function of the velocity ratio) in the post-transtion region. The constant Strehl ratio (in the fully developed region) is attributed to fast mixing that occurs within the core of the vortices

Fluid mechanical light guide

It is shown that a beam of light can be deflected continuously through large angles along a curved path using the gas flow from a convergent-divergent nozzle sector whose throat lies on the arc of a circle. The amount of deflection depends on the included angle of the nozzle sector and on the density gradients created by the expanding gas flow. For a 360 degrees included angle the beam can in principle be bent into a closed path with a radius of the order of a few tens of centimeters: Nozzle sectors with included angles of 30 degrees and 180 degrees are studied both theoretically and experimentally. It is demonstrated that a well-defined region exists in the flow wherein light rays are trapped in near-circular paths whose radii are close to the radius of the nozzle throat. The trapping region in effect constitutes a curved light guide. Very low light losses are predicted and measured, even for large deflections. Using nitrogen at a stagnation pressure of 110 atm as the working fluid, a laser beam is deflected as much as 180 degrees with a transmission approaching 100%.

Pulsed N 2 O molecular laser studies

The afterglow pulse-gain technique was used to measure the relaxation of the 00°1 upper laser level in N 2 O from 300 to 700°K. The rate constant k_{N_{2}O} (torr-1s-1) varies with temperature as \log 10 k_{N_{2}O} = 4.53 - 10.9T^{-1/3} . Measurements of the relaxation time at 300°K agree with the fluorescence technique results of Yardley [1]. Gain studies were also made with gas mixtures in a nonflowing system. Single-pulsed N 2 O + CO + He mixtures at 300°K showed considerable peak gain. However, subsequent pulses of these mixtures show that the gain is reduced because of the chemical reaction forming CO 2 . Measurements of this transformation from one molecular laser mixture to another are discussed.

Isotope separation and yield calculations for vibrationally enhanced oxidation of nitrogen

The separation of nitrogen isotopes by low temperature reaction of vibrationally excited nitrogen gas with oxygen has been studied, in which the formation of 15NO is theoretically favored. The potential yield and isotope separation coefficient β for this process were examined using a numerical simulation of the kinetic processes, which incorporated a steady‐state isothermal model of the 14N2 and 14N15N vibrational distribution functions coupled with a non‐steady‐state kinetic model of the chemical system including N*2, O*2, N, O, and their reaction products. In the absence of O2, the vibrationally enhanced rate coefficient for the reaction N*2+O → NO+N was observed to be inversely proportional to the concentration of O atoms, due to VT loading of the N*2 distribution function. O2 was also found to greatly reduce the rate coefficient due to efficient depletion of the highly excited species via the reaction N2(v)+O2(0) → N2(v−1) +O2(1). Computed reaction yield increases dramatically if both the O*2 and N*2 ...

Analysis of a collisionally induced dipole laser

An electric dipole moment is induced in homonuclear molecules during collisions, allowing radiative transitions which are otherwise forbidden. This effect, coupled with high‐pressure electrical excitation, may be used to achieve laser action in a homonuclear gas. Properties of this type laser include a very broad gain bandwidth and a new type of partial inversion. A mixture of H2 and Xe is analyzed as a prospective system wherein efficiencies of about 20% are predicted for picosecond pulses at 2.8 μm.