J. S. Whittier

Computer Modeling and Parametric Study for a Pulsed H 2 + F 2 Laser

A computer simulation of a pulsed HF laser pumped by the H(2) + F(2) chain reaction is presented. A chemical kinetic model encompassing sixty-eight reactions is used to approximate the reacting mixture contained within an optical cavity. For each vibrational level, a Boltzmann distribution for the rotational levels is assumed, with lasing on the vibrational band at line center of the transition having maximum gain. An analysis of cavity and chemical mechanisms yields a simple relationship between the pumping and depletion rates of the vibrational levels. This relationship is used to make a comprehensive study of the effects of cavity and chemical parameters on the laser pulse. The effect of changes in uncertain chemical reaction rate coefficients is assessed. A competition exists between vibrationtranslation deactivation reactions and stimulated emission. Laser performance is most sensitive to the relative rates of the vibrational-translational and pumping reactions. In addition, the effect of the photon flux on the chemical mechanisms is significant.

Effect of H2 Pressure on Pulsed H2+F2 Laser. Experiment and Theory

Stimulated emission predictions and measurements for an H2+F2 laser are compared for H2 pressures from threshold to stoichiometric, a range of several orders of magnitude. Slowly flowing, helium‐diluted, 50 torr mixtures are initiated photolytically. Two dilution ratios and two output couplers are considered, and good agreement is found for time‐to‐threshold and pulse duration vs H2 pressure. Spiking, relaxation oscillations, and possibly mode beating, features not modeled, are observable in some pulses; however, predicted intensity vs time generally agrees in pulse shape with laser output. Observed and predicted peak intensities nearly match for low H2 pressure, and the predicted increase of peak intensity with low H2 is followed fairly well. For H2 in the vicinity of one‐tenth stoichiometric, the peak intensity data show an abrupt leveling off, while the calculations predict a continuing increase. This disagreement most probably cannot be attributed to uncertainties in the kinetic model. All rate modifi...

Time-Resolved Spectroscopy of a Flash Initiated H2-F2 Laser

The time‐dependent output spectrum of a helium‐diluted H2–F2 chain‐reaction chemical laser has been observed. Reaction of a 50‐Torr mixture with mole ratio H2 : F2 : He = 1 : 1 : 60 was initiated by flash photolysis of the F2. Strong lasing was found from P‐branch vibration‐rotation transitions of the v = 1 → 0, 2 → 1, 3 → 2, and 4 → 3 bands of HF. Within some bands, the time sequence of transitions suggests non‐Boltzmann distributions of rotational states. No lasing from vibrational levels higher than 4 could be detected.

Closed-Form Solution to Rate Equations for an F + H 2 Laser Oscillator

Chemical lasers pumped by the reaction of atomic fluorine with molecular hydrogen emit power from rotation-vibration transitions of excited HF with upper levels as high as upsilon = 3. Collisional processes compete with stimulated emission for the energy of the excited HF. A simplified analysis is presented here for intensity, energy, and chemical efficiency of a class of such lasers. Results are obtained in closed form. A comparison with more exact computer solutions establishes the validity of the analysis despite its simplifications. A comprehensive parametric study examines the relative importance of initial conditions, optical cavity parameters, and rate coefficients for pumping and deactivation reactions.

Electron‐beam‐irradiated discharges considered for initiating high‐pressure pulsed chemical lasers

Volumetric irradiation by a short‐pulse electron beam has been used to trigger long‐duration spatially uniform electric discharges in gas mixtures of He and F2 or SF6. Uniform energy deposition to 300 J/liter has been observed for atmospheric F2–He mixtures at nominal electron‐beam currents of 3 A/cm2 and discharge currents up to 20 A/cm2. Operation suitable for efficient initiation of pulsed HF/DF chain lasers appears possible over a wide range of E/N and mixture ratios, limited by breakdown at large E/N and negligible field enhancement at low E/N. Approximate analytical plasma models are presented and used in conjunction with time‐resolved afterglow current measurements to obtain rate constants for F−‐ion F2+−ion recombination and F2+−ion electron recombination. Estimates of F2 dissociation fractions achieved in the experiments imply the possibility of scalable and efficient initiation of pulsed chemical lasers with such discharges.

Axially symmetric wave propagation in a two-layered cylinder

Abstract The linear theory of elasticity is used to investigate axially symmetric wave propagation in an infinitely long two-layered cylinder. Each material is taken to be homogeneous and isotropic. A perfect bond is assumed at the interface, while the inner and outer boundaries of the composite cylinder are treated as traction-free. The dispersion determinant relating phase velocity and wave number for a harmonic train of waves satisfying these boundary conditions is presented. The character of the dispersion equation is investigated analytically and numerically. Stress and displacement distributions are also presented for the numerical example. Comparisons are made with an approximate solution of the same problem obtained by means of a thin shell theory incorporating thickness-shear deformation of each layer.

An efficiently initiated pulsed H 2 + F 2 laser

Output pulse observations are presented for a helium-diluted HF laser pumped by the H 2 + F 2 chain reaction. Initiation of the reaction was accomplished with 180 transverse electrical discharges. Addition of a trace of O 2 to the gas mixture was found to enhance performance. Optimum performance was found with a 120-torr mixture with mole ratio F 2 :H 2 :O 2 :He = 1:0.7:0.08:15 initiated with a 22.5-kV 250-pF discharge; this gave a single-pulse output energy of 0.073 J. This corresponds to a ratio of laser output energy to electrical input energy of 115 percent.

Chain reaction pulsed HF laser: a simple model

With economical, yet accurate, predictions of pulsed H(2) + F(2) laser performance as a goal, a rate equation model is formulated that includes only the dominant kinetic mechanisms in the active medium. Effects of model assumptions are examined, and predictions of pulse characteristics are compared with results from a more comprehensive model presented in an earlier study. Computing costs for the present model are less than 1% of those for the comprehensive model; moreover, the present model yields laser pulse characteristics that are consistent with experiment and in excellent agreement with the more comprehensive model. In order to illustrate the models capability, the effects of initial gas mixture composition and cavity threshold on laser performance are studied over the regime of practical interest. Some possible extensions and applications of the model are also discussed.

Efficient pulsed chemical laser

Output pulse observations are presented for a helium‐diluted CO2 laser pumped by VV (vibration‐vibration) energy transfer from vibrationally excited DF produced by the D2–F2 chain reaction. Flash photolysis of the F2 served to initiate the reaction. A 290‐cm3 reaction chamber containing a 0.5‐atm mixture with mole ratio D2:F2:CO2:He = 0.33:1:8:10 gave a single‐pulse output energy of 2.8 J. Relative to the amount of D2 present in the reaction chamber, this corresponds to a chemical efficiency greater than 5%.

MOIRE INTERFEROMETRY WITH EMBEDDED GRIDS, EFFECT OF OPTICAL REFRACTION,

Optical refraction resulting from refractive index gradients in nonhomogeneously stressed models is investigated as a limitation on the usefulness of moiré interferometry employing embedded grids. Apparent displacements caused by internal refraction are estimated for static and dynamic cases; Boussinesqs problem, and the problem of a spherically expanding pulse, respectively. Refraction effects are found to be far larger in the dynamic case. Moiré observations of motions in models loaded by pellet impact are compared with independently measured motion histories. Anomalies in the moiré data are found to agree fairly well with predicted refraction effects for a spherically expanding pulse. It is concluded that optical refraction effects are potentially quite detrimental to observation of transient internal motions using moiré interferometry.