R. L. Kerber

Effect of cavity transients and rotational relaxation on the performance of pulsed HF chemical lasers: a theoretical investigation.

Two rate-equation models of a pulsed HF laser are presented. Calculations with these models reveal the time evolution of the gain and intensity of lasing on all lines in vibrational-rotational bands studied and the time histories of concentrations of the chemical species. The present formulation permits an accurate assessment of the effect of vibrational relaxation and rotational relaxation. The model predictions are compared with earlier models to illustrate their unique features. Multiline lasing on vibrational-rotational bands is predicted; however, the general trends of J shifting are similar to those of earlier models. It is found that lasing does not begin when the medium gain reaches threshold, but intensity increases sharply after the gain is far above threshold. The gain subsequently drops rapidly and oscillates near the threshold value. The effect of rotational relaxation on laser performance is shown for multiline laser operation, and the enhancing effect of cascading is clearly revealed. The present models are found to predict output and performance consistent with experiment.

MD4 - A kinetic model and computer simulation for a pulsed DF-CO 2 chemical transfer laser

The chemical and energy transfer reactions involved in the helium-diluted DF-COt chemical transfer laser system are discussed. Experimental and theoretical rate coefficient literature is reviewed and the most probable rates are selected for the kinetic model of the reaction system. Results of computer experiments show the relative importance of these reactions for pulsed laser simulation. Predicted laser performance is most sensitive to values of rate coefficients for DF-CO2 transfer, collisional deactivation of C02(O001) by DF, and collisional deactivation of DF(u) by D and F. A detailed investigation of the relationship of cavity and chemical mechanisms for two levels of initiation is presented for an initial composition of XF:lFz: 1D2:8C02:40He at 50 torr and 300 K with X = 0.1 and 0.01. For F = 0.01, a 260-ps pulse with six percent chemical efficiency is predicted, while the higher level initiation level (F = 0.1) produces a 52-ps pulse with eight percent efficiency. VALUATION of the performance of gas and chemical lasers requires an understanding of the kinetic and chemical mechanisms that pump and deactivate the energy levels associated with lasing. Normally, a simple model may be constructed that explains why lasing occurs for a particular species. However, a more detailed representation of the mecha- nisms is needed for an accurate prediction of laser per- formance. Recently, comprehensive theoretical models for computer simulation of laser performance have been developed 111-(4) Ohat utilize a detailed formulation of the kinetics of the lasing system. A review of the kinetics for the C0,-N, laser system is given by Taylor and Bitterman (5); Cohen (6) gives a similar review for the H, + F, chemical laser. With the discovery and development (7)-(16) of the DF-CO, chemical transfer laser, it is desirable to assemble a kinetic model that will facilitate future studies of this laser. This paper presents the resuIts of our efforts to gather and compile a complete set of the required rate coefficients based on data available in the literature, comparisons with similar reactions with known rate coefficients, or on various theoretical approaches. We have drawn heavily on (5) and (6); we include data prior to those studies only when necessary.

Rotational nonequilibrium mechanisms in pulsed H(2) + F(2) chain reaction lasers. 2: Effect of VR energy exchange.

The occurrence of pure rotational-to-rotational lasing from high J levels suggests that present rotational nonequilibrium mechanisms are inadequate to explain all lasing behavior of the HF laser. A possible mechanism for explaining this behavior is vibrational-to-rotational energy transfer. The usual assumption that vibrational relaxation occurs with rotational levels at equilibrium at the translational temperature is replaced with a near resonant multiquanta VR process that results in the formation of highly excited rotational states. Computer simulations incorporating VR relaxation predicted significant occurrence of rotational lasing. A simpler model that produced rotational nonequilibrium from pumping and P-branch lasing did not exhibit rotational lasing. Rotational lasing did not decrease energy available to P-branch lasing and produced effects resembling an increase in rotational relaxation rates. Rotational lasing is very sensitive to kinetics for both VR energy exchange and rotational relaxation.

Rotational nonequilibrium mechanisms in pulsed H 2 + F 2 chain reaction lasers. 1: Effect on gross laserperformance parameters

A rate equation model of a pulsed H(2) + F(2) chemical laser is used to examine the relative importance of rotational nonequilibrium mechanisms on laser performance. This computer model yields the time history of the first thirteen rotational levels and the first twelve vibrational-rotational P-branch transitions for the first six vibrational bands of HF. With this model, the general effects of rotational nonequilibrium on the H(2) + F(2) laser were found (1) to increase the number of transitions that lase simultaneously, (2) to lower the intensity of each transition, and (3) to extend the duration of lasing on each transition; these trends are similar to those observed earlier for the F + H(2) laser. The major thrust of the present work is to isolate the relative importance of the various rotational nonequilibrium mechanisms. To this end, we have examined and compared several approaches to modeling R-T and V-R relaxation, nonequilibrium pumping distributions, and line-selected operation. The effects of these mechanisms (and their relative importance) on the laser output are clearly revealed by the model. The character of the spectra for the H(2) + F(2) model is significantly different from that observed for the F + H(2) model. The ability of the model to predict spectra observed in experiments is assessed, and the model is found to compare well with discharge-initiated lasers. Additional calculations demonstrate the effect of multiquanta V-T deactivation of HF by HF.

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.

Simple Model of a Line Selected, Long Chain, Pulsed DF-CO 2 Chemical Transfer Laser

A rate equation model of a pulsed DF-CO(2) chemical transfer laser is Simplified by investigating the relationship of kinetic mechanisms. The model reduces to one equation which when Solved permits calculation of all pulse characteristics as a function of time. At low levels of initiation, predictions of this laser simulation model are in excellent agreement with those of a more comprehensive model presented in an earlier paper. Model predictions are also found to be consistent with experiment. The simplicity of the present model lends itself to easy physical interpretation and permits efficient and accurate prediction of transfer laser performance characteristics in regimes of practical interest.

Effect of vibrational and rotational relaxation mechanisms in pulsed H2 + F2 lasers.

A comprehensive model of the H2 + F2 pulsed chemical laser is used to investigate mechanisms important to laser performance. The model employs vibrational-to-rotational energy transfer and rotational nonequilibrium in an attempt to explain experimental observations of rotational lasing and P-branch lasing from high J states. The effect of partitioning energy between vibrational–translational (V–T) and vibrational–rotational (V–R) in the V–R, T mechanism was considered. Increased V–T produced smaller predicted pulse duration and laser power. The major effect of V–R is to populate rotational levels above J = 12; hence, it did not produce major changes in predicted P-branch lasing spectra. Increasing the dependence of rotational relaxation rate coefficients on rotational quantum numbers was effective in sustaining larger nonequilibrium populations at high J levels, but the effect was not as pronounced as an overall decrease in the rotational relaxation rate. The prediction of excessive energy in the hot bands of HF appears to be the result of insufficient vibrational deactivation from these levels. Both increasing the endothermic cold pumping reactions and the vibrational dependence of V–R rates were effective in decreasing hot-band lasing. Vibrational scaling of the V–R rate coefficients larger than V2.3 may be required to properly describe vibrational deactivation. Although the model compares favorably with experimental studies for the lower three P-branch bands, excessive P-branch lasing is predicted for high rotational levels in the hot bands. Insufficient experimental data on the pulsed H2 + F2 chemical laser exist to reconcile theory with experiment.

Possible high energy laser at 1.27 μm

Preliminary theoretical and experimental evidence is presented that suggests the potential of lasing from the strongly forbidden O(2)((1)Delta(g)) ? C(2)((3)Sigma (-)(g)) transition of molecular oxygen. A rate equation model is developed which predicts pulse energies up to several hundred joules/liter atmosphere for typical mixtures of 10(3):2N(2) activated by uv photolysis in less than 10 microsec. Preliminary results from a flash photolysis laser apparatus demonstrating 1.27-microm lasing are presented. Results from a computer analysis assessing the possibility of using this system as a multipass amplifier are also given.

Detailed characteristics of a pulsed H 2 +F 2 laser. 1: Experiment

An experimental investigation of a pulsed, flash photolytically initiated HF laser pumped by the H2 + F2 chain reaction is presented. Measurements of time-resolved spectroscopy, small signal gain, and total pulse energy are reported for two compositions (He:O2:F2:H2 = 20.8:1.0:4.6:1.2 and He:O2:F2:H2 = 22.0:1.0:2.7:1.0) and three mixture pressures (36, 102, and 331 Torr). Time-resolved spectroscopy results show individual pulse durations decrease with pressure and fluorine concentration while peak intensities increase with pressure and fluorine concentration. Small signal gain results show that increasing pressure or fluorine concentration increases individual peak gains and decreases gain durations. Total pulse energy is consistent with previous studies.

Detailed characteristics of a pulsed H 2 +F 2 laser. 2: Theory

A theoretical investigation of a pulsed, flash photolytically initiated, HF laser pumped by the H2 + F2 chain reaction is presented. The results of computer simulations of the laser kinetic mechanisms are compared to small signal gain, total pulse energy, and time-resolved spectral measurements. These comparisons show that a model assuming vibrational-to-translational (V–T) energy transfer as the dominant HF deactivation channel more closely predicts gain and spectroscopic durations, initiation, peak and termination times, plus peak gain magnitudes and total pulse energies than a similar model assuming vibrational-to-rotational (V–R) energy transfer as the dominant deactivation channel. This is contrary to the current understanding of HF kinetic mechanisms. It is concluded that the current understanding of these mechanisms is not sufficient to quantitatively predict either the time-resolved spectra or gain of the H2 + F2 laser. Model results also indicate that rotational relaxation is fast compared to other deactivation processes, but slow compared to stimulated emission. Finally, the model gives an estimated lower bound for the experimental [F]/[F2] ratio of 0.0025%.