Robert Hofland

Simplified Model of CW Diffusion-Type Chemical Laser

Abstract : A simplified analytical model of a diffusion-type HF chemical laser is presented. In this device, H2 is diffused into a supersonic stream to react with F atoms and form vibrationally excited HF that is made to lase. The reaction between H2 and F is assumed to commence at a flame sheet. A two-vibrational level model for the HF molecule is adopted. The dependence of laser amplifier and oscillator performance on diffusion rate, forward reaction rate, collisional deactivation rate, and radiative deactivation rate is determined. (Author)

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.

Flame-Sheet Analysis of C. W. Diffusion-Type Chemical Lasers.

Abstract : An analytical treatment of diffusion-type chemical lasers is presented. Chemical formation of the lasing molecule by laminar mixing and combustion of parallel streams of fuel and oxidizer is assumed to be diffusion controlled, and subsequent collisional deactivation of each vibrational level of the lasing molecule by nonreactive V-V and V-T energy transfer is found by a successive-approximation scheme. Radiative processes are considered only in zero-power an high-power limits. A laser employing the reaction H2 + F yields HF(v) + H is considered in detail with the use of a single-boundary-layer, flame-sheet model. Expressions for integrated zero-power gain, laser power, and efficiency are obtained. (Author)

Pulsed chemical laser with variable pulse‐length electron‐beam initiation and magnetic confinement

Performance of a pulsed HF/DF chain laser was investigated for the case of transverse initiation by a magnetically confined electron beam. Laser energy and beam quality are presented as functions of electron gun, magnetic field, gas mixture, and optical resonator parameters. The results include 79‐J/l HF output and electrical efficiency for conversion of total incident e‐beam energy to laser output energy of 45%.

Atmospheric-pressure H 2 -F 2 laser initiated by electron beam irradiated discharge

Abstract : The authors have measured pulse energy and power versus time for the output of an HF laser pumped by the H2 - F2 chain reaction. Noble-gas-diluted atmospheric pressure mixtures were initiated uniformly with the use of a transverse electrical discharge established by partially ionizing the gas mixture with a burst of high energy electrons. As primary objectives, the authors sought to demonstrate atmospheric pressure operation and a laser output comparable to the electrical energy supplied for initiation. Performance as a function of several parameters was studied.

Dissociation efficiency of electron‐beam‐triggered discharges for initiating atmospheric‐pressure H2‐F2 lasers

Time‐resolved uv absorption measurements of the rate of F2 disappearance have been compared with a theoretical pulsed chemical laser code to infer electrical dissociation efficiencies of electron‐beam‐irradiated discharges. The results indicate that a 400‐keV, 50‐nsec e‐beam of A/cm2 dissociates approximately 0.3% of the reactants initially present in dilute F2/H2 mixtures, producing four chain carriers per ionizing collision. With the addition of a discharge field at 80% of the self‐breakdown limit, initial reactant dissociation increases to approximately 1.1%, corresponding to 15 chain carriers per ionizing event and a dissociation efficiency of 7.5%. An earlier analytical plasma model of reactant dissociation that has been generalized to account for the presence of Ar and H2 suggests that heating of the negative ions of fluorine, leading to electron detachment in heavy‐particle collisions and direct electron‐impact dissociation of reagents by slow electrons become dominant mechanisms with the applicati...

Scaling laws for pulsed chain‐reaction chemical lasers

Scaling laws for pulsed chain-reaction chemical lasers are deduced with the use of a two-level vibrational model. The performance of a saturated laser depends only on the parameter K = t/sub cd//t/sub p/, where t/sub cd/ and t/sub p/ are the characteristic collisional deactivation and characteristic pumping times, respectively. The normalized output energy per unit volume per pulse of a saturated HF chain-reaction laser is 2E/Epsilon H/sub 2/, 0 = K(1 + 0(K)), where E is output energy per unit volume per pulse, Epsilon is energy per mole of photons, and H/sub 2/, 0 is the initial concentration of H/sub 2/ in moles per unit volume. In the range 0.02 < or = thi << 1 the normalized output energy from a saturated HF laser can be expressed as 2E/Epsilon H/sub 2/, 0 = thi, where thi approx. = (F/F/sub 2/)/sub 0/ 1/2(F/sub 2//H/sub 2/)0(1 + 0.094(F/sub 2//H/sub 2/)/sub 0/) to the minus 1/2 power. In the latter regime the product Et/sub e/ is a constant for a saturated laser (t/sub e/ = pulse length). Corrections for multiple vibrational levels are given in an Appendix.

Atmospheric‐pressure H2‐F2 laser initiated by electron‐beam‐irradiated discharge

Initial performance of an electric‐discharge‐initiated chemical laser pumped by the H2‐F2 chain reaction is presented. Uniform microsecond‐scale discharges in atmospheric‐pressure laser mixtures have been obtained after a short‐duration ionizing pulse of high‐energy electrons. Care in discharge pulse shaping is required to avoid late‐time arcs. Informaiton is presented on the effects of several parameters on laser performance. With a 3 H2 : 6 F2 : 54 He : 37 Ar mixture as much as 28 J/liter of discharge energy could be deposited in the mixture, and laser energies up to 42 J/liter were obtained. This corresponds to a chemical efficiency of 6.3%, based on the H2 content, and an electrical ’’efficiency’’ of 148%, based on the available lasing volume and total energy input to the medium.

Continuous-wave HF selected-line integral master oscillator power amplifier

Efficient lasing on multiple-selected lines has been recently demonstrated experimentally on a cw hf chemical laser. Multiple-selected-line operation is required to both enhance atmospheric transmission and to ensure high-power extraction efficiency on multiple vibrational hf levels. Seventy-five percent of the multiline power was measured on two selected hf lines using a confocal, unstable cavity with a high-efficiency (97%) intercavity diffraction grating. This measured fraction of the multiline power is consistent with theoretical calculations, which include the effect of rotational nonequilibrium. The two-selected-line hf unstable cavity was not prone to parasitic oscillations. A novel multiple-selected-line integral master oscillator power amplifier (IMOPA) concept was also evaluated. Line selection on two hf lines was demonstrated with the IMOPA, although the hole diameter had to be made sufficiently large to prevent parasitic oscillations within the amplifier. It was concluded from our experiments and theoretical calculations that, although the IMOPA concept was demonstrated at relatively low power (400 W), parasitics may be a problem at much higher values of the single-pass gain.

Performance of a Transverse Flow Gas Lens

Abstract The performance of a gas lens is presented in which a gas flow between two cooled (or heated) parallel plates generates a parabolic variation of index of refraction in the region between the plates. A laser beam, propagating normal to the gas flow, can then be diverged (or converged, or focused). A single module acts like a cylindrical lens. The use of two or three modules, arranged in an orthogonal manner, provides a spherical lens. A proof-of-principle experiment employing a single cooled (diverging) module was assembled and tested at gas-to-wall temperature differences of up to 99°C. For a plate separation distance of 0.569 cm this device yielded a beam divergence of 22.6 mrad for the maximum temperature differential. This corresponds to a twenty-two-fold increase in laser beam divergence over that produced by diffraction alone. Our measurements of beam divergence as a function of temperature differential are in excellent agreement with theoretical predictions. Laser beam divergence was shown to increase at a rate greater than linear with increasing temperature differential. This nonlinear behavior is a result of the parabolic index gradient which is established along the entire extended length (30 cm) of the beam path as it traverses the lens. The output beam was refocussed and scanned in the far field and was shown to be of high optical quality. In addition, the output beam is of high stability, both temporally and spatially.