Pulsed HF Chemical Laser Research Articles

Pulse energy sustainability of a burst mode closed-cycle pulsed HF chemical laser

Pulse energy sustainability of a burst mode closed-cycle pulsed HF laser is investigated with a simple theoretical model considering gas replenishment and chemical scrubbing. The effects of reactant gas consumption and de-excitation of excited HF by ground state HF are studied quantitatively. Also, experiments are carried out for comparison and validation. Simulation and experiments show that, in the burst mode operation, the energy loss caused by gas consumption is about 0.06–0.12 mJ per pulse while that induced by the de-excitation process is about 0.22–0.29 mJ per pulse, demonstrating that chemical scrubbing should play a more important role in the energy preservation of non-chain pulsed HF lasers.

An Installation for Fast and Safe Preparation of Working Mixtures in Reactors of Pulsed Chemical HF(DF) Lasers

A system for preparing working mixtures in reactors of pulsed chemical lasers based on the F2 + H2(D2) reactions is described. The design of a compact capillary mixer has been developed; its throughput of up to 2 (L atm)/s has been experimentally attained. A method for supplying gases using flow-control washers with holes operating in the critical mode is proposed. It provides a constant flow rate of mixed gases during the whole run-in. The described technology for preparing the working mixture of a pulsed chemical HF(DF) laser is theoretically analyzed and experimentally tested.

100 Hz重复频率脉冲中红外HF化学激光器

100 Hz重复频率脉冲中红外HF化学激光器

Pulsed chemical HF laser with a large discharge gap

The characteristics of the radiation emitted by an electric-discharge pulsed chemical HF laser with a discharge gap of 10 cm are studied. The discharge was stabilised by a semiconducting ferroelectric ceramic layer deposited on plane metal electrodes. The specific energy and technical efficiency were 3 J L-1 and 3.4%, respectively, for a laser operating on a nonchain reaction in SF6—H2 mixture and 25 J L-1 and 26%, respectively, for a laser operating on a chain reaction in F2—O2—SF6—H2 mixture.

A Compact Pulsed HF Laser Based on an Auto-Wave Photon-Branched Chain Reaction

Huge energy gain is detected theoretically in a pulsed chemical laser-amplifier based on a photon-branched chain reaction initiating in a gaseous disperse medium composed of H2–F2–O2–He and Al particles by focused external infrared radiation. It is shown that this effect is observed due to the other optical effect of diffractive multifocal focusing of the input radiation on a certain type of bicomponent optical system coupled structurally with the input mirror of an unstable telescopic laser cavity. Such a relatively simple bicomponent diffraction system, consisting of two plane screens with circular apertures on a given optical axis, enables one to focus the input beam without using classical refraction elements such as lenses and prisms. The focusing of the input signal ensures the minimization of the initially excited volume of the laser active medium and the appropriate sharp lowering of the necessary energy of the input pulse up to ∼ 10-6 J. This enables the laser system to reach a high value of the energy gain of 109. The huge laser energy gain allows us to use a master oscillator in the form of a small microjoule laser powered by an accumulator, and consequently makes it possible to construct a completely self-contained compact pulsed chemical HF-laser.

Self-contained compact pulsed laser based on an auto-wave photon-branched chain reaction

The possible construction of a self-contained and compact pulsed chemical HF-laser based on an auto-wave photon-branched chain reaction initiated in a gaseous disperse medium composed of H2-F2-O2-He and Al particles by focused external IR radiation is theoretically substantiated. It is shown that an autonomous system and minimization of the parameters of the main pulsed HF-laser units are achievable due to both the effect of ignition of the laser-chemical reaction in an auto-wave regime under the condition of external beam focusing and the effect of a huge laser energy gain of 1011. These effects provide strong reduction of the input pulse energy necessary for initiation up to ∼10-8 J, and make it possible to construct a self-contained laser with kilojoule output energy, which can be initiated by a small submicrojoule master oscillator powered by an accumulator. Due to an increase in the general pressure of working gases up to P=2.3 bar and optimization of the parameters of the dispersed component (Al particles with radius r0=0.09 μm and concentration N0=1.4×109 cm-3) and the composition of the working mixture, the HF-laser system will ensure an output energy up to ∼1.5 kJ in a pulse from the rather small volume of ∼2 L of the active medium.

Influence of the cavity losses on the energy and spectral characteristics of a pulsed chemical chain-reaction HF (DF) laser

The dependences of the energy and spectral characteristics of the radiation of a pulsed photoinitiated chemical chain-reaction HF (DF) laser, operating at atmospheric pressure, on the optical cavity parameters were investigated. The maximum specific output energies of the HF and DF chain lasers (53 and 43 J litre-1) and their fullest spectra (47 and 83 lines, respectively) were similar when the unsaturated gain exceeded the threshold by a factor greater than 50. An analysis was made of how the energy and spectrum of the radiation of the HF (DF) laser were influenced by the spatiotemporal inhomogeneity of the initiation and of the course of the chemical chain reaction. There was a marked effect of the CO2 molecules in the active medium and in the air filling part of the volume of the complex cavity of the DF laser on its efficiency and emission spectrum.

Laser amplifier based on a photon-branched chain reaction in an aerosol-evaporation reactor cavity

A new design is proposed and the characteristics are calculated of a pulsed chemical HF laser with a two-phase active medium and a photon-branched chain initiation mechanism. This laser includes an aerosol-evaporation reactor coupled structurally to an unstable telescopic cavity. Such a closed system reduces considerably (down to 0.217 ms) the time needed for the formation of a two-phase active medium with the required properties. When the working volume of the laser is ~7 litres, the following high output characteristics can be achieved: the maximum power per pulse ~2×1010 W, the pulse duration ~740 ns, the total output energy ~1.5 kJ, the specific output energy ~200 J litre-1 , and the energy gain 105 .

Lifetime of a two-phase active medium of a pulsed chemical HF laser

An analysis of the response of an aerosol and of the laser-chemical kinetics of a photon-branched chain reaction is used to determine the range of the maximum permissible parameters of the disperse component capable of supporting the operation of a pulsed chemical HF laser with a two-phase active medium (particle radius 0.09 — 0.4 μm, particle concentration 109 — 107 cm-3 ). It is shown that, in this range of parameters, the lifetime of the active medium of an HF laser with disperse Al particles is governed by coagulation, precipitation, and electrostatic scattering of particles, and that it amounts to 250 s. The main contribution to the degradation of the disperse component of the active medium comes from Brownian coagulation.

Degradation of the disperse component of the active medium of a pulsed chemical HF laser

The Brownian coagulation model and the Stokes particle precipitation law were used to analyse the dynamics of changes in the characteristics of a condensed disperse component of the chemically active medium of a pulsed HF laser. It was found that, if coagulation predominates over precipitation, the required average particle size (r0 ∼ 0.1 — 0.5 μm) and concentration (N ∼ 5 × 108 — 107 cm-3) of an aluminium aerosol in an H2 — F2 — O2 — He chemically active medium can be reached in ∼ 1000 s. The precipitation of particles begins to compete with the coagulation only for r0 ≥ 1μm.

Spectral optimization of a pulsed HF chemical laser for efficient energy delivery through a low-loss fluoride glass optical fiber

Pulsed HF chemical lasers oscillating in the midinfrared region of the spectrum where water can strongly absorb are suitable for the organic tissue ablation. For such a medical application, a flexible energy delivery system using an optical fiber is indispensable. The fluoride glass optical fiber may be used for low-loss optical energy delivery of the pulsed HF laser. Using an intracavity CO2 absorption cell, we have optimized pulsed multiline HF chemical laser spectrum for low-loss energy delivery through a fluoride glass optical fiber whose transmission loss is theoretically estimated to be minimum around 2.5 μm. The fractional output energy of P1-0(3) and P1-0(4) lines, which are sitting in the lowest loss spectral region of the practical fluoride glass fiber, exceeded half the multiline output energy using a line selective intracavity CO2 gas cell. The P1-0(4) line extracted from the resonator with a diffraction grating was successfully delivered through a 3-m-long commercial fluoride glass fiber (core/cladding diameter=450/500 μm). The delivered energy of 11.9 mJ was achieved with the corresponding energy fluence of 12 J/cm2 and peak intensity of 22 MW/cm2 at the exit core surface without optical damage.

Output spectral characteristics of a pulsed HF chemical laser using an SF6-HI-H2mixture and electron-beam pumping

We investigated the spectral characteristics of an SF6–HI–H2 laser pumped by an electron beam. The time-integrated output spectrum was studied as a function of the composition of the hydrogen-containing component of the working mixture. Lasing takes place under conditions far from rotational equilibrium.

Feasibility and conditions for the attainment of high output energy characteristics from a pulsed chemical HF laser utilizing an SF6–HI mixture

An investigation was made of the feasibility and conditions for the attainment of high output energy characteristics from an electron-beam-initiated SF6–HI laser. A specific output energy of 8 J/liter was achieved. The maximum output energy per pulse from an SF6–HI laser was found to be higher than that from an SF6–H2 laser. The presence of an external static magnetic field, inhibiting scattering of the initiating electrons, did not increase the maximum laser pulse energy.

Theoretical study of a supersonic high-repetition-rate H2+F2-pumped HF laser

A numerical model for calculating the performance of a premixed pulsed supersonic HF chemical laser, pumped by the H2+F2 chain reaction, is presented. The laser kinetic equations, which are coupled to the supersonic gas dynamics and to the chemical equations, are integrated along the flow direction. The model which has been shown to accurately predict the measured gain coefficients, the lasing induction times, and the laser power output, has been used for parametric studies leading to a better understanding of the laser operation. Conditions for maximum gain and maximum laser energy for each of the input parameters were investigated. The lasing zone length in the flow direction was studied. A comparison between the experimental and calculated results shows that the model can be used to evaluate unknown chemical and energy transfer rates.

Pulsed chemical HF laser using graphite continuous electrodes for transverse discharge stabilization

We report results obtained in the operation of a chemical HF laser using a mixture of SF6 and technical grade C3H8 reacting in a transverse high-voltage discharge in which a pair of inexpensive continuous electrodes of polycrystalline graphite are used as the only means for stabilizing the discharge and avoiding the formation of arcs during excitation of the laser medium. Energies of up to 60 mJ per pulse in stable operation are obtained from a 0.1-ℓ volume.

Phase conjugation of an optical wave in the case of a degenerate four-wave interaction in an amplifying medium of a chemical HF laser

The approximation of a constant intensity of reference waves is used in the case of low values of the reflection coefficients (less than unity) to investigate numerically the efficiency of optical phase conjugation of a wave from a pulsed chemical HF laser based on a chain reaction. It is shown that the reflection coefficient for the three strongest emission bands P1–0, P2–1, and P3–2 of a pulsed chemical HF laser can be 0.1.

Chemical HF laser initiated by evaporation of fine particles under the action of infrared radiation

It is shown that, in principle, it is possible to initiate a pulsed chemical HF laser by the evaporation of fine metal particles with the aid of infrared radiation. It follows from the reported calculations that the laser radiation energy may be 3–5 times greater than the initiation energy, which opens up the way for constructing purely chemical pulsed HF laser amplifiers.

State-selected molecular-beam reaction dynamics of K with HF (υ = 1, j = 5, 6 and 7)

The crossed-molecular-beam method has been used to measure relative integral cross sections σ r (υ = 1, j; E) of the endothermic (0.75 eV) reaction K + HF (υ = 1, j) → KF + H as a function of both the rotational quantum number j ( j = 5, 6 and 7) and the translational energy E (0.55 ⩽ E ⩽ 1.32 eV) of the reagents. The vibrational-rotational (υ, j) states of HF were populated by infrared pumping with the radiation of a properly tuned pulsed chemical HF laser. In addition, relative cross sections with υ = 0 but averaged with respect to the initial rotational-state distribution of the HF molecules were also determined. The results exhibit three remarkable features: (a) σ r (υ = 1, j; E) rises with increasing j, (b) the steepness of the ascent roughly decreases with increasing E, (c) at E = 0.98 eV reagent vibrational energy is about a factor of 200 more efficient in promoting the reaction than reagent translational energy. An intuitive dynamical model suggests that observations (a) and (b) are to be expected for a reaction governed by a potential-energy surface with a late barrier. This is consistent with the well-known interpretation of large vibrational effects as indicated by result (c).

Pulsed chemical HF laser utilizing a mixture of technical-grade C3H8and SF6and initiated by an electric discharge

An electrochemical HF laser was constructed utilizing a mixture of technical-grade propane and butane with SF6. A resistive anode was made of a Ge single crystal (ρ = 25 Ω·cm) and the mixture was subjected to ultraviolet preionization. The output radiation was highly stable. In the nonselective lasing regime a discharge volume of 86 cm3 yielded laser pulses of 220 mJ energy with a peak power of 3 MW. The energy corresponding to the individual P1(4–9), P2(3–8), and P3(3–8), transitions was in the range 15–43 mJ and the peak powers for these lines were 0.2–0.5 MW.

High efficiency, pulsed HF chemical laser using a mixture of H 2 and F 2 initiated by an intense electron beam

The stable gas mixing of H 2 and F 2 by adding a trace of O 2 at room temperature has been achieved. The electron beam (1 MeV, 20 kA, 70 nsec) initiated the HF pumping reactions by the dissociation of F 2 and/or H 2. The HF lasing has been obtained at a mixing ratio of [F 2]/[O 2]/[F 2]=1/0.3/0.3 and at total pressures up to 800 Torr. A typical HF laser energy was 180 J/pulse at an intrinsic laser efficiency as high as 200% and at a chemical efficiency (defined by the ratio of the HF laser energy to the total exothermicity of the pumping reactions between H 2 and F 2 as high as 8%. The theoretical results of the detailed HF laser computer code involving dissociation reactions, pumping reactions, deactivation reaction and lasing reactions coincided with the experimental results.