Laser-supported Detonation Wave Research Articles

Plasma formation resulting from the interaction of a laser beam with a solid metal target in an ambient gas

The formation and role of a plasma resulting from the interaction between a laser beam and a metal target in an ambient gas are reviewed in this paper. The plasma is initiated after the production of primary electrons by multiphoton absorption and thermionic photoemission mechanisms. Vaporization of the surface then occurs at a lower threshold than theoretically deduced, due to surface defects and impurities. Vapour ionization is first a thermal process, primary electrons gaining energy by inverse Bremsstrahlung than electron cascade growth occurs. The plasma propagates with absorption waves; laser-supported combustion or detonation waves depending on the laser irradiance regime. The ablation process is favoured with the shortest wavelengths, whereas ambient gas breakdown occurs with the largest wavelengths. The nature of the ambient gas must be adapted for the chosen application. Plasma development can be an inconvenience with target shielding effect when the ionization degree is too high and with laser-supported detonation wave generation occurring in this case. However, transparent plasmas provide optimum laser machining, as good surface coupling with plasma formation occurs which is useful for drilling or welding.

Ignition and maintenance of laser-supported detonation waves

Laser-driven detonation waves are being considered for use in ground to low-Earth-orbit launch applications. This paper addresses the problem of determining minimum laser irradiance for ignition and maintenace of laser-supported detonations waves. A means of igniting a detonation wave at low irradiance is to embed in the propellant thin metallic flakes that break down upon vaporization. A computational scheme has been developed for determining the structure and thickness of the waves. Results are presented for seeded H 2 O, LiH, and glass propellants in the density range 10 -4 to 10 -2 g/cm 3

Investigation of the dynamics of a laser-supported detonation wave using a self-consistent numerical model

Results are reported of a computational—theoretical analysis of the dynamics (propagation and decay) of laser-supported detonation waves (LSDW) in focused laser beams. The investigations were carried out using a self-consistent numerical model that takes account of the gasdynamic motion of the plasma and the refraction and absorption of the radiation in the plasma, with allowance for its real equation of state. An analysis is presented of the influence of the radial structure of the radiation on the propagation and decay of the LSDW, and of the conditions of the radiation transformation. Results on the dynamics of two-dimensional LSDW in the beam of an excimer laser are presented.

Comparison between laser-supported detonation and blast waves for laser propulsion

Comparison between laser-supported detonation and blast waves for laser propulsion

Propagation of laser breakdown and detonation waves in transparent droplets

A computer model is used to study recent experiments investigating laser-induced plasma formation and explosion of single liquid aerosol droplets. The model accounts for the formation and evolution of the plasma and the resulting fluid flow; it does not account for the initial breakdown. The model shows all the qualitative features of the experiment, including laser-supported detonation waves, which at high laser intensities rapidly convert the whole droplet into a plasma. In addition, quantitative agreement with measured velocities has been obtained to within approximately 50%.

Internal and external laser-induced avalanche breakdown of single droplets in an argon atmosphere

Laser-induced breakdown of a transparent micrometer-sized H2O droplet can be initiated only in the Ar gas in a strip outside the droplet shadow face. At a higher-input 0.532-μm Q-switched laser intensity, breakdown can also occur within the droplet. The resultant internal plasma blocks the laser from reaching the region outside the shadow face and absorbs more of the laser pulse to produce a shock wave and/or a laser-supported detonation wave. Various combinations of liquid and surrounding gas were investigated at different input intensities in order to provide information on the breakdown processes in a transparent droplet.

Mechanisms for thermally enhanced target coupling by repetitively pulsed lasers

This paper describes the results of computations of the heat and mass transfer to and from an aluminum target when irradiated by an intense 10.6 /mi laser pulse, for the case when a laser-supported detonation (LSD) wave is ignited. The results of the computations are compared to an experiment performed by Maher and Hall where a thermal-coupling coefficient of 17.5% was observed. The computations indicate that convective and radiative heat transfer alone are unable to explain the high thermal coupling; vapor recondensatibn is also investigated as a possible mechanism for explaining the observed coupling coefficient, and does appear to be compatible with the observations. Conclusions are drawn about the possible physical mechanisms at work both during the early phases of the pulse and during the time the LSD wave propagates away from the target.

A computational study of laser-supported detonation waves propagating up an oblique incident beam

We conducted a series of numerical experiments to study the propagation of laser-supported detonation waves (LSDWs) in the case that a CO2 laser beam strikes an aluminum surface obliquely in air. A reflected shock formed at the aluminum surface was more prominent at higher angles of incidence θ of the beam, but otherwise the hydrodynamics of the plasma and the LSDW were insensitive to θ. Furthermore, the total impulse delivered to the aluminum varied approximately as 1/cos θ, a result that can be modeled with elementary blast-wave theory.

Momentum transfer to a surface when irradiated by a high-power laser

A simplified model is developed to predict the impulse imparted to a surface when irradiated by a high-power laser. A high-pressure plasma is created by a laser-supported detonation (LSD) wave traveling toward the energy source. The multidimensional expansion of the high-pressure region is described by patching one-, two-, and three-dimensional blast wave solutions to the LSD wave. The patching times are obtained by conserving energy between the wave solutions and are shown to be different from those used by previous authors. The pressure- time-space results are integrated to determine the net impulse delivered to the surface. A simplified procedure is outlined for converting target failure criterion to laser parameter requirements. HEN a single-pulse high-power laser irradiates a target surface, a laser-supporte d detonation wave (LSD) forms above the target surface and propagates into the background gas. The high-pressure gas behind the LSD wave transfers momentum to the target and represents a potential damage mechanism. A measure of the efficiency with which momentum is imparted to the target is the coupling coef- ficient, defined as the impulse per unit laser energy. Several authors14 have sought to evaluate and optimize the coupling coefficient with respect to the laser beam diameter, pulse duration, and energy. The impulse imparted to the target during irradiation is a small fraction of the total impulse delivered. Hence, an ac- curate description of the momentum transfer must include the decay of the LSD wave into a hydrodynamic blast wave. The transition from an LSD wave to a blast wave has been described1'3'4 by patching the blast wave and LSD wave solutions together at an intermediate time. Hydrocode simulations2 have confirmed this approach but a unique choice for the patch time was not established. It is shown herein that various choices for the patch time results in variations in the energy in the blast wave. Since the energy in the blast wave must be equal to the laser energy deposited in the LSD wave, the patch time may be established by specifying the energy in the asymptotic blast wave. Using this technique to determine the patch time, it is shown that the laser energy required to deliver a fixed impulse to a target may, in some limits, be a factor of five greater than previously assessed.

Implications of applying a global energy balance to laser-supported and chemical detonation waves

A closer examination of the conditions needed to propagate a steady-state shock wave is made and generalized to include energy added in the vicinity of the wave via both laser deposition and chemical reactions. This analysis yields slightly different results than standard detonation theory, and these differences are explored. A case is made for the correctness of the present method by citing the results of other researchers.

Pulsed Laser-Generated Impulse on a Surface in Supersonic Flow

I is now well known that pulsed CO2 laser radiation can generate high pressures over a surface through formation of laser-supported detonation (LSD) waves at fluxes greater than 10 W/cm (Refs. 1-3). It is also possible for hot, dense plasmas to be produced in the strong laser-supported combustion (LSC) wave regime at fluxes between 10-10 W/cm (Refs. 4 and 5). The impulse generated at the surface by these phenomena are reasonably well understood. Previous analytical and experimental work has dealt with a static environment. The phenomenology should be different with a supersonic crossflow over the surface. This work examines the behavior of the laser/surface/supersonic flow interaction and presents impulse data from aluminum targets.

Explosive puncturing of metal plates by lasers

Substantial energy savings in puncturing metal plates by laser radiation can be achieved by heating the metal by cw radiation to a temperature closely below the melting point and then applying a sharp radiation pulse to push the hot metal plug through the plate. An analysis of this process is presented, in which the non-Newtonian viscosity behavior of metals is considered. Scale-up effects and efficiency calculations are presented for laser-supported detonation waves and point explosions. Flow characteristics as well as penetration criteria are given for aluminum, copper, and steel which are in good agreement with experimental data.

Two-dimensional modeling of aerosol-induced breakdown in air

The dynamics of aerosol-induced breakdown in air is described using a three-temperature nonequilibrium thermodynamic model of the air plasma and two-dimensional Lagrangian hydrodynamics. From a heated sphere of air used as the initial condition an expanding laser-supported absorption wave develops, provided a maintenance intensity-threshold criterion is met. The maintenance threshold is close to 6×106 W/cm2 for 10.6-μm radiation. Although the absorption wave develops a shock, it differs in important ways from a one-dimensional laser-supported detonation wave due to its two-dimensional character.

Propagation of a laser-supported detonation wave

Experiments were performed in order to determine the aerodynamic values of the gas velocity, pressure and temperature which are produced by a radiation driven wave in air. The wave is initiated on a plane target surface area of 2 cm 2 by the focussed 10.6 μ laser light energy (maximum of 280 joules in 3 μ sec). The resulting flux intensity which was approximately constant during 1 μ sec of the pulse, varied between 10 7 and 1.5 × 10 8 W/cm 2 as related to the emitted laser energy. Comparison of the incident and transmitted pulses permitted the determination of the deposited light energy on the target surface and of the characteristic time for plasma formation. The plasma front propagation velocity was measured by a streak camera. A piezoelectric gage of a relatively small diameter compared to the impact cross-section measured the pressure applied to the target during the interaction. The temperature was deduced from an analysis of the plasma spectra emission which was measured at different distances from the target. From measurements of laser line absorption at 488 nm and 633 nm the plasma density was estimated. The time between the beginning of the laser pulse and the beginning of the front propagation in air was found to be about 100 ns. This time did not seem to depend upon the target material. Considering that the cross-section was sufficiently large, these measurements give the necessary information to illustrate that, for the range of laser flux considered, a one-dimensional model of a detonation wave correctly describes the aerodynamic flow during the time of laser-light energy deposition.

Influence of laser-supported detonation waves on metal drilling with pulsed CO2 lasers

Drilling of highly reflective metals in an ambient atmosphere with single TEA-CO2-laser pulses of fluences between 300 and 6000 J/cm2 is reported. The drilling process was investigated by measuring the time-resolved laser power reflected specularly from the targets during the interaction and by analyzing the craters produced. Experiments were performed in ambient air, argon, and helium. Target damage was found to be strongly influenced by a laser-supported detonation (LSD) wave in the ambient gas. If the laser fluence exceeded a material-dependent damage threshold (copper: 300 J/cm2), drilling occurred, but the efficiency was inversely related to the duration of the LSD wave. Efficient material removal is possible if the LSD wave can be dissipated within a small fraction of the laser pulse duration. This was achieved by small-F-number focusing of TEM00 laser pulses of 5-μs duration. Replacing the ambient air at the target by a gas of lower density results in a further significant reduction of LSD-wave lifetime, and a correlated increase of the drilling yield. On copper targets a maximum drilling yield of 10−5 cm3/J was observed in ambient helium at a laser fluence of 1 kJ/cm2.

Impulse Transfer from Pulsed CO, Laser Irradiation at Reduced Ambient Pressures

EXPERIMENTAL and theoretical studies have been made of the impulse received by a target surface when irradiated by a high-power pulsed CO2 laser at reduced ambient pressures. Correlation of experimental and theoretical results for aluminum targets indicates a maximum impulse-energy coupling coefficient ap~2 dyn s/J and an effective thermal coupling coefficient a,—0.1. Both theory and experiment show an increase in ap with increasing ambient pressure. Structural deformation was experimentally demonstrated for an 1100 aluminum target. Contents Most of the previous work on impulse transfer from highpower laser pulses to surfaces, both experimental and theoretical, has been carried out for ambient air at or near 1 atm pressure, the measured impulse being dominated by the air environment.1 The air above the target surface can become ionized, forming an absorptive plasma. At laser intensities above about 107 W/cm2, this air plasma moves supersonically up the laser beam and away from the target, shielding the target surface from further direct radiation. This phenomenon is the well-known laser-supported detonation (LSD).2'3 At low ambient pressures (densities), on the other hand, impulse data are more limited in quantity, and theoretical predictions and correlations are comparatively less well established. As the ambient pressure is lowered, the air plasma absorption length for laser energy increases, becoming comparable to or exceeding the width of the laser beam; the LSD wave in air becomes harder to maintain and eventually disappears.3 In such cases, vaporization of the target material and the subsequent behavior of the vapor become the dominant factor in determining the impulse transfer. In the present work, both experimental measurements and theoretical calculations have been made of the impulse received by a target surface when irradiated by a high-power pulsed CO2 laser at reduced ambient pressures. The impulse experiments were carried out inside a 120-cmdiamx 160-cm-long vacuum tank. The pressure was varied from 10 to 0.01 Torr. The laser beam was admitted through a scintered NaCl window whose outside face was tilted to avoid reflecting energy back into the laser cavity. The 7.5-cm-diam beam was concentrated on the target by means of a copper mirror having a focal length of 114 cm located in the vacuum tank. By adjusting the distance between the mirror and the target, the laser spot diameter was varied from 0.38 to 2.3 cm.

Transient analysis of a laser supported detonation wave using Whitham’s rule

Analysis is made of a laser supported detonation wave using Whitham’s rule. The C+ characteristic is developed for absorption of laser energy immediately behind a shock wave. The characteristic is then coupled with the Rankine-Hugoniot equations relating conditions across the shock. In this way an equation for predicting the transient shock strength is obtained. The result is compared to experiment and to steady state laser supported detonation theory. The result of this comparison is that the derived equation does duplicate the transient response of the experiment and (when allowed to run to steady state) it does agree with many of the characteristics of the steady state theory.

Laser-beam optimization for momentum transfer by laser-supported detonation waves

Momentum transfer from a laser-supported detonation wave to a solid target has been studied by means of a time-dependent, two-dimensional, two-temperature Lagrangian hydrodynamics code. Its dependence on laser pulse length has been calculated and pulse length conditions obtained for optimizing impulse transfer to a target of finite area embedded in a larger plane surface, taking into account the laser coupling efficiency. Computer solutions are compared with existing analytical models for laser-induced surface pressures and impulse, and a summary is given of the pulse time regime of applicability of blast-wave models proposed by previous investigators. Scaling laws are given for optimized pulses that predict the laser parameters required to deliver a given impulse to a target of given area in an atmosphere of given density.

Laser-supported detonation waves and material processing with pulsed CO2 lasers

The processing of metals with short CO2 laser pulses is strongly influenced by an air-breakdown plasma (LSD-wave). Material removal is possible only if the duration τ of this LSD-wave can be kept short compared with the length of the laser pulse. A simple model for the calculation of τ is developed allowing the derivation of an analytic formula and leading to good agreement with experimental data.

Jet Flowfield behind a Laser-Supported Detonation Wave

Jet Flowfield behind a Laser-Supported Detonation Wave