Impulse Coupling Research Articles

Laser ablation of copper and aluminium in air

The ablation behavior of copper alloy and aluminium irradiated in air by 1.06 μm, 10 ns pulsed laser with power density of 6.4×109W/cm2 was studied using scanning electron microscopy (SEM), MCS-RBS and X-ray microanalysis. Evidence of bulk vaporization via bubble formation was observed for the copper alloy under the laser irradiation. Silver-enrichment microregions were found in the ablation crater created by the laser shots on the copper alloy sample. Material removal rates of these materials were determined by crater shape-profile measurement. Using self-similar solutions of the gas-dynamic equations, gas-dynamic parameters of the vaporization waves are obtained. These parameters are used to calculate material removal rates and impulse coupling coefficients of these materials under the pulsed laser irradiation. The calculated mass removal rates and the coupling coefficients are compared with the corresponding experimentally determined values. The surface kinetic energy of the irradiated area on the Al sample is estimated. Possible mechanisms for laser ablation of the materials under study are discussed.

Effects of Atrial Impulse Timing on AV Concealed Conduction in the Rabbit Heart

The influence of the timing of a nontransmitted or transmitted atrial impulse on the atrioventricular (AV) conduction time of the subsequent impulse was studied in nine isolated rabbit hearts. AV conduction curves were determined by applying the atrial extrastimulus test. The extrastimulus was delivered preceded or not by an interposed atrial impulse whose coupling interval with respect to the last atrial beat of a basic train was kept constant at 100, 120, 140, 160, 175, 200, 225, 250, and 300 msec. In all experiments, there was a "concealment interval," i.e., the AV effective refractory period was longer than the atrial functional refractory period, and in seven experiments was comprised between 100 and 160 msec. For any given extrastimulus coupling interval in the presence of an interposed nontransmitted atrial impulse, AV conduction time was significantly greater than in its absence; the increase was greater than the longer the nontransmitted atrial impulse coupling interval, i.e., the shorter the subsequent transmitted impulse coupling interval with respect to the previous interposed nontransmitted impulse. The AV conduction curves relating the extrastimulus AV conduction time to its coupling interval with respect to the last atrial impulse of the basic train fitted to a hyperbolic model both in the absence of the interposed atrial impulse and in its presence (mean square residual 31 +/- 23 msec2), and the interposed atrial impulse modified the constants of the functions; the slope of the linear transformations was progressively more negative as the interposed atrial impulse was delayed. Furthermore, the effects of the interposed atrial impulse--transmitted or not--on AV conduction time of the subsequent impulse were qualitatively similar, their magnitude depending on the time elapsed were qualitatively similar, their magnitude depending on the time elapsed between the two.

Enhanced vacuum laser-impulse coupling by volume absorption at infrared wavelengths

We report measurements of vacuum laser impulse coupling coefficients as large as 90 dyne/W, obtained with single μs-duration CO2 laser pulses incident on a volume-absorbing, cellulose-nitrate-based plastic. This result is the largest coupling coefficient yet reported at any wavelength for a simple, planar target in vacuum, and partly results from expenditure of internal chemical energy in this material. Enhanced coupling was also observed in several other target materials that are chemically passive, but absorb light in depth at 10-μm and 3-μm wavelengths. We discuss the physical distinctions between this important case and that of simple, planar surface absorbers [such as metals] which were studied in the same experimental series, in light of the predictions of a simple theoretical model. Ablation parameters for use with the model were determined in separate experiments near threshold in air and in vacuum. The transition from volume- to surface-absorption behavior in the infrared is described as being controlled by fluence-limiting and wavelength conversion in the uppermost target layer.

Impulse coupling to targets in vacuum by KrF, HF, and CO2 single-pulse lasers

We present a laser-target scaling model which permits approximate prediction of the dependence of ablation pressure, mechanical coupling coefficient, and related parameters in vacuum upon single-pulse laser intensity (I), wavelength (λ), and pulse width (τ) over extremely broad ranges. We show that existing data for vacuum mechanical coupling coefficient for metallic and endothermic nonmetallic, surface-absorbing planar targets follows this empirical trend to within a factor of 2 over 7 orders of magnitude in the product (Iλ(τ)1/2). The comparison we present is valid for intensity equal to or greater than the peak-coupling intensity Imax, where dense plasma formation mediates laser-target coupling. Mechanical coupling coefficients studied ranged over two orders of magnitude. The data supporting this trend represent intensities from 3 MW/cm2 to 70 TW/cm2, pulse widths from 1.5 ms to 500 ps, wavelengths from 10.6 μm to 248 nm, and pulse energies from 100 mJ to 10 kJ. With few exceptions, data approximating one-dimensional or planar expansions were selected. Previously, meaningful scaling of ablation pressure parameters with I, λ, τ was not possible because existing data concentrated in a small range of these parameters. Our own data, obtained in the low- and midrange of (Iλ(τ)1/2), completes the experimental picture. Since this new data was derived from five separate experiments with specialized character and purpose, detailed accounts of this work will appear separately. In this paper, we summarize the experimental conditions and select only those data which are relevant to the scaling issue. We find that laboratory-scale laser experiments can often give impulse coupling data which agree with results from much higher-energy experiments without much error, and at much lower cost. We review a theory of vacuum laser ablation, specialize it to a quantitative description of mechanical coupling, and show that the resulting model provides a simple physical description which comes quite close to the observed empirical trend. This is accomplished with minor elaborations of the theory as originally presented to account for the temperature dependence of plasma ionization states, while adhering to the premise that a simple and generally applicable treatment of laser impulse production should be available. The theoretical model can quantitatively predict vacuum ablation pressure for opaque targets without adjustable parameters to the factor-of-2 accuracy in which we are interested. Other published scaling models omit one or more of the important variables, lack broad applicability, or deviate more noticeably from the observed trend.

Impulse coupling to aluminum resulting from Nd:glass laser irradiation induced material removal

The properties of the impulse coupling coefficient for long-pulse Nd:glass laser irradiation to aluminum are reported. Most significant is an anticorrelation between this coefficient and the mass of the material removed by the irradiation. A model is suggested which shows that this property may be related to the convergence or divergence of the laser beam as it penetrates the material. When using long-pulse irradiation, the impulse coupling coefficient does not depend on the initial conditions of the surface finish. Data are presented which also demonstrate the dependence of the impulse coupling coefficient on both the incident intensity and energy density.

Ground Control Considerations for Remotely Piloted Spacecraft

To augment the capabilities of the space transportation system, NASA has funded studies aimed at developing a reusable, remotely piloted spacecraft capable of delivering and retrieving payloads at altitudes and inclinations beyond the reach of the present Shuttle Orbiters. Development of such a remotely piloted spacecraft involves five major categories of human factors design issues related to controllability: 1) mission conditions including thruster plume impingements and signal time delays; 2) vehicle performance variables including control authority, control harmony, minimum impulse, and cross coupling of accelerations; 3) maneuvering task requirements such as target distance and dynamics; 4) control parameters including various control modes and rate/displacement deadbands; and 5) display parameters involving camera placement and function, visual aids, and presentation of operational feedback from the spacecraft. To resolve these human factors design issues, the Martin Marietta Company is presently engaged in a program of research based on high fidelity simulations of remotely piloted spacecraft operations. Preliminary results of these studies will be presented in an interactive session at the 1984 Human Factors Society Annual Convention.

Transient Vaporization from a Surface into Vacuum

This paper is motivated by a general interest in pulsed energy addition to a surface in vacuum at high power fluxes (> 1 MW/cm absorbed). The theoretical treatment assumes a perfect gas without plasma formation, and examples are for aluminum. Transient heat conduction into the surface, vapor flux through a Knudsen layer at the surface, and transient one-dimensional flow of vapor into the surrounding vacuum are included as part of the modeling. Results show that the Knudsen layer remains choked so long as the absorbed power is constant or increasing, and it is unchoked when the absorbed power is decreasing. The problem is fully coupled in the latter case. Quasisteady vapor efflux from the surface is established within a fraction of the pulse time at sufficiently high power fluxes. Impulse coupling coefficients due to vapor recoil are noted.

Coupling of pulsed 0.35-μm laser radiation to aluminum alloys

Experiments and theoretical analysis have been performed to investigate the thermal and mechanical coupling of pulsed 0.35-μm laser radiation (τp≊10−6 s) to aluminum targets irradiated in vacuum. Thermal coupling coefficients measured for Al 2024 (as-received and polished) and pure aluminum, and impulse coupling data for polished Al 2024 are presented. We find low fluence (≲5.0 J/cm2) thermal coupling coefficients of 0.4 for as-received Al 2024, 0.2 for polished or laser-cleaned Al 2024, and 0.1 for pure aluminum. Neglecting initial surface layer effects, impulse coupling is shown to result from bulk target vaporization for pulse fluences greater than 10 J/cm2. Measurements are also presented for the plasma formation threshold above aluminum targets irradiated in vacuum at 0.35 μm. An intensity threshold of 6×107 W/cm2 is indicated for a pulse duration of 0.5 μs. The impulse coupling and plasma ignition threshold data are shown to be in good agreement with theoretical model predictions.

Interaction of a pulsed XeF laser with an aluminum surface

The interaction of XeF laser radiation at 353 nm with an aluminum target in air at reduced (0.1 Torr) and ambient pressures has been studied. The thermal coupling at moderate fluxes (106–108 W/cm2) and long pulse length at 0.1 Torr was found to be ∼20%. Impulse coupling was also determined.

Thermal and impulse coupling to an aluminum surface by a pulsed KrF laser

The amount of heat deposited on an aluminum target after irradiation in air by a KrF laser at 249 nm has been measured to be ∼40% of the incident pulse energy for long pulses (0.5 μsec), moderate flux levels (106–108 W/cm2), and spot diameter ⩾0.5 cm. The impulse generated by the laser has also been measured. Significant impulse levels were observed at low incident fluence levels.

Effect of vapor plasma on the coupling of laser radiation with aluminum targets

The effect of vapor plasma on thermal and impulse coupling of laser radiation with aluminum targets is studied to understand and explain experimental data showing anomalously high coupling to 10.6 μm laser radiation. Heating of vapor by inverse bremsstrahlung absorption of laser radiation, subsequent reradiation in the u.v. and deep u.v. by ionized species, and vapor layer growth are modeled. A computer code has been developed to solve the governing equations. Major conclusions include the following: (a) vapor plasma radiative transport can be an important mechanism for laser/target coupling, (b) aluminum vapor (density × thickness) ≈ 10 17 cm -2 (corresponding to about 0.01 μm of target material) can result in thermal coupling coefficients of 20% or more, and (c) too much vapor reduces the net flux at the target.

Impulsive loading of targets by HF laser pulses

Measurements of impulse coupling coefficients for 180-nsec HF (2.7 μm) laser pulses on aluminum targets are reported. The targets were irradiated both at atmospheric and reduced pressure. Coupling coefficients of up to 13 dyn sec/J were observed. These short-pulse 2.7-μm measurements are compared with 1.06-μm short-pulse measurements reported in the literature.

Impulse reaction resulting from the in-air irradiation of aluminum by a pulsed CO2 laser

The impulse reaction resulting from the in-air irradiation of aluminum by a high-intensity pulsed CO2 laser is experimentally examined in this study. Power densities of the order of 107−108 W/cm2 are obtained for focused beam energies between 100 and 700 J and pulse durations of between 15 and 50 μ sec. The targets are mounted in a sensitive pendulum in order to measure the total recoil impulse. The time history of the impulse during irradiation is determined by an interferometric technique. In addition, thermocouples are used to monitor the rear surface temperature of the thin (0.05 cm) targets. The thermal and mechanical response of the target is dominated by the effects of a high-intensity leading edge of the laser pulse. The precursor spike generates an opaque plasma at the target surface which effectively shields the target from the incident energy. Impulse and pressure transmitted to the target are related, therefore, to reaction of the expanding plasma against the target surface. Impulse coupling coefficients (I/E) between 5 and 7.8 dyn sec/J are measured for this mechanism of impulse production. The major contribution to the total impulse is associated with the plasma pressure during the initial stages of the plasma-surface interaction. The peak pressures generated during this time are found to depend on the second power of the initial beam intensity. It follows that the corresponding impulse coupling coefficients should increase linearly with beam intensity. However, a net increase in I/E is not realized at higher beam energies because of absorption losses (air breakdown) along the beam path.