摘要:

Production of a laser‐produced plasma plume off a metallic target is accompanied by a strong shock wave with shock Mach number in excess of 100. Time‐resolved spectroscopy and streak photography show that the shock front evolves from a cylindrical to a complex profile. Along the beam axis the shock speed remains constant initially and then sharply increases due to a movement of the shock epicenter. Radially but close to the surface, the shock speed decays as r−0.5, where r is the distance from the laser beam axis. The observed behavior is consistent with the notion that the plasma plume grows toward the laser beam and becomes supercritical shortly after the peak of the laser pulse.

ISSN:0094-243X    

DOI:10.1063/1.39424

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A laser thermal rocket uses the energy of a large remote laser, possibly ground‐based, to heat an inert propellant and generate thrust. Use of a pulsed laser allows the design of extremely simple thrusters with very high performance compared to chemical rockets. The temperatures, pressures, and fluxes involved in such thrusters (104K, 102atmospheres, 107w/cm2) typically result in the creation of laser‐supported detonation (LSD) waves. The thrust cycle thus involves a complex set of transient shock phenomena, including laser‐surface interactions in the ignition of the LSD wave, laser‐plasma interactions in the LSD wave itself, and high‐temperature nonequilibrium chemistry behind the LSD wave. The SDIO Laser Propulsion Program is investigating these phenomena as part of an overall effort to develop the technology for a low‐cost Earth‐to‐orbit laser launch system. We will summarize the Program’s approach to developing a high performance thruster, the double‐pulse planar thruster, and present an overview of some results obtained to date, along with a discussion of the many research question still outstanding in this area.

ISSN:0094-243X    

DOI:10.1063/1.39425

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Maintenance of a Chapman Jouguet laser supported detonation (LSD) wave requires complete absorption of the laser beam in an absorption front travelling supersonically with respect to the upstream gas. A requirement for LSD maintenance is that this distance be less than the radius of the beam. A computational scheme was developed to study the structure of LSD waves in various gases. A simultaneous solution of the mass and momentum conservation equations yields a (Maxwell) line when p is plotted versus 1/&rgr;, the slope of which is related to the velocity of the wave. The physical distance x in the wave corresponding to each point on the Maxwell line is determined by calculating the inverse bremsstrahlung absorption coefficient k as a function of the p and &rgr; and then inverting the equation Iabs(p, &rgr;)=Io(1–exp–∫xk dx’) for x. The method is used to calculate wave thicknesses for a variety of gases (H2O, LiH) that are being considered as propellant materials for laser propulsion. Results are presented for a laser wavelength of 10.6 &mgr;m and upstream gas densities in the 10−3to 10−4g/cc range. The effect of adding a low ionization potential seed to the propellant is explicitly calculated.

ISSN:0094-243X    

DOI:10.1063/1.39426

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Recent work in ground‐to‐orbit laser propulsion has focused on detonation wave thrusters driven by pulsed infrared lasers. In this concept, a pulsed laser is used to vaporize a small amount of solid propellant; a second laser pulse then drives a detonation wave through this vapor, substantially raising its temperature. Thermal energy is converted to thrust by the expansion of this hot vapor. Several aspects of the physics of these thrusters have recently been explored theoretically and with the aid of numerical models. Many of the theoretical predictions can be tested in the laboratory. We report here some results from on‐going experiments probing the physics of detonation wave thrusters. Included are spatially and temporally resolved measurements of radiance and opacity of absorption waves. We also present data on target mass loss and momentum transfer, and discuss the importance of the measured quantities in the design of a useful thruster.

ISSN:0094-243X    

DOI:10.1063/1.39427

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A pulsed high flux source of nearly monoenergetic atomic oxygen has been developed at Physical Sciences Inc. (PSI) to perform accelerated erosion testing of spacecraft materials in a simulated low earth orbit (LEO) environment. Molecular oxygen is introduced into an evacuated conical expansion nozzle at several atmospheres pressure through a pulsed molecular beam valve. A laser induced breakdown is generated in the nozzle throat by a pulsed CO2TEA laser focused to intensities ≳109W/cm2. The resulting plasma is heated in excess of 20,000 K by the ensuing laser supported detonation wave, and then rapidly expands and cools. The nozzle geometry confines the expansion to promote rapid electron‐ion recombination into atomic oxygen. The source generates an atomic oxygen beam with fluxes ≳1018atoms per pulse at 8±1.6 km/s with an ion content below 1% for LEO testing. For other applications the beam velocity can be varied over a range from 5 to 13 km/s by changing the discharge conditions. Materials testing has obtained the same surface oxygen enrichment in polyethylene samples as observed on the STS‐8 mission, and scanning electron micrographs of the irradiated polymer surfaces reveal an erosion morphology similar to that obtained on low earth orbit.

ISSN:0094-243X    

DOI:10.1063/1.39428

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

This paper describes the experimental and analytical investigation on the dynamics of cavitation vapor bubbles in cryogenic liquids. The cavitation vapor bubbles were generated by the irradiation of high intensity Ruby laser beam in a cryostat. The dynamics of the laser induced vapor bubbles in liquid nitrogen under the atmospheric pressure were studied by means of high speed frame camera. Furthermore a simple analytical approach based on the liquid compressibility and the polytropic change in the gas was conducted. The obtained results suggest that the vapor bubble dynamics for cryogenic cavitation are of great importance in the fields of future technology, as the water cavitation dynamics play in the present fluid mechanics. The technique of laser induced bubble experiments has proved to be of great use for such cryogenic cavitation researches.

ISSN:0094-243X    

DOI:10.1063/1.39513

出版商:AIP    

年代:1990

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.39429

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Measurements are reported of the electron populations behind high enthalpy shock waves in low density air. These results are compared with chemical kinetic models. The comparison shows that thermal nonequilibrium effects play a significant role in ionization at these conditions.

ISSN:0094-243X    

DOI:10.1063/1.39430

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Chemical reactions in shock waves can be strongly affected by minute impurity concentrations. Thus it is not adequate to take into account the additional impurity electron production in relaxation studies simply by global adjustment of the atom‐atom excitation cross section constant to the measured electron density.A definite improvement, however, can only be achieved if the ionization relaxation model is extended to comprise all relevant impurity atom reactions. Consequently we treated the real test gas as a mixture of Krypton and impurity Carbon atoms. A comprison with experimental electron density profiles yielded 3.0*10−6m2/J for the Kr‐Kr excitation cross section constant as well as values for the C‐Kr constants.For a temperature of about 8000 K and an impurity concentration of about 40 ppm it is shown that the impurity reactions dominate the electron production in the initial relaxation zone. This effect causes a pronounced decreasing of the relaxation time with increasing concentration.By comparing computational results of the Kr‐C model with those of the simplistic pure Kr model it is possible to explain the dependence of the Kr‐Kr excitation cross section constant on the impurity concentration and the plasma temperature.

ISSN:0094-243X    

DOI:10.1063/1.39431

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Finite‐difference simulations are carried out for bifurcated reflected‐shock waves in ionizing argon in a shock tube by solving the thin‐layer Navier‐Stokes equations with ionization reactions. Results are shown for reflection process of an ionizing shock moving at the Mach number of 12 in argon at 400 Pa and 298 K. Computed flow‐fields are compared with snap‐shot shadowgraphs for reflected‐shock waves in ionizing argon. The simulations and the experiments are in good qualitative agreement.

ISSN:0094-243X    

DOI:10.1063/1.39432

出版商:AIP    

年代:1990

数据来源: AIP