Laser propulsion, a unique new system in which thrust is produced by the absorption of remote laser energy, has several distinct advantages over existing forms of propulsion. This article is intended as an overview of the status of laser propulsion based on a review of the scientific literature. Included in the discussion is an evaluation of the advantages and limitations of laser propulsion, as well as the types of missions for which it can best be used. The various types of proposed thrusters are also discussed, focusing on operation, current research, and unresolved problems. Also included is a thorough list of references documenting the research in this relatively young field.

Concepts and status of laser-supported rocket propulsion

The optical emission spectra (180–760 nm) of plasmas produced by a flashlamp‐pumped dye laser focused on an aluminum target have been recorded and analyzed. In the incident intensity range from near plasma threshold to 5×107 W/cm2 the electron temperature was calculated from the relative emission intensity of Al(II) states (Te=8.0×103 K). The electron density was determined from Stark broadened linewidths of four Al(II) lines. Both the spatial and temporal dependence of the emission spectra were obtained providing a map of the electron density and temperature.

The UV‐visible spectroscopy of laser‐produced aluminum plasmas

The infrared photochemistry of silane has been investigated using CO2 TEA laser excitation. Energy deposition measurements show that only 1.5 photons/molecule are absorbed at a pressure of 1 Torr and a P (20) fluence of 0.8 J/cm2, while at 10 Torr 7.5 photons/molecule are absorbed. The visible luminescence that accompanies the dissociation of silane is due to molecular and atomic hydrogen. The thresholds for both luminescence and dissociation increase sharply as the pressure drops below 5 Torr, indicating that dissociation is a collisional process. Silane d1 and d2 have been produced by irradiating silane–deuterium mixes.

Infrared laser photochemistry of silane

We report results of an investigation into the roles played by thermal and radiative transport in the propagation of Laser Supported Combustion (LSC) waves We replace the volume radiation losses assumed by Raizer with detailed treatment of radiation in 19 frequency groups, the flux of each being calculated from the temperature in discrete cells along the LSC wave axis and allowed to deposit in other cells according to temperature dependent absorption coefficients for air Calculations of LSC wave structure and propagation velocity as a function of laser intensity are compatible with available experimental data a = c = CP = f = h = h = hi(a) = K = k = KV =

Role of Radiative Transport in the Propagation of Laser Supported Combustion Waves

An experiment was performed to determine the temperature distribution within a stationary plasma in atmospheric air produced by a continuously operating CO2 laser A peak temperature of 17000 °K (±5%) was found to exist at a point 11 cm ahead of the focal point The radiation surrounding the hot plasma core was found to consist primarily of the first negative system of nitrogen

Experimental study of a stationary laser−sustained air plasma

An extension to two dimensions of Raizer's analysis for the steady state, subsonic propagation of laser sparks is presented. A solution in closed form is obtained for a simplified, two dimensional energy equation, yielding both axial and radial temperature profiles as well as the relationship between incident laser intensity, laser radius and propagation velocity. The propagation mechanism is assumed to be thermal conduction and radiation losses are included in a phenomenological manner. The solution is obtained for a laser spark propagating in a channel and therefore can be used to investigate the effect of a boundary on spark propagation. The solution also allows calculation of the radial heat conduction parameter A introduced by Raizer for use in one dimensional approximations. Sample calculations are presented for air sparks of .15 and .50 cm radius.

Two Dimensional Generalization of Raizer's Analysis for the Subsonic Propagation of Laser Sparks

Threshold intensity for laser induced isotope separation in SF6

An energy balance analysis has been performed for a confined cylindrical arc column heated inductively by high-frequency fields. The analysis includes the convective energy transport due to a radial inflow in addition to the radiative and conductive transport considered in the Ellenbaas-Heller equation. The resulting non-linear, two-point boundary value equations were solved numerically. Calculations were performed for an argon arc operating at atmospheric pressure using experimentally determined transport properties. Temperature profiles were found to be in better agreement with measured values, when radial inflow was included. There was no indication that the arc attained some natural diameter based on a specific plasma radius to skin depth ratio.

The Electrodeless Arc with Radial Inflow

The plasma generated when a 2024 aluminum target was irradiated with the beam from a cold cathode electron beam laser (CCEBL) has been investigated with spectroscopic techniques. The target was designed to form the end plate of a differentially pumped slit assembly attached to a McPherson Model 225 visible-u.v.-v.u.v. monochromator. A pinhole in the center of the plate permitted plasma radiation to enter the monochromator from the back side of the target. Calibration with tungsten, deuterium and argon standards permitted quantitative plasma radiance measurements to be made in the region 115–600 nm using a range of diffraction gratings and photomultiplier tubes. The total radiant fluence observed (1 J/cm 2 ) was in reasonable agreement with the 1-1.2J/cm 2 of absorbed thermal fluence determined by independent thermal coupling measurements. Photographic spectra of the plasma (100–620 nm) were recorded and many features identified.

Dennis R. Keefer

Papers

Two Dimensional Generalization of Raizer's Analysis for the Subsonic Propagation of Laser Sparks

Threshold intensity for laser induced isotope separation in SF6

The Electrodeless Arc with Radial Inflow

The role of radiative transfer in pulsed laser plasma-target interactions

The role of radiative transfer in pulsed laser plasma-target interactions