FALCON is a high-power, steady-state, nuclear reactor-pumped laser (RPL) concept that is being developed by the Department of Energy. The FALCON program has experimentally demonstrated reactor-pumped lasing in various mixtures of xenon, argon, neon, and helium at wavelengths of 585, 703, 725, 1271, 1733, 1792, 2032, 2630, 2650, and 3370 nm with intrinsic efficiency as high as 2.5%. The major strengths of a reactor-pumped laser are continuous high-power operation, modular construction, self-contained power, compact size, and a variety of wavelengths (from visible to infrared). These characteristics suggest numerous applications not easily accessible to other laser types. A ground-based RPL could beam its power to space for such activities as illuminating geosynchronous communication satellites in the earth's shadow to extend their lives, beaming power to orbital transfer vehicles, removing space debris, and providing power (from earth) to a lunar base during the long lunar night. The compact size and self-contained power also makes an RPL very suitable for ship basing so that power-beaming activities could be situated around the globe. The continuous high power of an RPL opens many potential manufacturing applications such as deep-penetration welding and cutting of thick structures, wide-area hardening of metal surfaces by heat treatment or cladding application, wide-area vapor deposition of ceramics onto metal surfaces, production of sub-micron sized particles for manufacturing of ceramics, wide-area deposition of diamond- like coatings, and 3-D ceramic lithography.

Reactor pumped lasers have the potential to be scaled to multi-megawatt power levels with long run times. In proposed designs, the laser will be capable of output powers of several megawatts of power for run times of several hours. Such a laser would have many diverse applications such as material processing, space debris removal and power beaming to geosynchronous satellites or the moon. However, before such systems can be designed, fundamental laser parameters such as small signal gain, saturation intensity and efficiency must be determined over a wide operational parameter space. We have recently measured fundamental laser parameters for a selection of nuclear pumped visible and near IR laser transitions in atomic neon, argon and xenon. An overview of the results of this investigation are presented.

The key component technologies required for a high average power free-electron laser (FEL) are described. Some basic aspects of approaches for high average power (scalable to megawatt level) accelerators and FELs are presented. A short description of the Novosibirsk 100 kW average power near infrared (IR) FEL driven by a race-track microtron-recuperator is given. The current status and plans for this facility are provided by Institute of Nuclear Physics (Novosibirsk).

Beaming laser energy to spacecraft has important economic potential. It promises significant reduction in the cost of access to space, for commercial and government missions. While the potential payoff is attractive, existing technologies perform the same missions and the keys to market penetration for power beaming are a competitive cost and a schedule consistent with customers' plans. Rocketdyne is considering these questions in the context of a commercial enterprise -- thus, evaluation of the requirements must be done based on market assessments and recognition that significant private funding will be involved. It is in the context of top level business considerations that the technology requirements are being assessed and the program being designed. These considerations result in the essential elements of the development program. Since the free electron laser is regarded as the `long pole in the tent,' this paper summarizes Rocketdyne's approach for a timely, cost-effective program to demonstrate an FEL capable of supporting an initial operating capability.

The power requirements for a satellite power beaming laser system depend upon the diameter of the beam director, the performance of the adaptive optics system, and the mission requirements. For an 8 meter beam director and overall Strehl ratio of 50%, a 30 kW laser at 850 nm can deliver an equivalent solar flux to a satellite at geostationary orbit. Advances in diode pumped solid state lasers (DPSSL) have brought these small, efficient, and reliable devices to high average power and they should be considered for satellite power beaming applications. Two solid state systems are described: a diode pumped Alexandrite and diode pumped Thulium doped YAG. Both can deliver high average power at 850 nm in a single aperture.

Approaches are discussed for a direct solar-pumped semiconductor laser. Efficiencies of 35% should be achievable, an order of magnitude better than the efficiency achieved by other solar- pumped lasers. The output wavelength of a semiconductor laser will be well matched to the optimum conversion efficiency of a solar cell of the same material. Solar pumped semiconductor lasers are thus an excellent candidate for space-based energy transmission. Recently several designs for such lasers have been proposed. A critical parameter is the sunlight intensity required for lasing. This threshold has been calculated to be in the range of 2500 to 10,000 times solar intensity for conventional stripe laser designs, depending on assumptions. Several approaches have been recently proposed to reduce this threshold. The calculated minimum threshold is 25 - 50 times solar concentration, and could possibly be reduced even further with use of light-trapping.

This paper discusses the laser beam control system requirements for power beaming applications. Power beaming applications include electric and thermal engine propulsion for orbit transfer, station changing, and recharging batteries. Beam control includes satellite acquisition, high accuracy tracking, higher order atmospheric compensation using adaptive optics, and precision point-ahead. Beam control may also include local laser beam clean-up with a low order adaptive optics system. This paper also presents results of tracking and higher-order correction experiments on astronomical objects. The results were obtained with a laser beacon adaptive optics system at Phillips Laboratory's Starfire Optical Range near Albuquerque, NM. At a wavelength of 0.85 micrometers , we have achieved Strehl ratios of approximately 0.50 using laser beacons and approximately 0.65 using natural stars for exposures longer than one minute on objects of approximately 8^th magnitude. The resulting point spread function has a full width half maximum (FWHM) of 0.13 arcsec.

The need for a number of interferometric sensor systems during fabrication, assembly and operation of large segmented array mirrors, is discussed. Specifically, an interferometric approach is described for calibrating edge sensors and actuators, and aligning the segments during mirror assembly.

An analytical study of laser diode (LD) operation coupled to external cavity scattering elements, which function as variably coupling reflectors (VCRs), is carried out with the purpose of determining the interrelationship between cavity coupling and intracavity optical intensity which determine the current generated at the rear facet PIN detector. If the external cavity coupling is position sensitive it can allow the relative position between the LD and the external cavity to be determined from the PIN or other detector mounted with the LD. If the LD and external cavity element are placed on opposite edges of two adjacent adaptive optics segments they can provide the basis for a self aligning position sensor; the amount of current detected at the PIN or other detector will depend on the relative displacement between the LD and external coupling element. Schematics of the edge sensors, the basic electronic configuration, and the optics of the external cavity are given. The ratio of the internal cavity intensity, I_c, to the saturation intensity, I_s, is plotted as a function of the external cavity coupling. When this ratio approaches one, large-signal output is not a linear function of large-signal output. For operation well below saturation, the PIN detector current is directly related to I_c and may serve as a reliable detector.

This paper discusses the design issues and fabrication considerations specifically related to a large twelve meter, graphite-epoxy space truss that has been developed to provide support of the primary mirror system for the Space Laser Energy (SELENE) Beam Transmission Optical System (BTOS). Details of the optical system and wavefront corrector concepts have been discussed in prior papers. Specific issues which are addressed in this paper include optical performance needs, environmental requirements, and low-cost fabrication techniques.

Massively parallel algorithms and architectures for real-time wavefront control of a dense adaptive optic system (SELENE) are presented. We have already shown that the computation of a near optimal control algorithm for SELENE can be reduced to the solution of a discrete Poisson equation on a regular domain. Although this represents an optimal computation, due to the large size of the system and the high sampling rate requirement, the implementation of this control algorithm poses a computationally challenging problem since it demands a sustained computational throughput of the order of 10 GFlops. We develop a novel algorithm, designated as Fast Invariant Imbedding algorithm, which offers a massive degree of parallelism with simple communication and synchronization requirements. We also discuss two massively parallel, algorithmically specialized, architectures for low-cost and optimal implementation of the Fast Invariant Imbedding algorithm.

This paper presents the development and analysis of a wavefront control strategy for the Space Laser Electric Energy (SELENE) power beaming system. SELENE represents a substantial departure from most conventional adaptive optics systems in that the deformable element is the segmented primary mirror and the signal that is fedback includes both the local wavefront tilt and the relative edge mismatch between adjacent segments. The major challenge in designing the wavefront control system is the large number of subapertures that must be commanded. A fast and near optimal algorithm based on the local slope and edge measurements is defined for this system.

ECTIVE The objective of this project is to produce a proof-of-concept demonstration of an edge sen sor for the Space Laser Energy (SELENE) program. The operational specifications are: im full range in the z 10 nm resolution in z 100 jim 5p.mgap width between segments sign information 30 kHz bandwidth where z is the direction normal to the reflective surface of the mirror segment. Fabrication cost constraints also require that the edge sensor is affordable, easy to align, and integrable with seg ment materials. The small dimension of each hexagonal segment (3 cm from flat to flat, 1 cm thick) also requires that the sensor is small.

Accurate simulation of a ground-based adaptive optics system requires a realistic model of atmospheric turbulence. Conventional models of atmospheric phase distortions involve two- dimensional Fourier transforms, and are thus complex and slow. In the Kolmogorov-Taylor `frozen' approximation, there is an alternative way to generate a phase screen directly that is a close approximation to the theoretical phase structure. We approximate the exact atmospheric transfer function in a stair-step fashion using only integral powers of spatial frequency. Then conventional digital filter synthesis techniques are used to produce the phase screen directly. Examples are shown for several different atmospheric power spectral density functions, and their statistics discussed.

The PAMELA segmented optical surface concept uses the cellular automata paradigm to build up an active surface of individually controlled elements that maintain edge-match by relying on electronically sensed nearest neighbor edge-to-edge errors. The segments are controlled in tilt directly from a wavefront sensor (e.g., of the Hartmann-Schack type) in a separate parallel loop. The approach obviates the matrix operations needed in a typical multiple-input, multiple- out (MIMO) servo control system. In this manner, the segmented optical system is extensible to arbitrary aperture diameter by gradually building up the active surface using identical elements. This paper addresses methods to improve the real-time adaptive control of such a surface using hierarchical control architectures.

Some applications for high energy beam directors will require near diffraction limited performance at near visible wavelengths even in the presence of severe environmental conditions and operational requirements. Telescopes based on the PAMELA architecture appear to be the only viable candidate for such applications which necessitate wave front correction on a spatial scale of a few centimeters over an aperture of 10 to 12 meters. Past work under the sponsorship of DARPA and the SDIO has demonstrated the feasibility of constructing such telescopes based on light weight silicon carbide segments which are 7 cm flat to flat. More recent work has been directed at assessing the feasibility of producing 3 cm segments using single crystal silicon micromachining technology. This paper describes the latest activities and results of this work.

Laser power beaming of energy through the atmosphere to a satellite can extend its lifetime by maintaining the satellite batteries in operating condition. An alternate propulsion system utilizing power beaming will also significantly reduce the initial insertion cost of these satellites, which now are as high as $72,000/lb for geosynchronous orbit. Elements of the power beaming system are a high-power laser, a large diameter telescope to reduce diffractive losses, an adaptive optic beam conditioning system and possibly a balloon or aerostat carrying a large mirror to redirect the laser beam to low earth orbit satellites after it has traversed most of the earth's atmosphere vertically. China Lake, California has excellent seeing, averages 260 cloud-free days/year, has the second largest geothermal plant in the United States nearby for power, groundwater from the lake for cooling water, and is at the center of one of the largest restricted airspaces in the United States. It is an ideal site for such a laser power beaming system. Technological challenges in building such a system and installing it at China Lake are discussed.

The Nevada Test Site (NTS) is one excellent possibility for a laser power beaming site. It is in the low latitudes of the U.S., is in an exceptionally cloud-free area of the southwest, is already an area of restricted access (which enhances safety considerations), and possesses a highly skilled technical team with extensive engineering and research capabilities from underground testing of our nation's nuclear deterrence. The average availability of cloud-free clear line of site to a given point in space is about 84%. With a beaming angle of +/- 60 degree(s) from the zenith, about 52 geostationary-orbit (GEO) satellites could be accessed continuously from NTS. In addition, the site would provide an average view factor of about 10% for orbital transfer from low earth orbit to GEO. One of the major candidates for a long-duration, high- power laser is a reactor-pumped laser being developed by DOE. The extensive nuclear expertise at NTS makes this site a prime candidate for utilizing the capabilities of a rector pumped laser for power beaming. The site then could be used for many dual-use roles such as industrial material processing research, defense testing, and removing space debris.

Geosynchronous satellites use solar arrays as their primary source of electrical power. During earth eclipse, which occurs 90 times each year, the satellites are powered by batteries, but the heavy charge-discharge cycle decreases their life expectancy. By beaming laser power to satellites during the eclipses, satellite life expectancy can be significantly increased. In this paper, we investigate the basic system parameters and trade-offs of using reactor pumped laser technology to beam power from the Nevada Test Site. A first order argument is used to develop a consistent set of requirements for such a system.

A team of Phillips Laboratory, COMSAT Laboratories, and Sandia National Laboratories plans to demonstrate state-of-the-art laser-beaming demonstrations to high-orbit satellites. The demonstrations will utilize the 1.5-m diameter telescope with adaptive optics at the AFPL Starfire Optical Range (SOR) and a ruby laser provided by the Air Force and Sandia (1 - 50 kW and 6 ms at 694.3 nm). The first targets will be corner-cube retro-reflectors left on the moon by the Apollo 11, 14, and 15 landings. We attempt to use adaptive optics for atmospheric compensation to demonstrate accurate and reliable beam projection with a series of shots over a span of time and shot angle. We utilize the return signal from the retro- reflectors to help determine the beam diameter on the moon and the variations in pointing accuracy caused by atmospheric tilt. This is especially challenging because the retro-reflectors need to be in the lunar shadow to allow detection over background light. If the results from this experiment are encouraging, we will at a later date direct the beam at a COMSAT satellite in geosynchronous orbit as it goes into the shadow of the earth. We utilize an onboard monitor to measure the current generated in the solar panels on the satellite while the beam is present. A threshold irradiance of about 4 W/m² on orbit is needed for this demonstration.

A mission to recharge batteries of satellites in geostationary orbits (geosats) may be a commercially viable application which could be achieved with laser systems somewhat larger than present state-of-the-art. The lifetime of batteries on geosats is limited by repetitive discharge cycles which occur when the satellites are eclipsed by the earth during the spring and fall equinoxes. By coupling high power lasers with modern, large aperture telescopes and laser guide star adaptive optics systems, present day communications satellites could be targeted. It is important that a near term demonstration of laser power beaming be accomplished using lasers in the kilowatt range so that issues associated with high average power be addressed. The Laser Guide Star Facility at LLNL has all the necessary subsystems needed for such a near term demonstration, including high power lasers for both the power beam and guide star, beam directors and satellite tracking system.

W.J. Schafer Associates, Inc. has produced an interactive end-to-end model of a laser power beaming system designed to deliver electrical power from a ground based free electron laser (FEL) to a satellite. The model includes a description of pertinent FEL physics, realistic atmospheric propagation effects, photovoltaic interactions for various semiconductor materials, and satellite onboard power conditioning. A detailed orbital model with graphical output is available, which visualizes the effect of electric propulsion for orbital reboost or orbit transfer. This flexible tool has been applied to specific examples of satellite battery charging for geostationary communication satellites, as well as parametric studies of photovoltaic cell performance. Preliminary results of system wavelength/power/aperture diameter trades are presented.

A new method for providing power to space consists of using high-power cw lasers on the ground to beam power to photovoltaic receivers in space. Such large lasers could be located at cloud-free sites at one or more ground locations, and use large mirrors with adaptive optical correction to reduce the beam spread due to diffraction or atmospheric turbulence. This can result in lower requirements for battery storage, due to continuous illumination of arrays even during periods of shadow by the Earth, and higher power output, due to the higher efficiency of photovoltaic arrays under laser illumination compared to solar and the ability to achieve higher intensities of illumination. Applications include providing power for satellites during eclipse, providing power to resurrect satellites which are failing due to solar array degradation, powering orbital transfer vehicles or lunar transfer shuttles, and providing night power to a solar array on the moon.

It is proposed that a ground-based laser can beam power to commercial communication satellites in geosynchronous orbit and reduce battery depth-of-discharge during eclipses. Two laser system designs are presented which have the capability of reducing battery discharge by 100%. Both utilize a steerable beam director, with a mirror diameter of 4 meters in one case and 8 meters in the other. Both also use an adaptive optics unit within the beam train to provide real-time corrections for wavefront distortions caused by atmospheric turbulence. The required system power output is in the range of 100 to 200 kW for a transmitted wavelength just under 900 nm. Laser power beaming can nearly double the remaining lifetime of a satellite that uses NiCd batteries. However, by the time such lasers become available, nearly all NiCd satellites will be replaced by NiH₂ satellites, which stand to benefit much less from power beaming.

Solar cells may be used as receivers for laser power beaming. To understand the behavior of solar cells when illuminated by a pulsed laser, the time response of GaAs and Si solar cells to pulsed monochromatic input has been modeled using a finite element solar cell model.

Recent advances in the development of reactor-pumped lasers (RPLs) have stimulated renewed interest in the concept of laser-powered propulsion. This paper surveys a number of laser propulsion concepts and identifies the one that is most promising from the standpoint of practicality. It is proposed that a ground-based FALCON (fission-activated laser concept) RPL can provide primary power for this launch vehicle design. The laser-vehicle system could launch small payloads into low-earth orbit (LEO) with high repetition rates and at low costs per kilogram. For the favored design, thruster efficiencies are currently estimated to be about 50%, with 80% being seen as a potentially reliable goal after further design refinements. Laser launch system simulations indicate that, with a buy-in laser power of 10 MW, it will be possible to obtain specific impulses in the range of 600 to 800 seconds and payload-to-power ratios of 1 to 3 kg/MW.

The feasibility and practicality of using a ground-based laser (GBL) to remove artificial space debris is examined. Physical constraints indicate that a reactor-pumped laser (RPL) may be best suited for this mission, because of its capabilities for multimegawatt output, long run- times, and near-diffraction-limited initial beams. Simulations of a laser-powered debris removal system indicate that a 5-MW RPL with a 10-meter-diameter beam director and adaptive optics capabilities can deorbit 1-kg debris from space station altitudes. Larger debris can be deorbited or transferred to safer orbits after multiple laser engagements. A ground- based laser system may be the only realistic way to access and remove some 10,000 separate objects, having velocities in the neighborhood of 7 km/sec, and being spatially distributed over some 10¹⁰ km³ of space.

W.J. Schafer Associates has proposed an architecture for a laser system capable of not only beaming power from a ground site to space, but also capable of intercepting theater missiles during their boost phase for defense of ground troops in regional conflicts. The system comprises a ship-based multi-megawatt laser and beam control system, a relay mirror package mounted on a high altitude, long endurance, unmanned lighter-than-air vehicle, and a sensor package, mounted on the balloon, which directs the laser beam to the target and can also provide an early commitment of ground based kinetic energy interceptors. A system concept is presented, as well as an assessment of system effectiveness.

The material evaporation under laser radiation is followed by the erosion plume formation generating a momentum transfer on the irradiated surface. Such a laser jet engine, formed on small-sized space debris within the laser effect area, allows the alteration of the velocity of debris, transferring them to lower orbits where they burn up completely in the upper atmosphere. This is done with comparatively lower power requirements.

Design considerations and working formulas and graphs are presented for estimating the actuator requirements for adaptive optics correction of global tilt and residual piston error arising from atmospheric turbulence along a ground-to-space path. Frequency characteristics are calculated for several important crosswind conditions for the case where the active segments are very small compared to the full aperture; it is shown that the velocity profile has a strong effect on the power spectra and that high slew rates significantly increase the required high-frequency response and accentuate the effects of high-altitude turbulence. Predictions are given for the SELENE laser power beaming system which uses active control of a segmented primary telescope mirror.