Power beaming to satellites with a ground-based laser can be limited by clouds. Hole-boring through the clouds with a laser has been proposed as a way to overcome this obstacle. This paper reviews the past work on laser hole-boring and concludes that hole-boring for direct beaming to satellites is likely to require 1 - 50 MW. However, it may be possible to use an airborne relay mirror at 10 - 25 km altitude for some applications in order to extend the range of the laser (e.g., for beaming to satellites near the horizon). In these cases, use of the relay mirror also would allow a narrow beam between the laser and the relay, as well as the possibility of reducing the crosswind if the plane matched speed with the cloud temporarily. Under these conditions, the power requirement to bore a hold through most cirrus and cirrostratus clouds might be only 500-kW if the hole is less than 1 m in diameter and if the crosswind speed is less than 10 m/s. Overcoming cirrus and cirrostratus clouds would reduce the downtime due to weather by a factor of 2. However, 500 kW is a large laser, and it may be more effective instead to establish a second power beaming site in a separate weather zone. An assessment of optimum wavelengths for hole boring also was made, and the best options were found to be 3.0 - 3.4 micrometers and above 10 micrometers .

Large scale development of space is severely curtailed by two fundamental problems: (1) the very high cost, low efficiency, and low versatility of space transportation systems, and (2) the high cost and low efficiency of systems for providing large amounts of electric power in space. Indeed, the demise of the Space Exploration Initiative (SEI), NASA's hope for returning to the moon "to stay" and proceeding with the manned exploration of Mars, is directly traceable to extremely high costs derived from "business as usual" in space transportation and power. The $400 Billion price tag and 30 year timeline for the SEI as conceived in 1989 pointed to the need for creative and innovative advanced concepts to find paths that are in accord with NASA Administrator Dan Goldin's "quicker, better, cheaper" mandate ifthere is to be a future for large manned operations in space. This perception led to the creation in 1991 of a small advanced technology effort at NASA headquarters that can now report substantial progress toward totally new ways of achieving major space capabilities. This program is known as SELENE (for SpacE Laser ENErgy). Serendipitously, the same effort has matured a radical approach to the construction of very large telescopes that will revolutionize ground and space based astronomy, surveillance, and optical communications. This advanced optics approach, called PAMELA (for Phased Array Mirror, Extendible Large Aperture) is the result of a state-of-the-art marriage between silicon microtechnology and fast optical fabrication methods. All ofthese efforts are now seeking to coalesce in a proposed joint NASAl U.S. Navy effort known as the National Advanced Optics Mission Initiative (NAOMI). NASA's catalytic role in creating and pursing these efforts, together with a strong and production partnership with the Naval Air Warfare Center Weapons Division laboratory at China Lake, California, has led us to the threshold of major new applied optics capabilities. The present two day SPIE symposium and several previous meetings show that enthusiasm for these new optics opportunities is rapidly expanding. This paper will describe a power beaming system that enables lunar colonization, reduces the cost of orbital transfer, and greatly expands the capabilities of communication satellites for both civilian and defense users. Sections that follow then describe the principal near term supporting technology development effort (PAMELA) and the overarching NAOMI program that bridges these development efforts to the long term giant space telescope and power beaming development activities

Satellites are of vital importance to the Department of Defense and the Navy as well as to the civilian economy. For example, about 90% of the communications to the fleet are by satellite. Economical means for putting satellites into orbit and maintaining and extending their lifetimes in orbit are just as important for the military as for civilian industries. There is also a significant economic impact to the ability to repair rather than replace satellites that are malfunctioning or have been inserted into the wrong orbits. Laser power beaming can not only accomplish these tasks but also promises to move satellites in orbit quickly and inexpensively, provide boost power for degraded satellites or those which suffer intentional jamming from adversaries, remove space junk even in geosynchronous orbit and provide very high resolution pictures of objects in space by eliminating atmospheric disturbances.

Free-electron lasers (FELs) have long been thought to offer the potential of high average power operation. That potential exists because of several unique properties of FELs, such as the removal of `waste heat' at the velocity of light, the `laser medium' (the electron beam) is impervious to damage by very high optical intensities, and the technology of generating very high average power relativistic electron beams. In particular, if one can build a laser with a power extraction efficiency (eta) which is driven by an electron beam of average power P_EB, one expects a laser output power of P_L equals (eta) P_EB. One approach to FEL devices with large values of (eta) (in excess of 10%) is to use a `tapered' (or nonuniform) wiggler. This approach was followed at several laboratories during the FEL development program for the Strategic Defense Initiative (SDI) project. In this paper, we review some concepts and technical requirements for high-power tapered-wiggler FELs driven by radio- frequency linear accelerators (rf-linacs) which were developed during the SDI project. Contributions from three quite different technologies--rf-accelerators, optics, and magnets--are needed to construct and operate an FEL oscillator. The particular requirements on these technologies for a high-power FEL were far beyond the state of the art in those areas when the SDI project started, so significant advances had to be made before a working device could be constructed. Many of those requirements were not clearly understood when the project started, but were developed during the course of the experimental and theoretical research for the project. This information can be useful in planning future high-power FEL projects.

A table of free electron laser (FEL) projects worldwide is described. The status of FEL attributes leading to high-average power operation is reviewed with discussion of future directions. Arguments are given as to why a short undulator containing only a few periods may be advantageous for the high-power FEL.

Power beaming and industrial materials processing are two applications which require high average power lasers operating in the visible or near infrared. Although a handful of gas lasers in the hundred kilowatt range exist, free electron lasers (FELs) should be capable of producing even greater powers, and provide continuous tunability and higher beam quality. While these benefits were realized early in the development of FELs, the highest average power FEL to date has produced just over ten watts. Progress in achieving more average power has been hindered largely by a lack of appropriate accelerators. We believe that superconducting accelerators, which offer continuous operation at high gradients and high efficiency with excellent beam quality are ideal candidates as drivers for such a device. We discuss the challenges of operating both superconducting and room temperature accelerators at high powers and described solutions to these problems. We propose general guidelines along which a superconducting FEL capable of 100 kW to 1 MW could be built and discuss recent experimental demonstrations of these design principles. Finally, we compare the superconducting approach with other possibilities and outline areas requiring future research.

We discuss details of a technical plan that will lead to the development of a 1 MW, continuous wave (CW), efficient, near infrared electrostatic-accelerator FEL for power beaming applications. Among the advantages of our technical approach are: (a) true cw or pulsed operation is possible; (b) better than 20% wall-power efficiency can be achieved, (c) existing electrostatic accelerator technology will be employed, and (d) only low levels of unwanted ionizing radiation will be produced. We propose to achieve our final objective in three technical steps. The first step is currently under implementation at the University of Central Florida. The main thrust of the present program is to show 1 kW, CW, far-infrared FEL operation with a 1.7 MV electrostatic accelerator. The goal of the second step is to reach 60 kW FEL power in the middle-infrared region using the TANDAR (Argentinian Tandem) accelerator (V equals +17 MV). For the third step we propose using a combination of two high voltage tandem accelerators (V equals +25, -25 MV) to achieve the final goal of 1 MW, CW operation in the near-infrared region.

Advanced concepts of electrostatic accelerator free-electron lasers (EA-FELs) are discussed. The capabilities of electrostatic accelerators to produce continuous high energy electron-beams enable construction of efficient FELs, for generation of high average power of radiation at a wide range of the electromagnetic spectrum. Employing RF linacs for charging electrostatic accelerators provides transport of larger e-beam currents, and correspondingly producing even higher power. The unique features of EA-FELs make them excellent sources for applications, which require high-power radiation.

To provide the user facility for Siberian Center of Photochemical Researches in Novosibirsk the high power free electron laser is under construction. The project status and installation are described.

The computer program called Energy Stability in a Recirculating Accelerator (ESRA) Free Electron Laser (FEL) has been written to model bunches of particles in longitudinal phase space transversing a recirculating accelerator and the associated rf changes and aperture current losses. This energy-current loss instability was first seen by Los Alamos's FEL group in their energy recovery experiments. This code addresses these stability issues and determines the transport, noise, feedback and other parameters for which these FEL systems are stable or unstable. Two representative systems are modeled, one for the Novosibirisk high power FEL racetrack microtron for photochemical research, the other is the CEBAF proposed UV FEL system. Both of these systems are stable with prudent choices of parameters.

Even at the conceptual level, the strong coupling between subsystem elements complicates the understanding and design of a free electron laser (FEL). Given the requirements for high- performance FELs, the coupling between subsystems must be included to obtain a realistic picture of the potential operational capability. The concept of an Integrated Numerical Experiment (INEX) was implemented to accurately calculate the coupling between the FEL subsystems. During the late 1980's, the INEX approach was successfully applied to a large number of accelerator and FEL experiments. Unfortunately, because of significant manpower and computational requirements, the integrated approach is difficult to apply to trade-off and initial design studies. However, the INEX codes provided a base from which realistic accelerator, wiggler interaction, optics, and control models could be developed. The Free Electron Laser Physical Process Code (FELPPC) includes models developed from the INEX codes, provides coupling between the subsystem models, and incorporates application models relevant to a specific study. In other words, FELPPC solves the complete physical process model using realistic physics and technology constraints. FELPPC can calculate complex FEL configurations including multiple accelerator and wiggler combinations. When compared with the INEX codes, the subsystem models have been found to be quite accurate over many orders-of-magnitude. As a result, FELPPC has been used for the initial design studies of a large number of FEL applications: high-average-power ground, space, plane, and ship based FELs; beacon and illuminator FELs; medical and compact FELs; and XUV FELs.

We analyze and present numerical simulations of the so-called electron output scheme applied to the SELENE proposal of using a high power FEL to illuminate satellite solar cells. In this scheme, a first stage FEL oscillator bunches the electron beam while a second stage `radiator' extracts high power radiation. Our analysis suggests only in the case where the radiator employs a long, tapered undulator will the electron output scheme produce a significant increase in extraction efficiency over what is obtainable from a simple, single-stage oscillator, 1- and 2-D numerical simulations of a 1.7 micrometers FEL employing the electron output scheme show that large bunching fractions at the output of the oscillator stage but only approximately 1% extraction efficiency from the radiator stage.

This paper is dedicated to the discussion of some critical issues related to the choice of the appropriate driver for high power short wavelength CW FELs. The case for industrial need of high power short wavelength CW FELs is well established and will not be discussed here. This paper emphasizes issues related to power beaming requirements with an FEL wavelength of 0.85 micrometers and an average power level of 100 kW to 1 MW. Most of the conclusions are applicable for high power short wavelength ((lambda) < 1 micrometers ) CW FELs. Conclusions of this paper are not applicable to far infrared FELs where different technologies can be successfully used.

High-average power, short wavelength free-electron lasers (FELs) driven by RF accelerators require electron sources that are capable of producing beams that have both high brightness and high average current. In this paper we consider the electron source requirements for a putative 1 MW average power FEL operating at 1 micrometers wavelength. We examine the critical issues associated with developing an electron source suitable for such a device.

For high power free electron laser applications, such as power beaming, the electron accelerator must deliver a bright electron beam at high average current. The excellent beam brightness of the RF photoinjector is well established, however, until recently none has operated with an average current above 10 microamperes. This paper describes the operation of a RF photoinjector from 20 to 32 milliamperes of average current. The electron beam emittance dependence upon micropulse charge and the photocathode quantum efficiencies and operating lifetimes obtained during high duty factor testing are discussed.

The Phased Array Mirror, Extendible Large Aperture (PAMELA) prototype telescope phase I testing and verification has been completed. The prototype telescope is the first to have a fully adaptive primary mirror which consists of 36 hexagonal injection-molded Pyrex segments that are seven centimeters flat-to-flat. The segments are mounted on three long-throw voice-coil actuators for tip, tilt, and piston motion. The segments' tilts are measured with a Hartmann- Shack wavefront sensor, and the piston errors between adjacent segments are measured via inductive edge-sensors. The 0.5 meter telescope was successfully operated with simultaneously closed tilt and piston control loops for the entire array. Phase I test and verification results are shown for the closed loop operations.

Orbital transfer from LEO to GEO using beam power from the ground was first suggested in 1979, and a number of papers on the subject have been published since then. It is shown here that low thrust reusable orbital transfer vehicles with high descent and optimized ascent specific impulses are especially effective compared to conventional systems. They promise to permit the use of larger payloads and/or smaller launch vehicles, and the low specific mass and high power which are key to acceptable transit times can be achieved with laser-powered electric propulsion.

Adaptive optics (AO) segmented array systems require los cost nm (piston) and (mu) rad (tilt) range displacement edge sensors to assist in establishing phase continuity between adjacent segments by measuring relative AO edge displacements. Preliminary laboratory experiments using Si based integrated optic (IO) chips coupled to laser diode sources were carried out to determine their suitability as low cost, miniature, reliable and high precision piston and tilt edge sensors. Edge sensor displacement experiments demonstrate a resolution (sensitivity) on the order of 10 nm. The IO chip used is a micro-version (7.5 X 7.5 X 0.5 mm³) of a double beam Michelson interferometer with a phase shifter to allow direction determination. Operating at a wavelength 770 < (lambda) < 790 nm and a power output < 50 (mu) W, the wavelength stability, within a temperature range 15 to 35 degree(s)C, is (Delta) (lambda) ₀/(lambda) ₀ equals 10^-6. A 1 kHz frequency response was achieved for displacements on the order of 100 micrometers (measuring range) and 10⁶ Hz at 100 nm displacement, with a logarithmic frequency response increasing with decreasing measuring range. The maximum linear (moving) relative speed between adjacent segments is 200 mm/s or 200 X 10⁶ nm/s, allowing MHz sampling rates. Synchronously calibrated experiments of IO performance against a capacitance gauge, closed loop calibrated piezo stepper, and a HeNe source interferometer are presented. Systems integration design of the IO chip as a sensor is described.

We report on the design and operation of an integrated 1 meter adaptive optics system for compensation of a visible wavelength laser for satellite beamforming applications. A visible artificial laser guide star (frequency-doubled Nd:YAG laser with wavelength of 0.532 micrometers ) is used as the source for the reference wavefront. A shearing interferometer which uses a narrow optical bandwidth and has 500 subapertures is employed to sense wavefront distortion. These measurements are used to compute a conjugate wavefront to the distorted input light. The computed conjugate is then imprinted on a deformable mirror which consists of 500 square mirror segments. The deformable mirror is integrated with a 1 m Cassegrain telescope. The tracking system is designed to track and illuminate low Earth orbit (LEO) satellites. Computer control of both the adaptive optics and tracking systems are done via two terminals, and the entire adaptive optics/tracking system can be run by only two operators. We have used this system for both compensated imaging and compensated illumination applications. In this paper, we will present an overview of the system architecture and discuss computer control of the adaptive optics and tracking systems.

Adaptive and active optics in general, and laser power beaming in particular, requires the construction of large, complex mirror systems. Since these mirrors need to be tested with high accuracy to achieve optimum performance, their unusual shape and size often require the construction of special testing equipment. A full-surface interferometric scanning (FSIS) system for testing large cylindrical surfaces, which cannot be readily tested with current commercial interferometer systems, is described. The FSIS approach, using grating shearing interferometry, is based on interferometric slope measurement along the long direction of the cylindrical surface under test. The full surface measurement is synthesized from a discrete set of subaperture measurements obtained by scanning in the direction of the long side of the cylindrical surface. The FSIS is characterized by a high degree of immunity from the effects of vibration.

The ability to acquire, track, and accurately direct a laser beam to a satellite is crucial for power-beaming and laser-communications. To assess the state of the art in this area, a team consisting of Air Force Phillips Laboratory, Sandia National Laboratories, and COMSAT Corporation personnel performed some laser beaming demonstrations to various satellites. A ruby laser and a frequency-doubled YAG laser were used with the Phillips Lab Starfire Optical Range (SOR) beam director for this activity. The ruby laser projected 20 J in 6 ms out the telescope with a beam divergence that increased from 1.4 to 4 times the diffraction limit during that time. The doubled YAG projected 0.09 J in 10 ns at 20 Hz. The SOR team demonstrated the ability to move rapidly to a satellite, center it in the telescope, then lock onto it with the tracker, and establish illumination. Several low-earth-orbit satellites with corner- cube retro-reflectors were illuminated at ranges from 1000 to 6000 km with a beam divergence estimated to be about 20 (mu) radians. The return signal from the ruby laser was collected in a 15-cm telescope, detected by a photomultiplier tube, and recorded at 400 kHz. Rapid variations in intensity (as short as 15 microsecond(s) ) were noted, which may be due to speckles caused by phase interference from light reflected from different retro-reflectors on the satellite. The return light from the YAG was collected by a 35-cm telescope and detected by an intensified CCD camera. The satellite brightened by about a factor of 30 in the sunlight when the laser was turned on, and dimmed back to normal when the 50-(mu) radian point- ahead was turned off. The satellite was illuminated at 1 Hz as it entered the earth's shadow and followed for about 10 seconds in the shadow. In another demonstration, four neighboring GEO satellites were located and centered in succession with a 3.5-m telescope at a rate of about 16 seconds per satellite.

Using a large adaptive optical system designed for the correction of a visible wavelength laser, we report the results for the first known compensated laser illumination demonstration of an uncooperative low Earth orbit (LEO) satellite using an active point ahead mirror. The results of these tests using a low-power laser beacon at D/r_o equals 25 show improvement of over a factor of 5 in peak energy at the diffraction limited resolution of 0.1 arc sec for stellar images. The amount of light reflected from a large, diffuse LEO satellite was improved by a factor of approximately 2 using the adaptive optical system.

The influence of anisoplanatism due to angular aberration on the power beaming to satellites is considered in Rytov's approach. For universal two-parameter's model of atmospheric turbulence the correlation functions for phase and level are derived and discussed. Rigorous conditions of validity of geometrical approach in the problem discussed were obtained by straightforward expanding of diffraction results in short-wave limit.

The effectiveness of phase correction of high-power radiation propagating on a vertical path in the atmosphere, for which phase distortions are concentrated near the radiating aperture, have been investigated by means of modeling. The possibilities of modal and segmented correctors have been studied. This paper some factors such as the intensity profile of the initial beam, the number of degrees of freedom of modal and zonal correctors, as well as some variants of the simultaneous effect of these factors are investigated. A phenomenon of the wavefront dislocations of the optical irradiance propagating through the turbulent atmosphere and for the thermal blooming of a high-power beam is considered.

This paper addresses the potential augmentation of a quasi-stationary Unmanned Aerial Vehicle with a highly agile beam steering optical system. In addition to the primary application of relaying laser power from a ground station to low earth orbit satellites, applications include (1) precision tracking and ranging at distances of a few hundred kilometers, (2) covert communications to distances of 80 km utilizing only a modulable corner cube at the receiving end and (3) pollution detection and control and (4) continuous meteorological analysis of high altitude wind, CO₂ content, liquid water content, ice particle effective radius, effective drop size, optical depth and density, turbulence structure and emissivity profile.

According to the results of complex experimental investigations on optical characteristics of an irradiated surface and erosion plume, mass discharge, recoil momentum and calorimetric study of metallic and dielectric targets, the energy equation is solved, and the energy balance of a system is analyzed when intensity of a millisecond laser radiation and ambient pressure are varied from barometric to near vacuum.

The site for the proposed National Advanced Optic Mission Initiative (NAOMI) facility will be in the mountains near China Lake, California. This location has 260 clear days per year (more than any other feasible site in the U.S.). In 1993 there were 5 completely overcast days all year. The area near the proposed site is unpopulated. The solar insolation in this general area is the greatest of any area in the United States. The NAOMI system will be installed at an altitude of 5600 feet. Astronomical seeing there is excellent. Even at a less favored site than that planned for NAOMI the average Fried seeing coefficient r_o is 12 cm in the visible region and 20 cm values of r_o (comparable to the best observatories) are commonly observed. The area is centrally located in and entirely surrounded by one of the largest restricted airspace/military operating airspace complexes in the United States, 12% of the entire airspace in California. Electrical power is available from either the nearly Coso Geothermal plant, second largest in the United States, or from the even closer cogeneration plant at Trona, California. Cooling water can be obtained from the nearby area or from the lake itself. Although a dry playa, the lake has a high brackish groundwater level. Most of the commercial satellites over the U.S. could be reached by a laser/telescope system located on government land at the Naval Air Weapons Station (NAWS) military reservation at China Lake. This telescope/laser system will be a prototype for five other systems planned for around the world. The complex will provide laser power beaming to all satellites and put the United States into the position of world leader in satellite technology and power beaming to space.

The Birchum Mesa SELENE (Space Laser Energy) facility will be dual use facility as it provides for progressive development of high power Free Electron Lasers (FEL) and commercial laser beam power transfer to space-borne vehicles. The facility will be comprised of SELENE mainsite containing two laser system bays and supporting facilities with transport tunnels coupling to the Beam Transfer Optical System (BTOS) which is the active optical array space beam director with its supporting facility. The first generation commercial grade laser will operate at 100 kW of quasi-CW laser power with a planned growth to 10 MW of output power. The BTOS beam director will direct a focus compensated laser power beam to provide power service to space vehicles within a +/- 50 degree (half angle from zenith) tracking cone service field. An underground hardened site is proposed for this facility to mitigate any potentially hazardous effects from operation of a very high energy CW electron beam laser, to protect the facility from inadvertent weapons splashdown during range Test and Evaluation operations, and to create minimum environmental impact upon historical and ecological elements of the range.

The National Advanced Optics Mission Initiative (NAOMI) consists of two proposed programs, the SpacE Laser ENErgy (SELENE) which includes the site, and the Advanced Telescope Technology Integrated Large Array (ATTILA). The infrastructure of the SELENE facility requires a systems engineering approach. There are several large scale projects for the water, power, access, and communications all of which are interactive elements. These projects need to be designed and constructed concurrently while taking environmental concerns into account before the SELENE facility becomes operational.