¢¢ " O<,... # _:'.i>37.5 ,, /" NASA Contractor Report 194444 IEPC 93-208 28P Low Power Ground-Based Laser Illumination for Electric Propulsion Applications Michael R. LaPointe and Steven R. Oleson Sverdrup Technology, Inc. Lewis Research Center Group Brook Park, Ohio (NASA-CR-194444} LOW POWER N9_-23555 GROUND-BASEO LASER ILLUMINATION FOR ELECTRIC PROPULSION APPLICATIONS Final Report (Sverdrup Techno|ogy) Unclas January 1994 28 p G3/20 0203577 Prepared for Lewis Research Center Under Contract NAS3-25266 National Aeronautics and Space Administration LOW POWER GROUND-BASED LASER H,I,UMINATION FOR ELECTRIC PROPULSION APPLICATIONS Michael R. LaPointe and Steven R. Oleson Sverdrup Technology, Inc. LeRC Group, Brook Park, OH 44142 ABSTRACT l.,p specific impulse (s) LACE Low-Power Atmospheric Compensation A preliminary evaluation of low power, ground-based Experiment LEO Low Earth Orbit laser powered electric propulsion systelns is present- ed. A review of available and near-term laser, LLNL Lawrence Livermore National Laboratory photovoltaic, and adaptive optic systems indicates that Mo total initial spacecraft mass (kg) approximately 5-kW of ground-ba_d laser power can M v expended propellant mass {kg) be delivered at an equivalent l-sun intensity to an MIRACL Mid-lnfraRed Advanced Chemical Laser orbit of approximately 2000 kin. Laser illumination MPD magnetoplasmadynamic thruster at the proper wavelength can double photovoltaic Ny extended satellite life in orbit (y) array conversion efficiencies compared to efficiencies Nd:YAG neodymium:yttrium aluminum garnet obtained with solar illumination at the same intensity, PL laser power (W) allowing a reduction in alray mass. The reduced PIT pulsed inductive thruster array mass allows extra propellant to be carried with PPT pulsed plasma thruster initial beana radius (m) no penalty in total spacecraft mass. The extra propel- ro lant mass can extend the satellite life in orbit, allow- r._v,,, final beam radius (m) silicon ing additional revenue to be generated. A trade study Si using realistic cost estimates and conservative ground SPT stationary, plasma thruster station viewing capability was performed to estimate SWAT Short Wavelength Adaptive Techniques the number of communication satellites which must Z propagation distance (m) be illuminated to make a proliferated system of laser or= molecular absorption coefficient (nf _) aerosol absorption coefficient (m") ground stations economically attractive. The required cq number of satellites is typically below that of pro- molecular scattering coefficient (m q) aerosol scattering coefficient ( m"_) posed cornmunication satellite constellations, indicat- ing that low power ground-based laser beaming may atmospheric attenuation coefficient (m _) be commerciaUy viable. However, near-term advanc- AV velocity change, delta-V (m/s) es in low specific mass solar arrays and high energy Z wavelength (m) density batteries for LEO applications would tender T atmospheric transmittance the ground-based laser system impracticable. l -sun equivalent l-sun intensity (I.38 kW/nt) NOMENCLATURE I. LNTRODUCTION ACE Atmospheric Compensation Experiment APSA Advanced Photovoltaic Solar An'ay Laser power beaming has been advocated for a AVLIS Atomic Vapor Laser Isotope Separation variety of spacecraft power, propulsion, and commu- nication applications): Early research focnsed on the C_ cost of laser groutv, l station ($) DF deuterium fluoride design of multimegawatt space-based lasers, and the FL:L free electron laser development of suitable high power conversion tech- g acceleration due to gravity (9.8 nVs 2) nologies? Orbiting laser power stations were pro- GaAs gallium arsenide posed as a methtxl to deliver power to a multitude of GEO Geosynchronous Earth Orbit (3.6xl07 m) reusable orbit transfer vehicles, whose reuse could HF hydrogen fluoride amortize the expense of a space transportation infra- I intensity (W/m _) structure. s High power laser systems were envisioned 1o initial beam intensity (W/m 2) to occupy stationary lunar orbits, providing continuous power for bases, rovers, and ltmar mining operations, 6 radius after propagating a distance Z, and ,_ is the and were proposed as a method to deliver power from laser wavelength. The final beam or spot radius in Mars orbit to a base on the Martian surface as part of Equation i corresponds to the first zero in the diffrac- an ambitious program of planetary exploration. 7 tion pattern at the receiver, which contains 84% of the initial beam energy. For a laser wavelength of 1.06 Although attractive in terms of potential benefits, lum and an initial beam radius of 0.5 m, the spot several hurdles must be overcome in the design, radius at a distance of 500 km is approximately 0.65 development, and operation of muitimegawatt space- m due to diffraction, which corresponds to a beam based laser stations. High power chemical lasers are expansion half-angle (r_/Z) of approximately 1.3 well developed and routinely used for industrial and vrad. In the absence of atmospheric effects, larger military applications, t'_ but the need to continually initial beam radii or smaller laser wavelengths could replenish the chemical reactants would significantly be used to further reduce the beam radius at the increase the operating cost of a laser station in orbit. receiver. Solar-pumped lasers, which use solar radiation to induce lasant population inversions, have been suc- The effect of atmospheric turbulence on beam propa- cessfully demonstrated at sub-kW power levels, t'9 gation is a function of the atmospheric coherence However, a tremendous amount of work remains distance, _ which is a measure of the lateral distance before these concepts can provide the high power, over which atmospheric fluctuations do not signifi- closed-cycle, autonomous operation required for the cantly effect beam propagation. The atmospheric multimegawatt mission applications proposed above. coherence distance depends upon the path length, the Nuclear-pumped lasers, which utilize fission frag- beam propagation angle, and the refractive index ments to excite a iasant gas in a reactor core, have structure "constant', a complex and decidedly non- also been developed and tested at low power levels?'_° constant variable which approximates the strength of However, continual postponements in the development the atmospheric turbulence. For wavelengths of and deployment of space nuclear power systems make interest to laser power beaming, the coherence dis- it unlikely that a space-based nuclear-pumped laser tance is on the order of 0. I m, '4 which roughly corre- will be available in the near future. Free electron sponds to the maximum initial beam diameter that can lasers, in which coherent radiation is extracted from be propagated through the atmosphere without serious the periodic oscillations of high energy electron degradation. For initial beam diameters smaller than beams, I1 can generate high power levels but require the atmospheric coherence length, the final spot size significant development before they can provide is governed primarily by diffraction. For larger initial reliable, long-term operation in the remote space beam diameters, the final spot size is governed environment. primarily by atmospheric turbulence. An initial beam diameter larger than the atmospl'_eric coherence Many of the issues associated with using space-based distance will not decrease the beam expansion caused lasers are removed by keeping the laser stations on by atmospheric turbulence. Instead, the beam will the ground, where operating power is readily available spread as if the effective initial beam diameter were and the lasers are accessible for maintenance and equal to the atmospheric coherence distance. upgrade. Enthusiasm for ground-based power beam- ing was initially tempered by the difficulties associat- The final spot radius of a beam propagating through ed with propagating high power laser beams through the atmosphere is given by the addition of the beam the atmosphere. Atmospheric turbulence and temper- expansion due to diffraction and the beam expansion ature fluctuations change the atmospheric index of due to atmospheric turbulence. For the parameters refraction along the beam propagation path, _2produc- used in the above example, an initial beam radius ing distortions which can spread the beam and dra- equal to half the atmospheric coherence distance matically alter the intensity profile at the receiver. In yields a spot radius of approximately 6.4 m. corre- the absence of atmospheric distortion, the beam will sponding to an expansion half-angle of about 13 _trad spread as it propagates due to diffraction at the beam due solely to atmospheric turbulence. The final spot source such that: xadius for the example is then given by: r,m _ (0.61)(5x10 -_m)(I.O6xl0 -_ m)/(0.5 m) r,r,, = 0.61 Z )_ (m) (1) (2) + (5x10 "_m)(l.3xl0 "-_rad) - 7 m r0 where ro is the initial beam radius, r,_,, is the beam which is significantly larger than the 0.65 m spot radius calculated for diffraction effects alone. A cal signals and used to drive the deformable mirror. which compensates for the phase distortions and larger initial beam radius would reduce tile diff,'active flattens the incoming wave. With the use of adaptive component of beam spreading, but it would not alter optics, the inaage resolution can be nearly diffraction- the significantly larger beam expansion caused by limited. In addition to natural stars, laser guide stars atmospheric turbulence. The larger receiver