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ORIGINAl.CONTAIt S

COl,ORILI.USTRATIOt:IS

Laser PowerTransmission

Edmund J. Conway

study in the 1970s. These activities,although generally incomplete andsometimes contradictory, identifiedseveral themes:

Since their development, lasershave offered the potential of •projecting large amounts of poweronto a distant, small area. (Laserpower was once measured in"gillettes," the thickness in •number of razor blades it took tojust stop the beam.) Initially, thischaracteristic seemed good forweapons (e.g., the laser rifle) andmining (thermal fracture or •vaporization of rock). Actualapplications later developed in theareas of cutting (anything fromsheet metal to cloth), welding,scribing, and surgery.

One of the earliest proposalsfor the application of a high-powered laser in the civilianspace program was made byKantrowitz (1972). He proposedan Earth-to-orbit launch systemin which a laser on the groundsupplied thermal energy to asingle species of rocket propellant(such as hydrogen). The removalof the oxidizer, no longer neededto release chemical energy forpropulsion, reduced the lift-offweight of Earth-launched vehicles.

This and similar proposals onpower and propulsion generated agreat deal of speculation and

Lower cost power and propulsionis key to the development ofnear-Earth space.

Solar- and nuclear-poweredlasers have the characteristics

for high payoff in spaceapplications.

Expensive transportationapplications show high potentialfor cost reduction through theuse of remote laser power.

Economical power beaming inspace requires multiplecustomers who cannot useavailable (solar photovoltaic)power sources.

High laser conversion efficiencyis a key power-beamingchallenge.

NASA laser power requirementsare very different from those ofDOD and DOE, but NASA canbenefit from the breadth of basicresearch generated by theprograms of other agencies.

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https://ntrs.nasa.gov/search.jsp?R=19930007721 2020-07-31T07:01:09+00:00Z

A particularly complete study by

Holloway and Garrett (1981)

showed substantial payoff for bothlaser-thermal- and laser-electric-

powered orbit transfer vehicles. A

recent comparison by DeYoung

and coworkers (1983) suggests

that with a laser providing 100 kW

or more of power for electric

propulsion and for other onboard

utility needs, spacecraft will be able

to operate in low altitude, high drag

orbits and will be much lighter andsmaller.

From the studies, then, a general

set of requirements are emerging

for beaming power by laser to

currently envisioned spacemissions. First, the laser must be

capable of long-term continuous

operation without significant

maintenance or resupply. For this

reason, solar- and nuclear-poweredlasers are favored. Second, the

laser must supply high average

power, on the order of 100 kW or

greater for applications studied sofar. For this reason, continuous

wave or rapidly pulsed lasers are

required.

Since solar energy is the most

available and reliable power source

in space, recent research designed

to explore the feasibility of laser

power transmission between

spacecraft in space has focused

on solar-pumped lasers. Three

general laser mechanisms havebeen identified:

• Photodissociation lasing driven

directly by sunlight

• Photoexcitation lasing driven

directly by sunlight

• Photoexcitation lasing driven

by thermal radiation

Solar-PumpedPhotodlssoclatlon Lasers

Several direct solar lasers based

on photodissociation have been

identified, including six organiciodide lasants that have been

successfully solar pumped and

emit at the iodine laser wavelengthof 1.3 micrometers. (See figure 40

for a possible application of such a

laser.) Another lasant, IBr, has

been pumped with a flashlamp and

lased at 2.7 pm with a pulsed

power of hundreds of watts. One

organic iodide, C3F71, and IBr have

been investigated intensively to

characterize their operation.

Several reports on experimental

71

resultsandmodelinghavebeenpublished(ZapataandDeYoung1983,HarriesandMeador1983,WeaverandLee1983,Wilsonetal. 1984,DeYoung1986).Animportantcharacteristicofthephotodissociationlasersunderconsiderationis thattheyspontaneouslyrecombineto formthelasantmoleculeagain.BothC3F71 and IBr do this to a high

degree, permitting continuous

operation without resupplying

lasant, as is generally required for

chemical lasers. In addition, C3F71

absorbs almost no visible light and

thus remains so cool that it may

require no thermal radiator exceptthe pipe that recirculates the

lasant. A variety of other lasants

offering increased efficiency are

under study.

Figure 40

One-Megawatt Iodine Solar.PumpedLaser Power Station

This picture shows the elements of anorbiting laser power station. A nearlyparabofic solar collector, with a radius ofabout 300 meters, captures sunfight anddirects it, in a line focus, onto a lO-m-longlaser, with an average concentration ofseveral thousand solar constants. Anorganic iodide gas lasant flows throughthe laser, propelled by a turbine-compressor combination. The hot lasantis cooled and purified at the radiator.New lasant is added from the supply tanksto make up for the small amount of lasantlost in each pass through the laser.Power from the laser is spread andfocused by a combination of transmissionmirrors to provide a 1-m-diameter spot atdistances up to more than 10000 km.

Transmission

optics

Lasant supply ta

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Solar-PumpedPhotoexcitation Lasers

Another group of direct solar-

pumped lasers rely on theelectronic-vibrational excitation

produced by sunlight to power thelaser action. Two systems are

being actively studied. The first is

a liquid neodymium (Nd) ion laser,

which absorbs throughout the

visible spectrum and emits in the

near-infrared at 1.06 9m. Thislasant has lased with flashlamp

pumping and is currently beingtried with solar pumping, since

calculations indicate feasibility.A second candidate of this sort is

a dye laser, which absorbs in the

blue-green range and emits in

the red, near 0.6 ]Jm. These lasers

offer good quantum efficiencyand emission that is both of

short wavelength and tunable.

However, the lasers require

extremely high excitation toovercome their high threshold

for lasing, and the feasibility ofachieving this with concentrated

sunlight is still a question forfurther research.

Laser Power to a Lunar Base

In this artist'sconcept, a large receiver iscovered withphotovoltaic converters tunedto the laser wavelength. Such a systemcould produce electric power with anefficiency near 50 percent.

Artist: Bobby E. Silverthorn

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Indirect Photoexcitation Lasers

Photoexcitation lasers driven bythermal radiation produced by theSun are termed indirect solar-

pumped lasers. The lowerpumping energy implies longerwavelength emission than withphotodissociation lasers. Two!asers, the first blackbody-cavity-pumped laser (Insuik andChristiansen 1984) and ablackbody-pumped transfer laser(DeYoung and Higdon 1984), workon this principle. Molecules suchas CO2 and N20 have lased with

emission wavelengths between9 pm and 11 [_m. These lasersare inherently continuous waveand have generated powersapproaching 1 watt in initiallaboratory versions, with blackbodytemperatures between 1000 Kand 1500 K. While such lasers,powered by solar energy, may beused in space, they also offer greatpotential for converting to laserenergy the thermal energygenerated by chemical reactions,by nuclear power, by electricalpower, or by other high-temperature sources.

Laser-Powered Lunar ProspectingVehicle

This manned prospecting vehicle, far fromthe base camp, is receiving laser powerfor fife support, electric propulsion acrossthe lunar surface, and drilling. Since thispower is available during lunar night aswell as day, prospecting need not be shutdown for 14 Earth days every month.A mobile habitat module (not shown)accompanies the prospecting vehicle onits traverse.

Artist: Bobby E. Silverthorn

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References

DeYoung, R. J.; W. D. Tepper;

E. J. Conway; and D. H. Humes.1983. Preliminary Comparison of

Laser and Solar Space Power

Systems. Proc. 18th IntersocietyEnergy Conversion Eng. Conf.,

Aug.

DeYoung, Russell J. 1986. Low

Threshold Solar-Pumped IodineLaser. J. Quantum Electronics,

vol. QE-22 (July), pp. 1019-1023.

Inst. Elec. & Electron. Eng.

DeYoung, Russell J., and N. F.

Higdon. 1984. A Blackbody-

Pumped CO2-N2 Transfer Laser.NASA TP-2347, Aug.

Harries, Wynford L., and Willard E.

Meador. 1983. Kinetic Modeling of

an IBr Solar-Pumped Laser. SpaceSolar Power Review 4:189-202.

Holloway, Paul F., and L. B.Garrett. 1981. Comparative

Analyses of Space-to-SpaceCentral Power Stations. NASA

TP-1955, Dec.

Insuik, Robin J., and Walter H.

Christiansen. 1984. A Radiatively

Pumped CW CO2 Laser. J.Quantum Electronics, vol. QE-20

(June), pp. 622-625. Inst. Elec. &

Electron. Eng.

Kantrowitz, Arthur. 1972.

Propulsion to Orbit by Ground-Based Lasers. Astronaut. &

Aeronaut. 10 (May): 74-76.American Inst. Aeronaut. &

Astronaut.

Weaver, Willard R., and Ja H. Lee.

1983. A Solar-Pumped Gas Laserfor the Direct Conversion of Solar

Energy. J. Energy 7 (Nov.-Dec.):498-501.

Wilson, John W.; Y. Lee; Willard R.

Weaver; Donald H. Humes; and JaH. Lee. 1984. Threshold Kinetics

of a Solar Simulator Pumped Iodine

Laser. NASATP-2241, Feb.

Zapata, Luis E., and Russell J.DeYoung. 1983. Flashlamp

Pumped Iodine Monobromide

Laser Characteristics. J. Applied

Physics 64 (April): 1686-1692.

75

Conclusions

Henry W. Brandhorst, Jr.

It is abundantly clear that energy is

the key to utilization of space. Infact, bold programs are completely

dependent upon and in effect

hostage to the availability of

energy. We believe that, foreither the baseline scenario or the

alternative scenario that makes

use of lunar resources, there is

sufficient time to develop the

broad mix of power sources and

associated technologies

necessary for success.

A list of envisioned applicable

technologies related to power and

energy supply for space activitiesat various power demand levels isshown in table 6.

In general, stepwise development

of a variety of sources is

envisioned: First, an expanding

LEO space station with power

levels up to 10 MW powered bysolar or nuclear sources. Then,

lightweight photovoltaic systems

TABLE 6. Applicable Power Technologies

Power level Technology Application

1 - 100 kW Photovoltaic

100 kW - 1 MW

1 - 10MW

Radioisotope

Energy storage

P hotovoltaic

Solar dynamic

Direct solar heat

Nuclear

Waste heat rejection

Nuclear

Waste heat rejection

Power management

Lightweight arrays for satellites in GEO

Space station in LEOOn the lunar surface (day only) (hardware derived from space station)

Radioisotope thermoelectric generator (RTG) for lunar roverDynamic isotope power system (DIPS) for martian rover

Individual pressure vessel (IPV) nickel-hydrogen battery

Hydrogen-oxygen regenerative fuel cell (RFC)

Bipolar nickel-hydrogen battery

Flywheel

Solar electric propulsion for orbital transfer vehicle

Space station in LEO

On the lunar surface (day only) (hardware derived from space station)

Mirrors and lenses for processing lunar and asteroidal materials

SP-100 (safe, human-rated derivative) for lunar base

Liquid droplet radiatorBelt radiator

Rollup heat pipes

Nuclear electric propulsion for orbital transfer vehicle

or piloted spacecraft to Mars

Liquid droplet radiatorBelt radiator

RoIlup heat pipes

High-voltage transmission and distribution

Laser power beaming

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for GEO and lunar surfaceoperation. It is likely that lunarcamps staffed only during the daycould derive all their power(25-100 kW) from solar arrays.Lightweight electrochemicalstorage systems such ashydrogen-oxygen regenerative fuelcells would find use at GEO and,in concert with solar arrays, wouldpower surface-roving vehicles andmachines.

When full-time staffing becomesappropriate, we believe thatnuclear systems are the most likelysource of power. Power levels inthe 100-1000 kW range wouldbe derived from lunar-modifiedSP-100-class designs, whilepowers in the 1-10 MW rangewould be derivatives of civil and

military multimegawatt nucleardevelopments. These man-rated,safe nuclear systems would simplybe used as power demandswarranted.

Thus, for a lunar base, photovoltaic(or solar dynamic) systems wouldbe used initially for daytimeoperation, SP-100-class systemswould be used for full-timestaffing at power levels to 1 MW(by replication or design), andthese would be followed bymultimegawatt systems for the

1-10 MW needs. Similar progressis envisioned for either scenario for

GEO operations and asteroid andMars exploration. Attention mustalso be paid to the impact of thelunar, asteroidal, or martianenvironment on parameters ofthe power system.

We consider it unlikely that use ofnonterrestrial resources will affectpower system development before2010. It is rather the opposite:power systems will enable thedevelopment and use ofnonterrestrial resources.

Significant advances in the areasof nuclear power developmentand beamed power transmissionwill be made by both the militaryand the civilian space program.Full advantage must be takenof such corollary developments.

It should be noted that developmentof the 1- to 10-MW class of nuclear(or even solar) power systems willhave a profound influence on thestate and direction of the electricpropulsion programs. These powerlevels enable electrically propelledorbital transfer vehicles and

interplanetary explorers to travel tothe outermost fringes of the solarsystem with larger payloads andshorter trip times than chemical

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systems. In view of these

potentialities, a strong emphasis

on developing such propulsion

systems is warranted.

Assuming that current programs

in photovoltaics and in theSP-100 nuclear plant continue, the

following are considered critical

technological issues for further

research and development. They

are presented in order of priority.

By piggybacking atop and

augmenting existing programs,

we can ensure timely development

of the requisite systems.

, SP-100-derivative nuclear

power system capable of

providing power to 1 MW in anenvironment safe for humans

. Large-scale photovoltaic

arrays; solar dynamic power

conversion suitable for space,

using collectors that

concentrate sunlight

,

,

.

.

Solar furnaces and process

heat applications suitable for

processing space resourcesat high temperatures

7.

Multimegawatt (1-10 MW)

nuclear power-generatingsystems for electricity andheat

Thermal rejection systems to

reject waste heat from the

power conversion system,

processing, and environmental

conditioning (New conceptsfor efficient radiation are

required; the use of lunar

subsurface rejection should

be investigated.)

High-voltage electrictransmission and distribution

of multimegawatt power

Thermal energy control anddistribution for both manned

and unmanned systems

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8. Lightweight, rechargeablethermal and electrical storage

. Machine design includinghuman factors; robotics tosubstitute for humans inhostile environments

10. Laser technology for solar andinfrared sources to beam

power in space

11. Environmental interactions inspace associated with energysources, processing, andwork in space; i.e., the impactof foreign materials andpollutants

A broadly based program aimedat developing solar and nuclearpower systems to themultimegawatt level is of thehighest priority. For brevity'ssake, we have discussed only afew of the variety of long-range,innovative energy-relatedprograms supported by NASA,DOD, DOE, and industry. Toensure a broadly based,innovative program, a portion(up to 5%) of the funds allocatedfor space power research shouldbe devoted to areas that maypermit radical advance andextremely high payoff, albeit athigh risk.

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Heavy Lift Launch Vehicle

An unmanned heavy lift launch vehiclederived from the Space Shuttle to lowerthe cost of transporting material to Earthorbit would make it feasible to transportto orbit elements of a lunar base or a

manned spacecraft destined for Mars,Its first stage would be powered by twosolid rocket boosters, shown here after

separation, Its second stage would bepowered by an engine cluster at the aftend of the fuel tank that forms the centralportion of the vehicle. All this pushes thepayload module located at the forwardend. This payload module can carrypayloads up to 30 feet (9.1 meters) indiameter and 60 feet (18.3 meters) inlength and up to 5 times as heavy asthose carried by the Shuttle orbiter.

Artist: Dennis Davidson

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