An overview of previous studies related to laser power transmission is presented. Particular attention is given to the use of solar pumped lasers for space power applications. Three general laser mechanisms are addressed: photodissociation lasing driven by sunlight, photoexcitation lasing driven directly by sunlight, and photoexcitation lasing driven by thermal radiation

Conway, Edmund J.

English

NASA Technical Reports Server

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ORIGINAl.CONTAIt S
COl,ORILI.USTRATIOt:IS
Laser Power
Transmission
Edmund J. Conway
study in the 1970s. These activities,
although generally incomplete and
sometimes contradictory, identified
several themes:
Since their development, lasers
have offered the potential of •
projecting large amounts of power
onto a distant, small area. (Laser
power was once measured in
"gillettes," the thickness in •
number of razor blades it took to
just stop the beam.) Initially, this
characteristic seemed good for
weapons (e.g., the laser rifle) and
mining (thermal fracture or •
vaporization of rock). Actual
applications later developed in the
areas of cutting (anything from
sheet metal to cloth), welding,
scribing, and surgery.
One of the earliest proposals
for the application of a high-
powered laser in the civilian
space program was made by
Kantrowitz (1972). He proposed
an Earth-to-orbit launch system
in which a laser on the ground
supplied thermal energy to a
single species of rocket propellant
(such as hydrogen). The removal
of the oxidizer, no longer needed
to release chemical energy for
propulsion, reduced the lift-off
weight of Earth-launched vehicles.
This and similar proposals on
power and propulsion generated a
great deal of speculation and
Lower cost power and propulsion
is key to the development of
near-Earth space.
Solar- and nuclear-powered
lasers have the characteristics
for high payoff in space
applications.
Expensive transportation
applications show high potential
for cost reduction through the
use of remote laser power.
Economical power beaming in
space requires multiple
customers who cannot use
available (solar photovoltaic)
power sources.
High laser conversion efficiency
is a key power-beaming
challenge.
NASA laser power requirements
are very different from those of
DOD and DOE, but NASA can
benefit from the breadth of basic
research generated by the
programs of other agencies.
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A particularly complete study by
Holloway and Garrett (1981)
showed substantial payoff for both
laser-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 and
smaller.
From the studies, then, a general
set of requirements are emerging
for beaming power by laser to
currently envisioned space
missions. First, the laser must be
capable of long-term continuous
operation without significant
maintenance or resupply. For this
reason, solar- and nuclear-powered
lasers are favored. Second, the
laser must supply high average
power, on the order of 100 kW or
greater for applications studied so
far. 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 have
been identified:
• Photodissociation lasing driven
directly by sunlight
• Photoexcitation lasing driven
directly by sunlight
• Photoexcitation lasing driven
by thermal radiation
Solar-Pumped
Photodlssoclatlon Lasers
Several direct solar lasers based
on photodissociation have been
identified, including six organic
iodide lasants that have been
successfully solar pumped and
emit at the iodine laser wavelength
of 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
resultsandmodelinghavebeen
published(ZapataandDeYoung
1983,HarriesandMeador1983,
WeaverandLee1983,Wilsonet
al. 1984,DeYoung1986).An
importantcharacteristicofthe
photodissociationlasersunder
considerationis thatthey
spontaneouslyrecombineto form
thelasantmoleculeagain.Both
C3F71 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 except
the pipe that recirculates the
lasant. A variety of other lasants
offering increased efficiency are
under study.
Figure 40
One-Megawatt Iodine Solar.Pumped
Laser Power Station
This picture shows the elements of an
orbiting laser power station. A nearly
parabofic solar collector, with a radius of
about 300 meters, captures sunfight and
directs it, in a line focus, onto a lO-m-long
laser, with an average concentration of
several thousand solar constants. An
organic iodide gas lasant flows through
the laser, propelled by a turbine-
compressor combination. The hot lasant
is cooled and purified at the radiator.
New lasant is added from the supply tanks
to make up for the small amount of lasant
lost in each pass through the laser.
Power from the laser is spread and
focused by a combination of transmission
mirrors to provide a 1-m-diameter spot at
distances up to more than 10000 km.
Transmission
optics
Lasant supply ta
72
Solar-Pumped
Photoexcitation Lasers
Another group of direct solar-
pumped lasers rely on the
electronic-vibrational excitation
produced by sunlight to power the
laser 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. This
lasant has lased with flashlamp
pumping and is currently being
tried 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 efficiency
and emission that is both of
short wavelength and tunable.
However, the lasers require
extremely high excitation to
overcome their high threshold
for lasing, and the feasibility of
achieving this with concentrated
sunlight is still a question for
further research.
Laser Power to a Lunar Base
In this artist'sconcept, a large receiver is
covered withphotovoltaic converters tuned
to the laser wavelength. Such a system
could produce electric power with an
efficiency near 50 percent.
Artist: Bobby E. Silverthorn
73
Indirect Photoexcitation Lasers
Photoexcitation lasers driven by
thermal radiation produced by the
Sun are termed indirect solar-
pumped lasers. The lower
pumping energy implies longer
wavelength emission than with
photodissociation lasers. Two
!asers, the first blackbody-cavity-
pumped laser (Insuik and
Christiansen 1984) and a
blackbody-pumped transfer laser
(DeYoung and Higdon 1984), work
on this principle. Molecules such
as CO2 and N20 have lased with
emission wavelengths between
9 pm and 11 [_m. These lasers
are inherently continuous wave
and have generated powers
approaching 1 watt in initial
laboratory versions, with blackbody
temperatures between 1000 K
and 1500 K. While such lasers,
powered by solar energy, may be
used in space, they also offer great
potential for converting to laser
energy the thermal energy
generated by chemical reactions,
by nuclear power, by electrical
power, or by other high-
temperature sources.
Laser-Powered Lunar Prospecting
Vehicle
This manned prospecting vehicle, far from
the base camp, is receiving laser power
for fife support, electric propulsion across
the lunar surface, and drilling. Since this
power is available during lunar night as
well as day, prospecting need not be shut
down for 14 Earth days every month.
A mobile habitat module (not shown)
accompanies the prospecting vehicle on
its traverse.
Artist: Bobby E. Silverthorn
74
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 Intersociety
Energy Conversion Eng. Conf.,
Aug.
DeYoung, Russell J. 1986. Low
Threshold Solar-Pumped Iodine
Laser. 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. Space
Solar Power Review 4:189-202.
Holloway, Paul F., and L. B.
Garrett. 1981. Comparative
Analyses of Space-to-Space
Central 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 Laser
for 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 Ja
H. 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. In
fact, bold programs are completely
dependent upon and in effect
hostage to the availability of
energy. We believe that, for
either 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 activities
at various power demand levels is
shown 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 by
solar 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 LEO
On the lunar surface (day only) (hardware derived from space station)
Radioisotope thermoelectric generator (RTG) for lunar rover
Dynamic 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 radiator
Belt radiator
Rollup heat pipes
Nuclear electric propulsion for orbital transfer vehicle
or piloted spacecraft to Mars
Liquid droplet radiator
Belt radiator
RoIlup heat pipes
High-voltage transmission and distribution
Laser power beaming
76
for GEO and lunar surface
operation. It is likely that lunar
camps staffed only during the day
could derive all their power
(25-100 kW) from solar arrays.
Lightweight electrochemical
storage systems such as
hydrogen-oxygen regenerative fuel
cells would find use at GEO and,
in concert with solar arrays, would
power surface-roving vehicles and
machines.
When full-time staffing becomes
appropriate, we believe that
nuclear systems are the most likely
source of power. Power levels in
the 100-1000 kW range would
be derived from lunar-modified
SP-100-class designs, while
powers in the 1-10 MW range
would be derivatives of civil and
military multimegawatt nuclear
developments. These man-rated,
safe nuclear systems would simply
be used as power demands
warranted.
Thus, for a lunar base, photovoltaic
(or solar dynamic) systems would
be used initially for daytime
operation, SP-100-class systems
would be used for full-time
staffing at power levels to 1 MW
(by replication or design), and
these would be followed by
multimegawatt systems for the
1-10 MW needs. Similar progress
is envisioned for either scenario for
GEO operations and asteroid and
Mars exploration. Attention must
also be paid to the impact of the
lunar, asteroidal, or martian
environment on parameters of
the power system.
We consider it unlikely that use of
nonterrestrial resources will affect
power system development before
2010. It is rather the opposite:
power systems will enable the
development and use of
nonterrestrial resources.
Significant advances in the areas
of nuclear power development
and beamed power transmission
will be made by both the military
and the civilian space program.
Full advantage must be taken
of such corollary developments.
It should be noted that development
of the 1- to 10-MW class of nuclear
(or even solar) power systems will
have a profound influence on the
state and direction of the electric
propulsion programs. These power
levels enable electrically propelled
orbital transfer vehicles and
interplanetary explorers to travel to
the outermost fringes of the solar
system with larger payloads and
shorter trip times than chemical
77
systems. In view of these
potentialities, a strong emphasis
on developing such propulsion
systems is warranted.
Assuming that current programs
in photovoltaics and in the
SP-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 an
environment 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 resources
at high temperatures
7.
Multimegawatt (1-10 MW)
nuclear power-generating
systems for electricity and
heat
Thermal rejection systems to
reject waste heat from the
power conversion system,
processing, and environmental
conditioning (New concepts
for efficient radiation are
required; the use of lunar
subsurface rejection should
be investigated.)
High-voltage electric
transmission and distribution
of multimegawatt power
Thermal energy control and
distribution for both manned
and unmanned systems
78
8. Lightweight, rechargeable
thermal and electrical storage
. Machine design including
human factors; robotics to
substitute for humans in
hostile environments
10. Laser technology for solar and
infrared sources to beam
power in space
11. Environmental interactions in
space associated with energy
sources, processing, and
work in space; i.e., the impact
of foreign materials and
pollutants
A broadly based program aimed
at developing solar and nuclear
power systems to the
multimegawatt level is of the
highest priority. For brevity's
sake, we have discussed only a
few of the variety of long-range,
innovative energy-related
programs supported by NASA,
DOD, DOE, and industry. To
ensure a broadly based,
innovative program, a portion
(up to 5%) of the funds allocated
for space power research should
be devoted to areas that may
permit radical advance and
extremely high payoff, albeit at
high risk.
79
Heavy Lift Launch Vehicle
An unmanned heavy lift launch vehicle
derived from the Space Shuttle to lower
the cost of transporting material to Earth
orbit would make it feasible to transport
to orbit elements of a lunar base or a
manned spacecraft destined for Mars,
Its first stage would be powered by two
solid rocket boosters, shown here after
separation, Its second stage would be
powered by an engine cluster at the aft
end of the fuel tank that forms the central
portion of the vehicle. All this pushes the
payload module located at the forward
end. This payload module can carry
payloads up to 30 feet (9.1 meters) in
diameter and 60 feet (18.3 meters) in
length and up to 5 times as heavy as
those carried by the Shuttle orbiter.
Artist: Dennis Davidson
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