The technological feasibility of using multimagawatt lasers for space power transmission is discussed. Candidate lasers include electric discharge lasers, direct optically pumped lasers, and free electron lasers

Cassady, P.

Klosterman, E.

Taussio, R.

English

NASA Technical Reports Server

SOLAR DRIVEN LASERS FOR POWER SATELLITE APPLICATIONS
R. Taussig, P. Cassady and E. Klosterman
MSNW, Inc., 2755Northrup Way, Bellevue, Washington 98004
"_// /
22722
Introduction. Multi-megawatt lasers appear to be technologically feasible
for space power transmission in the 1990s time frame. Solar driven lasers
based on conventional gas dynamic and electric discharge laser concepts (GDLs
and EDLs) have been investigated to determine the feasibility of using exist-
ing laser technology for this application. With conventional solar photocells
as the power source the GDL and EDL lasers do not appear to be as efficient as
microwaves in transmitting power back to earth. However, for relatively new
and untested laser concepts such as the solar optically pumped laser (OPL) and
free electron laser (FEL) much higher laser efficiencies may be achievable,
leading to a laser SPS system competitive with the microwave SPS. Because of
their compact transmitters and receivers, lasers may have an advantage in bet-
ter economies of scale for smaller SPS sizes than the microwave SPS. Further,
laster light wavelengths longer than 2.5 microns may be considerably safer
and have less impact on the earth's environment (e.g., communications) than
microwaves. For these reasons one should consider the laser as a serious power
transmission option.
Results from a brief survey of solar powered, space-based lasers are given be-
low to gain some perspective on the types of lasers reasonable for power trans-
mission. A preliminary selection of candidate lasers for SPS application was
made on the basis of scalability to high powers (l MW and greater), relative
weights, efficiency (better than I%), and safety (wavelengths greater than
2.5_). The preliminary list includes CO and C02 EDLs, direct optically pumped
lasers (e.g., CF31, etc.), indirect optically pumped lasers (e.g., C0/C02 mix-
ing laser), and free electron lasers (FELs).
Electric Discharge Lasers. Several previous studies have focused on solar
powered, closed cycle EDLs for power transmission. The EDL requires electric
power both to drive the high voltage discharge which pumps the laser medium to
an excited state before it lases, and to circulate the lasant through a cool-
ing loop which extracts the waste heat and returns it to its original state.
Either photocells or perhap_ a more efficient solar thermal power cycle can be
used to produce the electricity. Table l summarizes the characteristics of the
EDL and other l MW cw solar powered lasers, where electricity in each case is
assumed to be produced by a 25% efficient solar thermal Brayton cycle power
system. Monson has shown how to optimize the closed cycle flow conditions for
minimum flow loop compressor power per unit laser power output. His results
are employed to keep the total laser system weight small.
Direct Optically Pumped Lasers. A preliminary consideration of the direct op-
tically pumped lasers in a previous study suggested that the only class of
direct optically pumped lasers with reasonable efficiency capable of high power
operation are those utilizing CF31, C3F_I, C,Fgl, and (CF3)2AsI. These molecules
photodisscoiate in the near UV of sunlight. With proper filtering, only the
solar wavelengths appropriate for pumping the molecules need to be focused on
the lasant. Concentration ratios on the order of lO0 appear adequate to pump
the heavier versions of these iodine molecules leaving an excited I* which lases
at 1.315_. Unfortunately, this wavelength is smaller than the safety limit of
2.5u for retinal damage. The dissociated fragments of these molecules also
polymerize so that in a closed cycle lasant for space, a substantial amount of
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energy must be invested to break apart the polymerized fragments and reconsti-
tute the original molecule. The overall efficiency of this type of laser, not
including the energy needed for reconsitution of the lasant, is approximately"
0.5% (laser power ÷ solar power).
Indirect Optically Pumped Lasers.
been examined:
o Static C02 laser
o Flowing CO laser (subsonic)
o Mixing C0/C02 laser (subsonic)
Three indirect optically pumped lasers have
Of these three only the last one combines the features of scalability, high
efficiency, and laser wavelength suitable for atmospheric transmission. At
this stage CO lasers appear to transmit successfully only on isolated lines
and, since the CO laser is relatively inefficient when operated in a single
line mode, it has not been considered further. The indirect optically pumped
laser (IOPL) uses a solar heated blackbody cavity to pump the lasant. The
advantages of the cavity are reduced radiative losses, a downshift in the peak
of the pump radiation toward the infra-red absorption lines of CO and C02, an
increase in the irradiation of the lasant to a full 4_ steradians, and refill-
ing the spectral lines absorbed by the lasant through radiative re-emmission of
a full blackbody spectra by the cavity walls. As in the direct optically
pumped case, the !OPL needs electricity or mechanical power only for circulat-
ing coolants and moving the lasant through the laser optical cavity. From
Table l it appears initially that this class of laser will be quite lightweight.
The mixing gas version is shown in Figure I. Initial gain experiments have
been performed indicating the viability of the blackbody radiation pump method.
Further research is needed to demonstrate the complete laser concept.
Free Electron Lasers. We have also investigated three possible versions of the
free electron laser: the CATALAC FEL, the Double FEL, and the Storage Ring FEL.
The CATALEC FEL, illustrated schematically in Figure 2, is based on a concept
developed at LASL to help recapture some of the energy left over in the elec-
trons as they exit from the laser cavity. These electrons are recirculated
through the rf-linac 180 ° out of phase with the next bunches of electrons to be
accelerated. The electrons are declerated and return most of their remaining
energy to the accelerating field. The linac therefore behaves as a catalyst
for transferring the energy of decelerating electrons to those being acceler-
ated. The spent electrons are collected at the other end of the linac with
approximately 8 MeV energies and the accelerated electrons emerge with energies
on the order of 50 MeV.
No high power FELS have been built in the wavelength range suitable for atmos-
pheric propagation so that this laser technology must be regarded as extremely
tentative. Elementary gain and oscillator experiments have been performed by
Madey and his co-workers at Stanford which indicate that the principle will
work. Several larger FEL experiments for l_ lasers are now in the planning
stage and are due to come on line in late 1980 or 1981. Nevertheless, a sub-
stantial amount of theoretical analysis has been performed which permits us to
carry out elementary scaling calculations; the results of these are included in
Table l for the CATALAC FEL which operates essentially as a once-through device
with good energy recover, and for the storage ring FEL. The double FEL is, at
present, too sensitive to assumptions made regarding low losses of the standing
EM wave used as the virtual wiggler field.
269
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Solar driven lasers for power satellite applications

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