Nuclear pumping of lasers by fission-fragments from nuclear chain reactions is discussed. Application of the newly developed lasers to spacecraft propulsion or onboard power, to lunar bases for industrial processing, and to earth for utilization of power without pollution and hazards is envisioned. Emphasis is placed on the process by which the fission-fragement kinetic energy is converted into laser light

Thom, K.

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NUCLEAR PUMPED GAS LASER RESEARCH
K. Thorn
NASA Headquarters ^76*21 F51 8
A nuclear-pumped gas laser is excited by the interactions of energetic particles from nuclear
reactions with a laser gas. Such nuclear reactions can be of various natures, such as fission in a
nuclear chain reaction, or radioactive decay resulting in fast particles, for example, from polonium
210. For practical reasons, I confine my discussion to the nuclear pumping of lasers by
fission-fragments from nuclear chain reactions, because this mechanism promises to result in the
highest laser power level. The NASA work on the nuclear pumped laser has evolved as a sideline of
plasma core nuclear reactor research, in the course of which it was realized that a fissioning gaseous p
medium should emit em radiation in a nonequilibrium spectral power distribution and, hence, ••
provide the basic conditions for lasing.
In principle, the conversion of fission-fragment kinetic energy into laser light is the direct
conversion of nuclear energy into work in a controlled fashion. It is a breakthrough in the usage of
nuclear energy, avoiding thermalization and the employment of a thermodynamic cycle with all its
limitation of efficiency and temperature tolerances.
Nuclear pumped lasers are only a few months old. On 2 October 1974, NASA announced a *
nuclear pumped helium-xenon laser (ref. 1) and the Sandia Corporation announced a nuclear
pumped carbon monoxide laser on 11 October 1974 (ref. 2). The efficiency in both of these
experiments is quite low, if one relates the laser power with the power of the nuclear reactor that
provides the neutron fluxes needed to induce the nuclear pumping. However, when one envisions
nuclear power stations in space, that beam their power via laser beams to customers at various
locations — to other spacecraft for propulsion or onboard power, to lunar bases for industrial
processing, and, perhaps, back to Earth for utilization of power without pollution and hazards - a ,
direct-pumped nuclear laser system need not be very efficient to be competitive with laser systems ^
that involve more conventional pumping methods, and whose power is derived from the
conventional conversion of nuclear heat into electricity. One can easily see that an efficiency of a
few percent would already establish a quite attractive system, in addition to the potential of
significant savings of mass and cost.
A schematic of the NASA experiment (ref. 3) is shown in figure 1. The Sandia experiment is
somewhat different in the geometrical configuration, but very similar in its functional components.
e.
In figure 1, one sees basically an arrangement in which fission fragments are produced and are
made to interact with a gas for studying optical radiation. Fission reactions occur in a foil of
enriched uranium by neutron capture and a good part of the fission fragments can escape from the
foil and penetrate into the gas inside a laser tube. In the NASA experiment, a mixture of 1 part
xenon in 20 parts of helium at 300 torr pressure lased at a wavelength of 3.5 n, during a pulse of
about 0.1 msec. The integrated energy at these first experiments was about 10 ml, measured by
means of a gold-doped germanium detector, 10 m away from the laser. The laser beam was bent
three times by 90° in order to locate the detector behind heavy shielding against nuclear radiation *
from the reactor.
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https://ntrs.nasa.gov/search.jsp?R=19760014430 2020-03-22T15:07:13+00:00Z
Left in the figure is the Los Alamos "Godiva" fast burst reactor, the origin of the neutrons
that cause the fissions in the uranium coating in the laser tube. In this device, two half-critical
masses of enriched uranium 235 are pneumatically shot into each other, whereupon the machine
becomes supercritical and starts a chain reaction, running up to 10,000 MW of power. The system
expands immediately by heat, thus slightly increasing its surface. In that moment, enough neutrons
can escape to interrupt the chain reaction and shut down the reactor. Only a very small fraction of I
the nuclear fuel is burned, but nevertheless about a megajoule of energy is released — which makes
the fraction of a joule of laser light that comes out of the laser tube, indeed, appear to be a very
poor effect.
Figure 2 shows the Godiva reactor as a handsome little research machine, which it is, as long as
one stays far away from it at operation. Also seen is a little laser tube, wrapped in a polyethelene
moderator.
Looking back at figure 1, one recognizes immediately the reason for the very low efficiency of
this arrangement. Only the neutrons of the reactor are used for the nuclear pumped laser
experiment, and those neutrons, particularly when moderated, carry practically no power. What is
needed, apparently, is to combine the laser tube and the reactor into one unit, that is, to make the
laser tube nuclear critical, or, in other words, to make a gaseous core reactor that lases.
If everyone would be entirely biased and perfectly objective, and if he thus could see things as
I do, he would immediately join us in the research on nuclear-pumped lasers and the related gaseous |>'
fueled reactors!
Gaseous core reactor research is well underway under a NASA interagency agreement with the
Los Alamos Scientific Laboratory.
As mentioned before, the nuclear-pumped laser research is closely connected with this gaseous
core reactor work. In the following, only our experimental program on nuclear lasers will be
discussed. We plan to test a high pressure xenon laser for nuclear pumping with the Godiva. Before T"
we achieved lasing with the helium-xenon mixture, numerous luminosity measurements were •-
conducted which indicated that the generation of light by fission-fragment, gas-atom interactions
increases with shorter wavelength. Because of insufficient instrumentation, we could not yet make
measurements in the uv spectrum. However, we suspect that most of the conversion of
fission-fragment energies into light occurs in the ultraviolet. Colleagues of the JPL will hopefully
soon be ready to join our friends at the LASL in conducting nuclear-pumped, high pressure xenon
experiments.
When increasing the pressure, the fission-fragment stopping distance will eventually become *•
smaller than the diameter of the laser tube diameter. In that case, the technique of using an uranium
foil inside the laser tube becomes less effective. Instead of a surface source of energetic nuclear
particles, a volume source must be used (ref. 5). One possibility is an admixture to the laser gas of
He , which upon the capture of a neutron undergoes the reaction He^ (n T)p, where the triton and
proton carry about 700 keV energy. From other laser systems it is known that helium will not
quench lasing in certain systems. Another option for a volume source of energetic nuclear particles
would be enriched uranium hexafluoride that undergoes fission in a neutron bath. -. -
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may quench laser action, because of its many degrees of internal freedom; perhaps it does
not do so, however, and instead may offer many lasing opportunities. This is a subject of
forthcoming research.
For the research involving enriched UFg and He , we have shipped our ballistic piston
compressor from the University of Florida to Los Alamos. This compressor and the Godiva reactor I'
seem to be an ideal match to produce, for a fraction of a millisecond, all conditions that one
possibly could desire for nuclear-pumped laser research: the cannon will compress gas mixtures to
almost any degree of needed density and temperature. A schematic of this cannon is shown in
figure 3. Very fortunately the free flying piston in this device, an old 50 mm Navy gun barrel, is
driven by compressed air and not by gun-powder; had the latter been the case, the reactor people of
LASL would have let the gun not come near the Godiva! Sometime in the near future we will
compress enriched UFg or He-* in the cannon and expose it to the high neutron flux from the
Godiva reactor. The high neutron fluence combined with the large uranium particle density should p
give us an estimated 1000 joules of fission energy generated in the gun!
A good faction of this energy could be converted into laser light. Or that part of this energy
could be used to recharge the air compressor — such that the machine would run like a Nuclear Otto
Engine!
REFERENCES |f
1. NASA RELEASE NO. 74-269, Sanchez, Gerald; Nuclear Source Converts Directly into Laser Light. 2 Oct.
1974.
2. Sandia Laboratories News Release, 11 Oct. 1974.
3. Helmick, H. H.; Fuller, J. L.; and Schneider, R. T.: Direct Nuclear Pumping of a Helium-Xenon Laser. Appl.
Phys. Lett., Vol. 26, no. 6, 15 Mar. 1975, p. 327-328.
4. Schwenk, Francis C.; and Thorn, Karl H.: Gaseous Fuel Nuclear Reactor Research. Frontiers of Power
Technology, 9-10 Oct. 1974. Energy Environment and Engineering, Oklahoma State University
Extension, College of Engineering.
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LASER GAS
(fti-Xf)
LASL GODIVA FAST BURST REACTOR »" „,
110.000 Mw. 1o"ncm2-stc| FIIM\
/ \
Fist Neutrons
sec!
Slow.
f i s s i o n
I ta jments
»i
Polyethelene
M o d e r a t o r
I10.J)
"""" I " I
««S« HO ««74 1HIJ |Ji 1 1 2 7 4
Figure 1.- Nuclear pumped laser research.
Figure 2- The "Godiva" reactor.
F
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OPTICAL ENERGY OUT
OPTICAL MIRROR AND
W.NDOW
H, - PRESSURE TUSES
UFA AND V/ASTE
Figure 3.— Nonequiiibrium nuclear reactor system.
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DISCUSSION
Abe Hertzberg, University of Washington — Karl, even though I'm very impressed with the success of these
experiments, all those nuclear radiation signs on the apparatus tend to keep me at arms length!
u
Stuart Bowen, Stanford University —. Are the pumping requirements of this device in any way compatible with the —
usage of reactor nuclear waste material? That is, can the waste actinides, etc., be used for this?
Answer: No, we wouldn't pump with it — we would hope to put it back into the reactor and burn it. The
trans-uranium actinides, of course, are fissionable.
Abe Hertzberg, University of Washington — But that's not really connected with your laser. What you're indicating, of
course, is that if you could build a UFg gas core reactor, you could recycle some of these wastes?
Answer: Yes.
Dick Stirn, J. P. L. — What is your feeling about the materials problems of mirrors, etc., at these high radiation
levels?
Answer: It may be a big problem. Of course we might use aerodynamic windows and there are other ways of
promoting self-anealing of materials. Such work at United Aircraft showed that some radiation damage can be
self-anealed by heat. There are various possibilities.
FAbe Hertzberg, University of Washington — What mixture did the group at Sandia use?
Answer: They used carbon monoxide. We believe we have to go to high pressure xenon to get better efficiency and
shorter wavelength.
Abe Hertzberg, University of Washington — Have you made any calculations of what are the possible efficiencies of
such a system?
Answer: Many attempts have been made. However, the problem is that the real cross sections involved are not
known. Our approach will be to measure some of these and then use them to assist the theory. *~
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