Initial studies of nuclear pumping concentrated on molecular gas lasers such as CO2 because early theoretical predictions indicated lasing with low pump (neutron flux) thresholds of ~1010 n/cm2 s [6, 7]. Indeed, experiments at the University of Illinois initiated during this period showed an enhanced output from an electrically excited CO2 laser with nuclear pumping superimposed [11]. Experiments at the Sandia National Laboratories also demonstrated reactor-ionized electrical excitation of the CO2 laser [6]. However, it was later found that lasing in CO2 was prevented by dissociation of the molecule during irradiation. Thus, other NPLs were sought.

In their search for a NPL, Sandia workers led by David McArthur obtained, in early 1974, a small-signal gain and lasing in CO gas cooled to 77 °K [7]. This provided the first clear observation of lasing in the United States with pure fission — fragment excitation. Preliminary systems studies were then performed to study NPL scaling to large sizes and large powers with a source configuration that retained existing pulsed reactor technology [8, 18]. An early attempt at scaling of the cooled CO laser used a “folded path laser” apparatus, which lased at a power ~100 W.

Lasing was subsequently observed in rare gas mixtures and in CO gas mixtures at room temperature.

Shortly thereafter, a University of Florida team lead by Richard Schneider obtained pumping in a noble gas laser (He-Xe at 3.51 qm) in collaborative experi­ments at the Los Alamos Scientific Laboratory [19]. Subsequently, much was learned about the physics of nuclear pumping of noble gas NPLs through a series of studies by the NASA-Langley Research Center group led by Frank Hohl and Russell Deyoung (supported via Karl Heinz Thom in NASA headquarters). This work involved lasers with wavelengths generally in the near infrared. The NASA studies used a fast-burst reactor at the Army Aberdeen Test Laboratory and the 3He (n, p)3H reaction for pumping. Specifically, lasing was achieved in 3He-Ar (1.27, 1.79 qm), 3He-Xe (2.026 qm), 3He-Kr (2.19, 2.52 qm), and 3He-Cl (1.58 qm) using thermal neutron pulses of 200 ps full width and approximately 1016 n/cm2 s [20]. These systems lased on atomic transitions where inversion occurred by collisional radiative recombination of the noble gas atomic ions. A record peak laser power of 3.7 W was achieved from a relatively small 3He-Ar (1 % Ar) (1.79 qm) NPL at a total pressure of 4 atm. However, further increases in laser power were prevented by the small fraction of neutrons interacting with either 3He or coated walls in the cell. Thus, a new laser geometry was devised to intercept more of the reactor neutrons. The laser volume was expanded to 40 x 30 x 3 cm by use of internal mirrors to create a “zigzag” type optical path through the excited 3He-Ar media as shown in Fig. 13.1.

Aiianrenl user

GoWo’ Aluminum
Plane Mirror

Neutral Density Filler

brevister ^ngle Window

Dielectric Output Mirror

-A) Array Detector

Fig. 13.1 Nuclear pumped multiple pass box laser [21]

An output of 1.1 kW was achieved with a 3He-Ar mixture at 2,300 Torr total pressure (0.74 % Ar) at 4.3 x 1016 thermal n/cm2 s [21]. This system demonstrated the ability to scale the NPL power with increased laser gas volume. During this period, much progress was made in the theoretical interpretation of the pumping mechanism with stress on the secondary electron energy distributions created by the nuclear reaction products. Studies at the University of Illinois [22—24] employed both Monte Carlo and analytic solutions for a variety of gases, including the possible superposition of a weak electric field. This work is discussed in more detail in the theory section later in this chapter. Intensive numerical studies were also done at North Carolina State University [25]. These results defined the source term for reaction rate calculations. Next, attention turned to kinetic calculations for both noble gases [20] and excimers [26].

Another important pumping reaction considered at NASA Langley is the 235UF6(n, ff)FF reaction, yielding 160 MeV of kinetic energy via the two highly charged fission fragments (FF5). The importance of this reaction is the possibility of using a gaseous core reactor to combine the reactor and laser as one integral system. With this in mind, experiments were done in the U. S. Army Aberdeen Test Center Fast Burst Reactor to excite a mixture of 235UF6-Ar-Xe (600 Torr). This produced lasing at 2.65 qm in Xe [27]. UF6 gas is an effective laser quencher; thus, only small amounts could be added. A uranium coating, deposited from the UF6 condensing on the internal laser cavity wall also produced fission fragments that excited the gas. Some results from this experiment are shown in Fig. 13.2.

With 3 Torr of UF6 in Ar-Xe, 38 % of the energy deposited came from gaseous UF6 and the remainder from the a 235U coating formed on the cavity wall by UF6

decomposition. In summary, both 3He and 235UF6 have been effective for pumping noble gas lasers. This coating reaction does not scale with volume as desired. Hence, if an ultra-compact fission core laser is desired, additional research is needed to find efficient systems for using UF6 to achieve volumetric reactions.

One alternate approach that could avoid the quenching limits on the UF6 concentration uses an aerosol of fissile microspheres in a fluorescer gas, making it an attractive approach to a higher-power nuclear-pumped flashlamp [28]. Thus work at the University of Missouri addressed an aerosol reactor concept called Aerosol Reactor Energy Conversion System (ARECS), illustrated in Fig. 13.3. The key component is a nuclear-driven flashlamp. Flashlamps do not require a long photon mean free path as do lasers. Thus, a media with relatively poor optical transport properties can be used to provide “pump” photons for lasers.

The ARECS fuel consists of fissile microspheres mixed with a fluorescer gas as an aerosol. The transport efficiency of the radiation from the nuclear fuel to the fluorescer medium strongly depends on the micropellet size distribution and the uniformity of pellet density. For example, as the radius of the micropellets increases, the transport efficiency decreases. While there have not been any experi­mental observations of a fissioning aerosol at high temperatures and flow, some key points are clear. A relatively high particle density with sizes averaging 2-10 qm in radius at temperatures of around 1,000 °K is required. Other experience with aerosols suggests that these conditions are achievable.

The fluorescer medium must channel the energy absorbed from the nuclear radiation to an excited state (versus gas heating). Irradiation of rare gas and rare gas halide mixtures can efficiently generate excimer fluorescence at relatively short wavelengths; thus, these gases are prime candidates for the fluorescer.

Several options are possible for coupling the light out of the fluorescer. One is to cause lasing directly in it. The other is to transport the light out via a “non­concentrating” cell geometry such as in Fig. 13.3 (light concentrating geometries are also possible). The coupling efficiency of a non-concentrating geometry can range from 30 to 80 % (the more complicated concentrating geometry could achieve even higher efficiencies).

The ARECS concept is potentially attractive for several applications including: photolytic lasers, photo-chemical production, and photoelectric or photo­
electrochemical generation of electricity. These processes can be achieved at both high efficiency and high temperature. Consequently, they can serve as topping cycles for an even more efficient, integrated energy conversion system. To illustrate the potential performance, Boody and Prelas considered a photolytic laser system [29]. Such lasers are relatively efficient energy conversion systems, primarily due to resonance transitions. Boody and Prelas estimated an overall system efficiency of 0.03 using a photolytically pumped XeF* laser. This result compares well with other high power laser alternatives which have predicted system efficiencies in the 0.005-0.05 range.