During the earlier work on NPLs in the United States, a number of groups provided important insightful theoretical studies that clarified the physics of this new field. This work generally tackled one of the fundamental issues involved—the details of the source term created by the incoming radiation, the transfer of energy to the laser media, the reaction kinetics of the species in the lasing medium, and the optics associated with the laser, especially special effects such as thermal blooming encountered in such lasers. There is a complex matrix where these issues overlap with applications and a variety of possible laser media such as noble gases, excimers, molecular gases, liquids, etc. This matrix is far from full, so much work remains to be done to fully understand all of the possible routes that NPLs may take. The main goals are to obtain improved efficiencies, and ways to take advantage of the large energy but relatively low peak powers available with this form of pumping the laser.

Mark A. Kushner reported a number of important theoretical studies of nuclear — pumped lasers while at the University of Illinois. A summary of his early studies with NPLs is presented next in the section.

Nuclear-pumped lasers are characterized by high-threshold neutron fluxes and low gain. The closer the fissile gas can be coupled to the actual lasing process, the more efficient the laser will be. One approach is to seek laser systems such as chemical lasers that have low threshold power requirements. Kushner presented a model for a nuclear-pumped HF laser using a 235UF6-H2 gas mixture [30]. This model predicted a peak gain of about 50 %/m and a threshold neutron flux of < 1014/ cm2. He provided recommendations concerning the optimum use of this unique system. However, as discussed in other work by Wilson and DeYoung [31] and Maceda [43], this and other lasers requiring 235UF6 are difficult to achieve experi­mentally, so the practicality of such lasers remains to be determined.

Kushner also sought other classes of lasers well suited to nuclear pumping. In study [32], he proposed study of a class of NPLs in which the laser levels are populated by negative ion-positive ion neutralization. He analyzed two particular systems, 3He/N2O/O2 and 3He/N2O/N2. Laser action was predicted in atomic oxygen and nitrogen as a result of O~ — O+ and O~ — N+ neutralizations. The source of negative ions is a dissociative attachment to N2O. A threshold neutron flux of «1015 cm-2 s-1, peak power of >500 W/cm2, and a fission power to laser power conversion efficiency of «0.4 % were predicted. The electron density in the attachment process was evaluated, and the highly selective nature of the ion-ion neutralization in producing product-excited states.

Kushner studied the excitation source term along with average energies going into excitation and ionization [W-values] for nuclear pumping with fission fragments. This is particularly important to nuclear pumping because of the unique highly non-Maxwellian characteristic of the electron energy distribution created during the slowing down process. In study [33] Kushner and Moratz described a method for calculating excitation and ionization rates in a plasma generated by the slowing of fission fragments in a gaseous medium such as neon. The energy distribution of the fission fragments as they slow down and the excitation and ionization of neon due to collisions with the fission fragments was calculated. Effective W values for ion­ization and excitation of neon directly by the fission fragments of uranium were found to be about 71.6 and 11 eV, respectively. To illustrate the non-equilibrium characteristic of this process, they compared the source function for electrons produced by an e-beam to the source function of electrons produced by heavy ions which are shown to have had a lower average energy.

In another study [34], Kushner and colleagues presented a model for the heavy — ion pumping of a XeF (B! X) laser by uranium fission fragments. The model is a self-consistent accounting of the generation and transport of the fission fragments through the fission foils, slowing of the fragments in the gas, evolution of the secondary-electron-source function and distribution, and the XeF laser plasma kinetics. By stimulating the same quantities for an e-beam-pumped plasma, direct comparisons can be made for laser performance. Kushner and colleagues found that the secondary-electron source generated by the e-beam was more energetic than that for direct ionization by fission fragments due to a more favorable mass ratio for momentum transfer collisions with orbital electrons. This difference in the electron — source functions significantly affects W values (energy/ion pair) and excitation fractions. The impact on laser performance, though, is not large due to the high efficiency of channeling deposited energy to the upper level in XeF lasers. For conditions typical of fission fragment excitation (power deposition 1-3 kW cm-3, pulse length «200 qs), this model predicted 10-15 % higher gain with e-beam excitation than heavy-ion excitation.

Later, Kushner et al. pointed out that excimer lasers excited by electron or ion beams having energy deposition of 100 s J/1-atm over many microseconds experi­ence a temperature rise of hundreds of degrees (K) [35]. The increase in gas temperature can significantly impact both the kinetics and spectroscopic para­meters. In this study, Kushner and colleagues considered the high-temperature (<900 K) plasma kinetics and absorption in He and Ne buffered gas mixtures for particle beam pumped XeF lasers. They found both gain and absorption depended differently on gas temperature in these mixtures (absorption decreased in He mixtures, but increased in Ne mixtures). The differences were attributed to a reduction in diatomic absorbing species with increasing temperature and differences in the temperature dependence of the optical absorption cross-sections for NeXe+ and Xe2+.

Later, Kushner and colleagues discussed the effects of He addition on the performance of fission fragment excited Ar/Xe lasers [36]. This was viewed as an important laser system because both Russian and Sandia researchers were studying such systems experimentally. The intrinsic power efficiency of the atomic xenon laser depends upon the electron density because of the mixing of the laser levels by electron collisions while the electron density in high-pressure particle-beam excited plasmas increases with increasing gas temperature. Therefore, in order to reduce the amount of electron collisional mixing when operating at high-energy loadings (>100 s J/1-atm), mixtures having a high-heat capacity are required. In particle — beam excited Ar/Xe mixtures, which typically yielded the highest intrinsic laser efficiencies, increasing the gas pressure to increase the heat capacity was not always practical due to the high-stopping power of the gas mixture. For this reason, Kushner and colleagues investigated adding He to Ar/Xe mixtures for a fission — fragment excited atomic xenon laser. Adding He increased the heat capacity without appreciably perturbing the favorable kinetics resulting in efficient opera­tion of the laser in Ar/Xe mixtures. They found that when adding He to Ar/Xe mixtures the dominant laser transition wavelength switched from 1.73 to 2.03 ^m without significantly decreasing the efficiency. The laser pulse length also increased, an effect attributed to a lowering of both the electron temperature and gas temperatures.

Subsequent studies of the infrared atomic xenon laser (5d! 6p) by Kushner et al. found it to be an attractive candidate for fission fragment excitation, providing low-power deposition (l—100 W cm-3), long pulse lengths (1-10 ms), and high — energy deposition (100’s J/l) [37]. The optical gain at 1.73 and 2.03 ^m was measured in a reactor-excited xenon laser and yielded values exceeding 0.03­0.05 cm-1 at power depositions of less than 10 s W/cm3. The gain was also found to rapidly terminate before the peak of the pump pulse for some experimental conditions. A computer model was developed to predict gain in fission-fragment- excited xenon lasers. It was found that the termination of gain was most likely attributable to gas heating which increased the electron density, leading to electron collision quenching. The specific dependence of gain on pump rate suggested that a reduced rate of recombination of molecular ions with increasing gas temperature was partly responsible for this behavior.

In another study pertaining to this system, Kushner and colleagues predicted gain of the 1.73 ^m [5d(3/2)1-6p(5/2)2] and 2.03 ^m [5d(3/2)1-6p(3/2)1] atomic xenon transitions for gas temperatures between 290 and 590 K [38]. Fission — fragment excitation was used to generate the gain in Ar/Xe, He/Ar/Xe, and Ne/Ar/Xe gas mixtures at a pump power of 8 W/cm3. For constant gas density, the gain exhibited an approximate T~ngas dependence with n between 2 and 3. The primary reactions responsible for the temperature dependence of the gain were identified as dimer formation, dissociative recombination, and enhanced electron collisional mixing of the laser manifold due to an increased electron density.

In a following study [39], Kushner and Shon discussed excitation mechanisms and gain modeling of the high-pressure atomic Ar laser in He/Ar mixtures. The high-pressure (>0.5 atm) atomic Ar laser (3d! 4p) oscillated on four infrared transitions (1.27-2.4 qm). Quasicontinuous oscillation on the 1.79 qm transition was obtained using electron beam and fission fragment excitation over a wide range of power deposition and gas pressure. In this regard, a computer model was developed to investigate excitation mechanisms of the Ar laser. Results from the model suggest that the upper laser level of the 1.79 qm transition [Ar(3d[1/2]1)] is dominantly populated by dissociative recombination of HeAr+. In contrast, the dissociative recombination of Ar+2 was believed to predominantly produce Ar (4 s) states. Electroionization from Ar metastables at moderate to high pump rates was likely to be responsible for the high efficiency of the Ar laser. Gain and laser oscillation were discussed and compared to experiments for He/Ar gas mixtures using various Ar mole fractions and total pressures. These results showed that the optimum Ar mole fractions in He/Ar mixtures are ~0.1-5 % for quasicontinuous pumping.

In a study aimed at understanding the pumping source term for coated wall type lasers, Miley and colleagues studied the transport of heavy charged particles produced by the 10B (n, a) nuclear reaction using a mean-range straight-flight model [23]. The slowing down of these particles in a gas adjacent to the coating where they are born is described in terms of their flux energy spectrum, scalar flux, average energy, and energy-loss rate. These results were used in a plasma kinetics model which was then compared to measurements of metastable excited state densities in helium and neon plasmas created by the heavy charged particles. The resulting plasma kinetic model was shown to agree reasonably well with measure­ments involving 10B-coated gas containers inserted in the Illinois pulsed TRIGA reactor. Ne(1S3) and Ne(1S5) densities ranging from 2 x 108 to 5 x 109 atom/cm3 and 3 x 109 to 4 x 1010 atom/cm3, respectively, were obtained in neon at 16.5 Torr with fluxes from 1012 to 1015 n/(cm2 s). Maximum He(21S) densities of about 10~4 atom/(cm3 unit flux) were obtained in helium at pressures above 200 Torr. Such measurements, not previously available, were the first attempt to directly evaluate excitation processes in a plasma created in a nuclear reactor core.

In a related study, P. E. Theiss and Miley developed a method to evaluate the spatial dependence of the energy spectrum of charged particle currents entering a process fluid from a bordering solid fuel region [40]. This enabled calculation of spatially dependent ionization-excitation rates including the general case where the formation rate varies with particle energy. Non-dimensional plots permit appli­cation of the results to various fluid-fuel combinations. Also, with adjustment of two parameters, these results can also be used for various particles including fission fragments, betas, alphas, and protons. The example of excitation of helium by alpha irradiation is also presented in some detail in study [40].

Another concept that is important to future NPLs involves uranium-pumped nuclear plasmas. In a study focusing on this, Maceda and Miley developed a model for the atomic levels above ground state in neutral, U0, and singly ionized, U+, uranium based on identified atomic transitions [41]. They found 168 states in U0 and 95 in U+. A total of 1,581 atomic transitions were used to complete this process in this study. Atomic inverse lifetimes and line widths for the radiative transitions as well as the electron collisional cross sections were presented. They identified 285 transitions in U0 and 278 transitions in U+, and during the course of the research employed 1,581 atomic transitions. The final compilation of these data is given in study [41].

As one approach to the use of an uranium plasma for NPLs, Y. R. Shaban and Miley proposed a hybrid uranium-core-laser reactor (UCLR) for advanced space missions that could radiantly transfer energy to a propellant or alternately active laser action [42]. The propellant mode would be employed in the phases of the mission requiring a higher thrust. However, for the bulk of the travel, the propellant would be turned off and the ultrahigh specific impulse laser mode of operation would be employed. The concept was further developed, research and development issues were identified, and the different operational modes were discussed in study [42].

In another study concerning uranium plasma pumping, Maceda and Miley [43] computed the opacity and radiative-energy current due to line radiation was calcu­lated for a U235 plasma with a temperature range 5,000-8,000 K. Also, a variation in the neutron flux of 2 x 1012 to 2 x 1016 neutrons/(cm2 s) was considered. The plasma formed a cylinder with a diameter and height of 1 m. Because the electron states in uranium lie below 5 eV, recombination is the principal excitation mecha­nism. Inversions were found above 6,000 K and, at all temperatures, the line radiation at line center is greater than the corresponding blackbody radiation. An example of this is the 28,763-5,762-cm~1 transition in neutral uranium, where the Planck function at 5,000 K is 6.49 x 10-6 ergs/cm2, and the calculated radiative — energy current is 1.492 x 10~4 ergs/cm2. Negligible changes in the radiative-energy current were observed for changes in the neutron flux at a given temperature. As a benchmark, it is noted that the opacity at 5,000 K agrees well with earlier calcu­lations done by Parks and colleagues [44], while recombinational excitation explains the variation in the opacity with temperature.

Beam profile control is an extremely important problem for NPLs due to the lensing effect caused by heating during pumping, especially for a wall coating type system. Thus several theoretical studies of this effect were performed at Illinois [45]. NPLs potentially offer an attractive method for high power laser applications such as a space power beaming. However, thermal gradients created by the pumping of the ion medium produce a dynamic “thermal blooming” (lensing effect due to temperature profile effects) which must be understood and controlled for accurate beam focusing. In paper [45], Petra and Miley performed basic experi­mental studies of thermal blooming under a variety of conditions presented. It was found that the laser pulse length could be severely restricted by thermal blooming, but some kinetic control is possible by changing the driving pulse shape. One promising approach found for reducing the effect was to employ a combined volume-wall pumping technique. Petra provided calculation of optimal coating and 3He concentrations to achieve minimum thermal blooming.

In study [46], Petra and Miley considered the issue of control of the extracted beam quality of a NPL. One possible control method involved changing the spatial power deposition profile, which is a function of the gas pressure and the pumping

3 10 235

mechanism. Thus, we proposed a combined He and wall coating (e. g., B or U ) technique. Then, the pump power deposition profile could be adjusted by varying the 3He partial pressure. To examine this technique, measurements were performed with an in-core cell by imaging the radiation-induced fluorescence onto a linear diode array located some distance from the cell. Results from these measurements are presented in reference [46], and other related issues such as gas dynamics, index of refraction effects, and efficient beam extraction from a large-volume cavity are discussed.

In an effort to simplify calculational methods, R. H. Lo and Miley developed an integral balance technique for calculation of the electron energy distribution in a radiation-induced plasma [47]. Results predicted W-values reasonably well and compared favorably with more complicated Monte-Carlo calculations. As expected, results showed that the electron energy distribution was non-equilibrium with a high energy tail but lower average energy. This distribution differs from one using a normal electrical discharge and favors excitation over ionization. This then provides insights into how to seek NPLs which use a laser medium with energy levels receptive to such a distribution, illustrated in Fig. 13.4.

In addition to the source electron energy distribution, other reaction kinetics such as recombination play an important role in laser performance, as illustrated by a Monte Carlo simulation model for radiation-induced plasmas with nonlinear properties due to recombination [24]. This Monte Carlo model employed a piecewise-linearized predict-correct technique. Several variance reduction tech­niques were used, including antithetic variants. The resulting code was applied to the determination of the electron energy distribution for a noble-gas plasma created by alpha-particle irradiation. Results are presented in reference [21] for helium with an electron source rate from 1014 to 1018 electrons/(cm3 s), initial energies from 70 to 1,500 eV, pressures from 10 to 760 Torr, and electric-field-to-pressure ratios from 0 to 10 V/(cm Torr). The low-energy portion of the distribution function approached a Maxwellian for zero electric field and Druyvesteyn’s distribution with an applied electric field. However, above the ionization potential and extending to the source energy, a parabolic-shaped distribution (tail) occurred. This distribution is not too different from the one found in [47], although the effect of recombination on it is clearly discernable.

Due to pump source restrictions, even with a fast burst reactor NPLs typically exhibit relatively long (micro — to millisecond) pulse lengths with modest peak powers but with very high total energy [48]. These pump power restraints seriously limit the choice of laser media, favoring energy storage type lasers. One way to avoid this constraint is to employ a Nuclear-Driven Flashlamp (NDF) for the primary pumped element in the system. The fluorescence from this NDF can then be used for pumping a laser or for other high intensity light applications. The first experimental example of this approach was a 3He-XeBr2 NDF employed by Williams and Miley [49] to pump a small iodine laser. Study [50] discussed issues

ENERGY (»V)

involved in scaling such an NDF up to high power levels. Possible optimum configurations included use of microsphere or fiber pump elements dispersed in the NPF media. Analysis of such possibilities is presented along with consideration of special reflecting surface designs in references [49—51].

Another very important potential NPL laser involves the pumping of the iodine line. Miley reevaluated the existing data in study [52]. At the time, the chemically pumped O2-I2 transfer laser was gaining interest as a high-power laser for a variety of applications. Thus the nuclear-pumped version would be very significant and as already discussed seems to offer an electron energy distribution favorable to this system. Indeed, Garschadin had previously shown that an electrically pumped version faced obstacles due to competition between ionization and unwanted

ionization and excitation. (After years of work, additives to combat this problem have finally enabled an electrically driven version, termed the “e-COIL” [53].) An interest in alternate pumping techniques for some situations remained, however. In study [54] Miley reviewed earlier work on the potential use of nuclear pumping, either for direct pumping of O2 or for pumping of an excimer flashlamp in a photolytic iodine laser system. Such lasers are attractive for space power beaming systems where the compact, high-energy system achievable with a pulsed reactor is desired. For a lower pressure transfer laser system, direct nuclear pumping of O2 to create O2(1A) is the simplest, yet efficient approach. For higher-pressure lasers, e. g., ^ 1 atm, the use of a nuclear-pumped excimer appears to be attractive. In that case, a transfer laser can be developed through photolysis of O3 to generate O2(1 Д), or the flashlamp can be used directly in a photolytic iodine laser system. The choice among these options largely depends on the specific application as this translates into system requirements.