Rare gases are a component part of virtually all gas NPLs. Helium, neon, or argon are used as buffer gases, while lasing is achieved at a number of transitions of Xe, Kr, Ar, and Ne atoms, so the luminescence spectra of rare gases are of the greatest interest. Unfortunately, in the scientific literature one can to find information only about the luminescence spectra in the visible and near IR spectral ranges (X < 1.2 pm), which may be explained by the absence of sensitive photodetectors for the longer-wave radiation. Table 4.12 shows a list of the studies in which the most significant results were achieved.

Despite the fact that study of the luminescence characteristics of rare gases excited by ionizing radiation (electron and ion beams, nuclear reaction products) began more than 50 years ago, investigations in this area cannot be deemed complete. As a rule such investigations were conducted under different experimen­tal conditions for a limited number of gas media and pressures, which makes it difficult to compare results. Studies [40, 82, 87, 88,91] carried out measurements of the absolute intensities of spectral lines.

The most complete information is contained in [89—91], in which all the rare gases and their binary mixtures that were of most interest from the standpoint of NPLs were studied under identical experimental conditions. The gas mixtures were excited by U fission fragments escaping from a thin uranium layer when the U nuclei interacted with neutrons. A VIR-2M-pulsed reactor was used as the neutron source. As a result of the investigations, information was obtained on the intensity of the roughly 300 spectral lines belonging to the transitions of Hel, Nel, ArI(II), KrI(II), and XeI(II). Also observed were lines belonging to transitions of the molecules N2 and N^, and the atoms O, N, and C, which were there because of the presence of impurities N2, CO2, CH4, O2, H2, and H2O in the rare gases, with a total concentration of no more than 5 x 10~4 %. The intensities of certain of them (for example, the 777.4 and 844.6 nm lines of the O atom) were comparable with the intensities of the basic lines of rare gas atoms.

The calibration of spectral apparatus performed in study [91] by using a refer­ence source made it possible to determine the absolute values of radiating powers on individual lines in the range of 740-1,100 nm, while the conversion efficiencies obtained for the mixtures He-(Ar, Kr, Xe) conform well with the results of studies [87, 88]. Virtually all of the most intensive lines belong to the transitions (n + 1)p-(n +1)s of the atoms Ar, Kr, Xe (n = 3,4, 5 for Ar, Kr, Xe respectively). The levels (n + 1)p are the lower levels for numerous laser IR transitions in the 1-4 pm region, arising on excitation of rare gases by nuclear radiation (see Chap. 3, Sect. 3.1). The maximal conversion efficiencies (~0.1 %) were registered for the lines 912.3; 965.8 nm (Arl, mixture of He-Ar); 892.9 nm (KrI, mixture of He-Kr) and 980.0 nm (Xel, mixture of Ar-Xe).

The neutron flux from the nuclear reactors used in experiments involving NPLs varies noticeably at distances that considerably exceed a laser cell’s transverse dimensions [2—14]. Thus, it can be assumed that fission density distribution in a uranium layer, as well as gas temperature and density distribution at any given point within a cylindrical cell, are essentially only dependent upon the radial coordinate, r, and the axial coordinate, z, which coincides with the axis of symmetry.

More precisely, the nonstationary thermal gasdynamic processes taking place in a laser of the type under conditions only permit modeling to be performed using a computer. But even for a geometry that is dependent upon two spatial variables, such calculations prove to be quite difficult and require large outlays of computer time. Therefore, the first investigations of this type were conducted for alternatives involving dependence upon just one spatial variable; the primary objective of these investigations consisted of determining the scale of the inhomogeneities occurring

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S. P. Melnikov et al., Lasers with Nuclear Pumping,

DOI 10.1007/978-3-319-08882-2_8

and establishing the regularities of their dynamics. For simplicity’s sake, it was assumed that all physical parameters, including the distribution of specific energy release sources in the gas and in the uranium layer, are only dependent upon a single radial coordinate, r. Consequently, it was decided that the distribution of the thermal neutron fluxes initiating uranium nucleus fission is not dependent upon coordinates and equals the average value over the length of a cell.

It should be noted that the effect of viscosity forces is negligible. Actually, the order of magnitude of the gas velocity scale comes to u ~ І/Тщ, where І is a typical cell dimension (a transverse dimension of d ~ 1 cm or a longitudinal dimension of L ~ 1 m), and; т1/2 is the excitation pulse duration. The Reynolds number for the gas flow over the course of excitation is then Re = iu /v ~102-107 (here v represents kinematic viscosity, which for rare gas mixtures over the temperature and pressure variation ranges actually used comes to ~10-1 cm2/s), suggesting the negligible effect of viscosity on gas flow.

All the initial calculations were performed using the standard VNIIEF program described in study [15]. In ignoring viscosity, it becomes possible to undertake the numerical solution of the thermal gasdynamic problems described by:

the motion equation

du _ JV-1 dp

dt ds ds = 1pd(rN);

dr

the continuity equation

d (1=p) = Is 1u)’

and the energy equation

Here, E is the internal energy per unit mass of gas; Fp is the internal source power per unit mass; ki is the thermal conductivity coefficient; u is matter velocity; p is pressure; r is the matter particle Eulerian coordinate; s is the matter particle Lagrangian coordinate; T is temperature; and p is density. The N parameter char­acterizes system geometry: N = 1 for a planar geometry, N = 2 for a cylindrical geometry, and N = 3 for a spherical geometry.

Over wide temperature and pressure variation limits, the rare gases that serve as NPL active media can be quite accurately described as ideal. Therefore, a state equation was given in the form

P =(y — 1) cvpT, (8.1)

E = CvT, (8.2)

where у = cp/cv; cp, cv represents specific heat capacities at a constant pressure and a constant volume, respectively.

At the contact boundaries of the gas and the uranium-containing active layer, as well as the active layer and the metal tube (the substrate) to which this layer is applied, the equality of temperatures and heat fluxes was postulated. The substrate’s outer surface was considered to be heat-insulated. The heat capacity and thermal conductivity coefficients of the active layer and the substrate were approximated by a quadratic dependence upon temperature. Based on data [16], the dependences of the gas thermal conductivity coefficients upon temperature were described by a fourth-degree polynomial.

Over the course of irradiation, the gas distribution density in the cell is transformed with the passage of time. Accordingly, because the fragment energy losses per unit range are unequivocally linked to the density of the stopping medium, the specific energy deposition profile along the laser cell cross-section is also deformed. During early calculations, specific source distribution, Fp, in the gas was tied to Lagrangian coordinates in order to take this factor into account, and was given in the form

Fp(s, t) = Fp0(s)q(t).

Here Fp0(s) is the specific power deposition at a point with a Lagrangian coordinate of s in the unperturbed gas (at the reference time) attributed to fission output per uranium layer unit volume, and q(t) is the fission output per unit volume of this layer. The spatial distribution, Fp0(s), was determined using the method described in study [17]. A comparison [18, 19] of this approach to the technique in which the precise dependence of the Fp function on the gas density distribution at any given moment in time is used revealed that discrepancies in the density and temperature calculations using both procedures over typical durations of t1/2 < 3 ms do not exceed 10 % (also see Chap. 7, Sect. 7.2).

After specific energy deposition calculation techniques were worked out for the media with variable densities [19, 20] as mentioned above, the program was modified to make it possible to perform calculations with strict allowance for the effect of the density redistribution on the energy deposition.

The advantages of RLs over other types of laser systems are provided by high specific and absolute power capacities, which are practically unattainable in other types of lasers, a wide range of radiation wavelengths (from the near ultraviolet region to the infrared), the absence of intermediate steps in energy conversion, and flexibility of control. The consequences of this are a relatively simple construction, compactness and reliability, the system’s high self-containment, operability for 20 years, the potential to develop mobile systems with minimal mass and dimen­sions at a given energy, especially in range of high laser powers. Multiple repeat startups do not require additional fuel, laser materials, or other materials.

Without chemically aggressive media and a large amount of uranium, the RL is safe for storage, use, and the environment. The disadvantage connected with the presence of nuclear radiations during RL operation is sufficiently neutralized by biological protection methods.

Proposals for potential uses of NPLs and RLs already started to appear in the first publications, which discussed the potential for the direct transformation of nuclear energy into a laser radiation. A review [12] gives a list of published works and notes on potential uses for NPLs and RLs. Some noted are:

1. Long distance communication.

2. Energy transfer (obtaining energy for space probes or satellites from generating stations placed on the earth or moon).

3. Photochemistry (for synthesis of chemical compounds on an industrial scale).

4. Fog dispersion (resulting from the evaporation of drops of liquid).

5. Illumination of surfaces (illumination of cities is possible with a stationary orbit).

6. High temperature heating of materials at a distance.

Issues concerning the use of NPLs and RLs in stationary as well as pulsed and pulsed-periodic modes are discussed in almost all reviews (see Chap. 1) and in more detail in specific studies. The following variants have been proposed for using NPLs and RLs:

• To supply energy to satellites and other space objects and stations on the dark side of the moon [19, 38, 39].

• To clean space of “debris” [19, 38, 40, 41].

• Laser drivers to transfer space devices from one orbit to another [19, 42] and delivering loads to a near-earth orbit [19, 43, 44].

• Reactor laser drivers for inertial confinement fusion systems [45—47].

• Heating and processing the surfaces of different materials (welding and cutting, changing surface properties, and deposition of thin coatings) [19, 38].

• Technological processes destined to the production of nanoparticles [19, 38].

• Development of new types of neutron detectors [48, 49], which can be used, for example, in systems for rapid nuclear reactor protection systems [50]. In this case, the detector is a NPL, which is a threshold system actuated at a specific level of neutron flux.

The area of use of an RL depends, naturally, on its characteristics, primarily its power, wavelength, and laser beam divergence, and also on the mode and duration of its operation. At the present time, the most realistic options for RLs with thin films of uranium fuel have an overall efficiency (nrl) with respect to the nuclear energy released in the core that does not exceed 0.4 %: qrl = є x Ці, where є < 20 % is the share of the total energy absorbed in the gas laser medium, and ці < 2 % is the laser efficiency (ratio of output laser power to power deposition in laser medium). To increase Пгі and expand the area of application for RLs, it is necessary to search for more effective laser media in different spectral ranges and for RL designs that have more effective transfer of nuclear energy to the laser media.

In experiments with NPLs based on pulsed reactors, the specific power deposition q < 5 kW/cm3 (specific energy deposition up to 1 J/cm3), which can lead to a marked increase in the temperature of the gas medium, of up to ~1,000 °K at pressures of ~1 atm. An increase in the temperature in the process of pulse pumping can influence the output power and possibly be the reason for a cessation of lasing until the moment of attainment of the pumping pulse maximum for some mixtures (for example, see [8, 45, 66]). Marked heating of the active medium can occur in powerful stationary nuclear-laser devices, so that a study of the influence of heating temperature of the NPL active medium on their characteristics is of great interest.

The first experiments in this field were carried out at VNIITF for NPLs operating on transitions of the Xe atom. By now the influence of temperature on NPL characteristics has been studied for mixtures of He-Xe, He-Ar-Xe, Ar-Xe [33, 67, 68]; 3He-Ar-Xe [69]; Ar-Xe, He-Ar-Xe, Ne-Ar-Xe [70] and Ar-Xe, He-Ar-Xe, He-Ar, He-Ne-Ar [71]. In all the experiments, an electric heater was used to change the temperature of the gas media. In the experiments, a marked reduction (by a factor of ~2) in the energy parameters was recorded at comparatively low temper­atures (see Table 3.7). Substantially different data were obtained in study [72], in

which no changes were noted in the laser parameters up to a temperature of 900 K for an electroionization laser using a mixture of 3He-Ar-Xe at a pressure of 1.5 atm with preliminary ionization of the gas medium by nuclear reaction products 3He(n, p)3H.

Analysis of the data of Table 3.7 shows that the influence of temperature on the NPL parameters depends on the pressure (gas density), composition of the gas medium, and laser wavelength. The reasons for this influence have not been finally ascertained to this point, and are the subject of discussion.

The most likely reasons considered for the reduction in the energy laser param­eters with the growth in temperature were the following: the influence of temper­ature on the processes of formation of the population inversion, for example, collisional “quenching” and “mixing” of laser levels by the atoms of the buffer gas [69]; the reduction in the rate of formation of heteronuclear ArXe+ ions and “mixing” of laser levels by electrons [70]; and the destruction of ions ArXe+ during collisions with atoms of the buffer gas [73].

Another possible cause is contamination of the laser medium by outside gas impurities (in particular water vapor) as a result of their desorption from the cell walls as the temperature increases [68, 74, 75]. The electron attachment to mole­cules of H2O can lead to a decrease in the concentration of electrons, and conse­quently to a reduction in the populating rate of the upper laser level as a result of reduction in the rate of recombination processes.

Interesting information was obtained in studies [76, 77], in which, given the excitation of lasers using mixtures of Ar-Xe (A = 1.73; 2.03 pm) and He-Ar-Xe (A = 1.73 pm) by a beam of 32S9+ ions with an energy of 100 MeV (duration of rectangular pulses 20-50 ps, pulse repetition frequency 30-45 Hz), it was possible to study the separate influence of the temperature of the medium and H2O impu­rities on the laser parameters. The energy parameters of a laser using an Ar-Xe mixture decrease both with an increase in the active medium without H2O impu­rities, and with an increase in the content of H2O at constant room temperature. Thus for the mixture Ar-Xe (Р = 0.16 atm; 0.3 % Xe), the output power decreases by a factor of 2 when the temperature increases to 400 K. When there is an increase in the water vapor content in the Ar-Xe mixture (Р = 0.5 atm; 0.5 % Xe), a similar reduction is observed when the concentration of H2O is about 1 x 1015 cm-3. For mixtures of He-Ar-Xe (Р = 0.2 atm; 23 % He; 0.3 % Xe), an output power reduction by a factor of 2 occurs at a temperature of 520 K. In the opinion of the authors of [76, 77], the influence of water vapors lies not only in the reduction of electron concentration, but also in the collisional “quenching” of the upper laser level 5d[3/2]10 of the Xe atom, because the rate constant of the “quenching” process is very high and amounts to 4 x 10~9 cm3 s-1. The output power reduction observed in the experiments [77] with growth in temperature for mixtures without impurities of H2O testifies that the gas temperature influences the processes of formation of the inverse population of laser levels, which were noted previously.

Apart from the water vapors, the NPL active media also contain the impurities N2, CO2, O2, H2, etc., which can appear both because of an inadequate degree of evacuation of the laser cells and the desorption processes of these impurities from the cell walls. At high power depositions (q > 10 kW/cm3), which are achieved with the use of electron beams, the addition of molecule impurities sometimes leads to an increase in output power, which is due primarily to the reduction in the electron temperature [78]. In the case of NPLs operating at q < 5 kW/cm3, the presence of molecular impurities leads to a reduction in the energy parameters and an increase in the laser thresholds [79].

The largest number of studies were dedicated to analysis of the lasing mechanisms and calculations of the characteristics of NPLs using a He-Cd mixture radiating at transitions of the Cd+ion (A = 441.6, 533.7, and 537.8 nm) [4,106—112]. A diagram of levels of the Cd ion and atom with laser transitions is shown in Fig. 3.5 (see Chap. 3, Sect. 3.3).

At present one can consider the basic populating processes of upper laser levels to be established:

• Levels 4/2F05/2j7/2, which are the upper levels for transitions with X = 533.7 and 537.8 nm, are populated by means of charge-transfer processes

He+ + Cd! (Cd+) * + He (5.17)

with the formation of higher levels 6f, 6 g, 8d, 9 s, etc. [113] and subsequent cascade transitions in 4f states;

• Levels 5s22D5/23/2, which are upper levels for transitions with X = 325.0 and 441.6 nm, are populated by means of charge-transfer and Penning processes

He2+ + Cd! (Cd+) * + 2He (5.18)

He * (21:3S) + Cd! (Cd+) * + He + e (5.19)

As a result of processes (5.17)-(5.19), various states of the Cd+ ion can arise. In a number of studies, for example [108, 109], it was proposed that by means of these processes, all energy-allowed states are populated with a probability proportional to the statistical weight of the given state. This approximation is inapplicable to charge-transfer processes, the probability of which is maximal if the potential energy of the starting ion, with an accuracy up to kTg, is equal to the excitation energy of the final state of the Cd+ ion. At present there is some information about partial rate constants of processes (5.17)-(5.19) for various states of the Cd+ [ПО— 115] ion. Study [112], based on examination of the populating kinetics of individual levels of the Cd+ ion, concluded that the contribution of the charge-transfer reaction (5.18) to populating of levels 5s22D5/23/2 and 6s2S1/2 is roughly an order of magnitude greater than the Penning reaction (5.19).

Relaxation or quenching of laser levels can occur as a result of the ion conver­sion process

(Cd+) * + Cd + He! Cd2+ + He, (5.20)

of radiative transitions, inelastic collisions with electrons, and (for levels above state 6s2S1/2), the Penning process on its own atom

(Cd+) * + Cd! Cd+ + Cd+ + e. (5.21)

Reaction (5.21) was not studied experimentally. In models [4, 106], its cross­section was assumed to be equal to the cross-section of polarization capture.

Among the transitions of the Cd+ ion, much attention was given to the transition with X = 325.0 nm, which is similar to the transition with X = 441.6 nm [112,116] in terms of its luminescence and other parameters. However, lasing under the conditions of nuclear pumping was obtained only in the second of them [116, 117], although for pumping with electron beams, lasing was observed at the 325.0-nm line both in the pulsed mode [118] and in the CW mode [119, 120], while the minimal specific power deposition at the laser threshold was just 10 W/cm3 [120]. The reasons for the lack of lasing at the 325.0-nm line in nuclear pumping conditions have not as yet been finally ascertained. Possible reasons that have been considered include absorption of radiation at the 325.0-nm line by excited atoms Cd*(3P2), quenching of level 5s22D3/2 by helium atoms and plasma electrons and the influence of uncontrolled impurities [107, 110, 117, 120].

Neutralization of plasma in the He-Cd mixture occurs basically by means of dissociative recombination processes of Cd2 molecular ions with electrons. As a result of recombination processes, the formation of an inverse population is possi­ble between certain excited levels of the Cd atom. In experiments, lasing was registered at the lines 1.43 and 1.65 ^m [116] and 361.0 nm [118] with pumping of the He-Cd mixture by uranium fission fragments [116] and an electron beam [118]. The kinetic model [107] included processes with the involvement of excited Cd* atoms, and an attempt was made to calculate some laser characteristics for the 1.65 ^m line. In the opinion of the authors [107], the two-humped laser pulse observed at the 1.65 ^m line may be explained by quenching of the upper laser level by electrons.

Differences in the results of calculations for various models for NPLs based on a He-Cd mixture are generally explained by the shortage of data on rate constants of many important processes. The maximal computed efficiency was obtained at the 441.6-nm line and is < 1 % [106, 111].

Lasing during pumping of a He-Zn mixture by uranium fission fragments was observed at transition 4s22D5/2-4p2P3/2 (A = 747.9 nm) (see Table 3.8), and during pumping by an electron beam, it was observed at transition 5d2D5/2-5p2P3/2 (A = 610.2 nm) [121]. Lasing mechanisms of NPLs based on the transitions of Zn+ and Cd+ ions largely coincide. A kinetic model of NPLs based on a He-Zn mixture was examined in studies [4, 106, 121].

Just as for the He-Cd mixture laser, the main processes of populating the levels 4s22D5/2 3/2 of the Zn+ ion are the charge-transfer and Penning reactions of He2 ions and metastable atoms He*(213S) during interaction with Zn atoms, while for the higher states, it is the process of charge-transfer of He+ ions on Zn atoms. Depopulating of lower laser levels with A = 747.9 nm occurs due to radiative transitions, and that of the transition with A = 610.2 nm by means of the Penning reaction on its own atom (Zn+)* + Zn! Zn+ + Zn+ + e and collisional quenching by electrons. The results of calculation of characteristics of He-Zn mixture NPLs in optimal modes are given in Table 5.10.

Investigations of optical inhomogeneities in sealed NPLs have revealed that flowing of the gas medium through the laser cavity is a necessary condition for achieving CW lasing in the stationary and quasi-stationary excitation modes. Gas heating is a no less important factor that requires the implementation of gas flowing. So, for example, at a thermal neutron flux density of Ф «1014 cm~2 x s_1, the average specific power deposition for helium with an average density of p0 = 3.2 x 10~4 g/cm3 (at initial pressure and temperature of P0 = 2 atm and T0 = 300°K) that fills a cylindrical laser cell with an internal radius of r1 = 1 cm at a metallic uranium active layer thickness of <5u = 2 .8 x 10 4 cm comes to

2 , qc = dqc/dt = OfN^E^e ~ 10 W/cm.

During stationary pumping in the absence of gas replacement, this specific deposition will result in a gas temperature increase to T > 104°K over a period of 1 s.

One method for implementing continuous gas replacement is longitudinal gas flowing [1] (in the direction of a channel’s optical axis), similar to the one used in high-temperature gas-cooled reactors.

Two studies [40, 62] considered a nuclear-laser device that operates in pulsed — periodic mode with a 1 Hz frequency and a pulse duration of about 1 ms (Fig. 11.11). The schematic (Fig. 11.12) shows the photon path from its origin in the NOC to its absorption in the laser material. The gas medium is excited in the NOC by fission fragments escaping from the fissile material: uranium aerosol particles of several microns in diameter. Luminescent photons reflected by particles from the fissile material and NOC walls pass through the window and are then transmitted to the active laser medium using a light-conducting tube. The laser medium is manufactured as a plate and is a so-called “active mirror” (see, for example, [63]). A design analysis [40] of different configurations for the “con­tainer-tube” shows that the extraction efficiency of the light radiation from the NOC container and its transport to the laser medium may reach 70-80 %.

Amplifier based on an adaptive mirror

Fig. 11.12 Schematic of the transmission of luminescent radiation from the NOC container to the laser medium [40]

The reactor core consists of two parts which contain a mixture of aerosol uranium fuel (2-5 qm UO2 particles) and a luminescent gas medium. Between the parts of the core, there are three disks that contain 235U, 10B, and Be and rotate at different frequencies. These disks, which are modulators of the core reactivity, are used to realize the pulsed operation mode of the reactor. We note that a similar method for obtaining periodic bursts of fissions using external reactivity modula­tion is used in IBR-type pulsed reactors [64]. The parameters of the proposed reactor in studies [40, 62] are: core volume 2.3 m3; uranium load 0.5 g/cm3; pulse repetition frequency 1 Hz; average power 400 kW (170 W/cm3); and the peak neutron flux density 2 x 1016 cm-2 s-1.

Using light-conducting tubes, the luminescent radiation from the two parts of the reactor core is withdrawn beyond the biological shielding. Two flat mosaic blocks of laser elements are installed at the ends of the light-conducting tubes. Each of
these blocks consists of tens of “active mirrors” manufactured from 0.5 cm thick GSGG:Cr3+:Nd3+ crystals up to 20 cm in diameter. The suggested luminescent media is a gas mixture based on excimer alkali molecules with an assumed conversion efficiency of 30 %. Under these conditions, the estimated peak power of the laser radiation is 12 MW (average power is 12 kW) with a 3 % total conversion efficiency of the nuclear energy released in the reactor core into laser radiation.

1.1 Chronology of Main Events. Initial Phases of Research

This section briefly examines the last 50 years of research in the field of transfor­mation of the kinetic energy of products of nuclear reactions into laser radiation, that is, nuclear-pumped lasers (NPL). NPLs include lasers in the optical spectral range, which are excited directly by nuclear radiation or by using intermediate devices, for example, nuclear-optical converters (nuclear-excited plasma, scintillators).

The discussion of the problem of direct conversion of nuclear energy into laser radiation began in the 1960s, immediately after creation of the first lasers. Interest in this problem was prompted by the possibility of using powerful, compact, high — energy sources of nuclear energy (nuclear reactors, nuclear explosions) for laser pumping, and accordingly, the fundamental possibility of creating powerful lasers. At present, the concept of the reactor-laser (RL) is the most common embodiment of this new technology. The core of the RL consists of fissionable material and the laser medium. There the direct conversion of the escaping nuclear energy into laser radiation occurs, bypassing the intermediate stage of thermal energy.

Early experimental and theoretical studies of NPLs were primarily carried out in the United States and Russia. Table 1.1 shows the basic phases of NPL develop­ment. In some cases, the dates of events are provided (if they are reliably known to the authors), but not the dates of articles about these events. Articles sometimes appeared considerably later, since roughly until the end of the 1980s, because research on NPLs in certain laboratories was carried out in accordance with classified programs. The sequence by which individual articles reached the open press may be traced from the list of references.

In Russia, research on problems of NPLs began in the late 1960s virtually simultaneously at three institutes—VNIIEF (Sarov), the Kurchatov IAE (Moscow), and the MGU Institute of Nuclear Physics (Moscow). The neutron sources were pulsed reactors of VNIIEF and IAE. Roughly from the start of the 1970s,

© Springer Science+Business Media New York 2015 S. P. Melnikov et al., Lasers with Nuclear Pumping, DOI 10.1007/978-3-319-08882-2_1

Note: Nomenclature of scientific organizations

UAC United Aircraft Corporation (United States), ANL Argonne National Laboratory (United States), NL Northrop Laboratories (United States), UI University of Illinois (United States), UF University of Florida (United States), NASA, NASA Langley Research Center (United States), NARC North American Rockwell Corporation (United States), GA General Atomic (United States), MC Martin Company (United States), VNIIEF All-Russian Scientific Research Institute of Experimental Physics (Sarov), MGU Moscow State University (Moscow), IAE Kurchatov Institute of Atomic Energy, IYaF SO AN Institute of Nuclear Physics, Siberian Department, Academy of Sciences (Novosibirsk), GOI Vavilov State Optical Institute (St Petersburg), LLNL Lawrence Livermore National Laboratory (United States); Sandia: Sandia National Laboratories (United States), LANL Los Alamos National Laboratory (United States), FIAN Lebedev Physics Institute, USSR Academy of Sciences (Moscow), MIFI Moscow Engineering and Physics Institute (Moscow), VNIITF All-Russian Scientific Research Institute of Technical Physics (Snezhinsk), IOFAN General Physics Institute, USSR Academy of Sciences (Moscow), FEI Physics and Power Engineering Institute (Obninsk)

investigations into the problems of NPLs began to be conducted at MIFI (Moscow), from the end of the 1970s at VNIITF (Snezhinsk), and from the start of the 1980s at FEI (Obninsk). Some experiments using a stationary reactor and calculations of the kinetics of certain types of NPLs were carried out at the Nuclear Physics Institute of the Academy of Sciences of Kazakhstan (Alma-Ata).

In parallel with the experiments, theoretical investigations of NPLs were carried out, in which FIAN (Moscow) and IOFAN (Moscow) participated, in addition to the institutes mentioned previously. Experimental investigations were carried out with the use of electron beams at FIAN (Moscow), the Institute of High Current Electronics of the Siberian Division of the Russian Academy of Sciences (ISE SO RAN) (Tomsk), the Scientific Research Center for Technological Lasers (Moscow), and the Institute of Electrophysics of the Ural Division of the Russian Academy of Sciences (Yekaterinburg).

Elsewhere, experimental and theoretical investigations of NPLs were mainly carried out in the United States (see Table 1.1). The first proposals regarding NPLs and the first experimental investigations began somewhat earlier than in Russia. The most substantial contribution to the development and creation of NPLs was made by investigations carried out at the Sandia National Laboratories, University of Illinois, NASA Langley Research Center, and University of Florida. In addition to the laboratories and universities indicated in Table 1.1, a number of NPL investigations were carried out in the United States at the University of Missouri — Columbia, the Idaho National Engineering Laboratory, Oak Ridge National Labo­ratory, North Carolina State University, and Vanderbilt University. Some individ­ual investigations of NPLs were carried out in France (Laboratoire de Physique des Gaz et des Plasmas), China (Institute of Nuclear Physics and Chemistry of CAEP), and Japan (University of Tokyo, Tokyo Institute of Technology).

To model the conditions that occur in gas media of NPLs, in joint experiments carried out in Germany (Technische Universitat Miinchen) and in the United States (Rutgers University), beams of heavy ions (4He+, 32S+, 132Xe+, etc.) were used. A number of interesting studies (basically of a high-pressure xenon laser) were also carried out using fast electron beams at the University of Twente (The Nether­lands), Stanford University (United States), Naval Research Laboratory (United States), and Science Research Laboratory (United States).

The majority of published data relate to the search for NPL media and the study of their characteristics. Basic efforts were directed toward obtaining maximal efficiency of conversion of deposited nuclear energy into laser radiation (ці) and low lasing thresholds (qth). The maximal values of ці and the lowest lasing thresholds were registered for a laser based on IR transmissions of the atom Xe: П = 2-3 % (VNIIEF, 1976 [6, 11, 16]; Sandia Laboratory, 1989 [36]), qth ~0.02 W/cm3 (VNIIEF, 1985 [6, 11, 16]).

Several key issues were uncovered and addressed in the problem of searching for and studying active NPL media. These included specific features of gas medium pumping with heavy charged particles and the problem of severe optical non-uniformities. Computational and theoretical investigations [25, 48—50], and then later also experimental investigations of this type were begun at VNIIEF in

1970. Later on there were reports about the conduct of analogous investigations in the United States [34, 35].

Apart from the work related to the search for NPLs and study of their various characteristics, one should also note other issues studied relating to development of powerful NPLs:

• Investigations into the properties of nuclear-excited plasma (track structure of plasma, kinetics of plasma processes, luminescence characteristics).

• Selection of radiation-resistant optical and structural materials for NPLs.

• Development and creation of various nuclear-laser devices.

• Development of the fabrication method of thin-film uranium fuel (radiator of fission fragments and fuel for the reactor-laser (RL).

• Selection of the optimal design for a RL and creation of its experimental model.

• Evaluation of possible applications of nuclear-laser devices of various types. Many of the studies done in these areas are also considered in this book.

In the history of development of NPLs, some special scientific conferences have been organized to discuss the results of research into their characteristics, prospects for development, and possible applications. The most significant of these were:

1. Second Symposium on Uranium Plasmas: Research and Applications, Atlanta (USA), 15-17 November 1971.

2. First International Symposium on Nuclear Induced Plasmas and Nuclear Pumped Lasers, Orsay (France), 23-25 May 1978.

3. The specialist conference Physics of Nuclear-Excited Plasma and Problems of Nuclear-Pumped Lasers, Obninsk (Russia), 26-29 May 1992.

4. The second international conference Physics of Nuclear-Excited Plasma and Problems of Nuclear-Pumped Lasers, Arzamas-16 (Russia), 26­30 September 1994.

5. The third international conference Problems of Nuclear-Pumped Lasers and Pulsed Reactors, Snezhinsk (Russia), 16-20 September 2002.

6. The fourth international conference Problems of Nuclear-Pumped Lasers and Pulsed Reactors, Obninsk (Russia), 18-21 September 2007.

The widest international representation was involved in the last four confer­ences, in which scientists from Russia, the United States, Germany, France, China, and Japan participated.

Despite the large volume of research carried out with regard to NPL problems, the number of survey papers is comparatively small, namely: K. Thom, R. T. Schneider [2] (1972); V. Ya. Pupko [51] (1981); N. W. Jalufka [3] (1983); R. T. Schneider, F. Hohl [52] (1984); D. A. McArthur [13] (1991); A. I. Miskevich [53] (1991); G. H. Miley [40] (1992); E. G. Batyrbekov [54] (1994); A. A. Sinyanskiy [6] (1995); A. V. Karelin, A. A. Sinyanskiy, S. I. Yakovlenko [28] (1997); A. A. Sinyanskiy, S. P. Melnikov, [11] (1998); E. P. Magda [55] (1998). Most of the surveys were published some time ago; many of them contain infor­mation only about research by individual laboratories. The most complete survey

[28] is dedicated to the energy and threshold characteristics only gas NPLs excited with the use of pulsed reactors, and to certain problems of the RL.

To create effective NPLs, it is necessary to use high pressures of the gas medium, close to atmospheric, because it is at such pressures that effective absorp­tion of the kinetic energy of nuclear particles (uranium fission fragments, a parti­cles) becomes possible at the path length of several centimeters, which is comparable with the characteristic transverse dimensions of laser cells. This is why, in the initial stages of research into NPLs, when atmospheric pressure gas lasers were still unknown, primary attention was given to liquid — and solid-state laser media. Studies dealt primarily with the condensed media known at the time: ruby and neodymium lasers, liquid lasers based on organic solvents activated with europium, and inorganic aprotonic solvents activated with neodymium. Attempts to achieve lasing during pumping of condensed media with nuclear radiation did not yield positive results, although in VNIIEF’s experiments with the pulsed TIBR reactor, superluminescence was registered (X ~610 nm) with the use of a solution of Eu(BTFA)4 HDPhH in acetone [6,7]. The principal reason for the absence of lasing with the use of condensed media was their radiation damage: radiation defects of the crystal lattice in solid-state lasers, radiolysis and the formation of gas bubbles on the tracks of nuclear particles in liquid lasers. The problems of such NPLs are examined in detail in Chap. 11.

Interest in the study of gas NPLs grew in the late 1960s, when powerful gas-discharge CO2 lasers (X = 10.6 ^m) were developed with a gas pressure of around 1 atm (for example, see monograph [56]) and lasing was obtained in the IR spectral range with excitation of atmospheric-pressure rare-gas mixtures with an electron beam [10]. The results of experiments performed before 1972, in which attempts were made to pump gas media with nuclear radiation, are shown in Table 1.2. As is noted in the review paper [2], none of these experiments unequiv­ocally demonstrated the presence of lasing, although indirect evidence was cited. Table 1.2 also includes experiments on the study of the influence of nuclear radiation on the parameters of gas-discharge CO2 lasers, which can be viewed as a preliminary phase on the path to finding gas NPLs.

The initial unsuccessful attempts at direct pumping of gas media with nuclear radiation did not stop studies to find gas media for NPLs, and soon after, in 1972 [6, 11] (Russia, VNIIEF, He-Xe mixture) and in 1974 [12, 13] (United States, LANL, SF6-C2H6) lasing was achieved when gas media were pumped with uranium fission fragments and with y-radiation from a nuclear explosion, respectively.

NPLs excited by products of nuclear reactions occurring at the interaction of the nuclei of several isotopes with thermal neutrons were considered earlier in this chapter. There is yet another method of pumping, using fast neutrons. It was proposed at VNIIEF and was used to excite NPLs operating on transitions of the Xe atom [181]. This method is based on elastic scattering of fast neutrons on atoms (nuclei) of the medium, and makes it possible to homogeneously excite gas media at pressures of tens and hundreds of atmospheres. For the efficient transfer of

energy from fast neutrons to recoil nuclei, it is advisable to use buffer gases with a low atomic weight, for example helium or hydrogen. Direct ionization and excita­tion of the gas medium is carried out by recoil nuclei. For helium at a pressure of 100 atm, the flux of fast neutrons with an energy of 1 MeV is weakened by a factor of e over a length of approximately 60 cm, so that it is possible to realize a quite homogeneous pumping of gas media at pressures of ~100 atm with characteristic transverse dimensions of ~1 m.

Gas lasers were first pumped with fast neutrons at VNIIEF in 1981 in experi­ments with the BR-1-pulsed reactor; the results were published in 1990 [181]. Suc­cessful pumping of the lasers became possible thanks to the BR-1 reactor, in which, in one of the operating modes, the reactor core can be separated to create an internal gap (slot) up to 10 cm in height.

The experiments used a cylindrical laser cell with an internal diameter of 3.5 cm and an active length of 40 cm, placed in the gap between the halves of the reactor core (see Fig. 2.1c). The fluence of the fast neutrons with an energy of more than 0.5 MeV was equal to 5.6 x 1014 cm-2, while the neutron flux density at the pulse maximum was Фтах = 6.4 x 1018 cm-2 s-1.

The specific power deposition near the axis of the laser cell at the pulse maximum may be evaluated from the expression q = [He]a! ФтахЕа ([He] is the concentration of helium atoms, us = 6.5 x 10-24 cm[9] is the cross-section of elastic scattering of neutrons with an average energy of En = 1.3 MeV in helium nuclei, Ea is the average energy of a recoil nucleus). With isotropic scattering, Ea = 2AsEn/ (As + 1)2, where As is the mass number of the atoms of the medium. The specific power deposition is proportional to the pressure of the helium and for the conditions cited above, q « 80 W/cm[10] atm. The energy deposition owing to the absorption of у radiation was approximately two orders of magnitude lower.

The mixture He-Xe was used to prove the possibility of pumping high-pressure gas lasers with fast neutrons; it was quite well studied from pumping with nuclear reaction products (see Sect. 3.1) and involves a buffer gas of low atomic weight. In experiments, the changes in laser power and threshold characteristics of a He-Xe laser (A = 2.65 pm) as a function of the pressure and composition of the mixture were studied. An oscillogram from one of the experiments is shown in Fig. 3.8. The dependence of the laser parameters on the helium pressure at optimal xenon pressure of around 4 Torr is shown in Table 3.11. A maximal output power of 290 W (ni~0.2 %) and minimal value of ФгА = 1.9 x 1018 cm-2 s-1 were registered for the mixture He-Xe (950:1) at a pressure of 5 atm. These energy parameters evidently are not maximal, since the laser cavity parameters were not optimized.

At present, among NPL applications being considered is the possibility of creating powerful reactor-laser facility (see Chap. 10), in the reactor core of which the kinetic energy of fission fragments is converted into laser radiation. Investigations in the field of neutron pumping of lasers can be of interest for future power plants based on controlled thermonuclear fusion, in which the bulk of the energy is released in the form of kinetic energy of fast neutrons.

Fig. 3.8 Oscillograms of fast neutron pulse of reactor BR-1 (a) and laser pulse (b) for the He-Xe mixture (2,000:1) at a pressure of 5 atm [181]

7.1 Comparison of Basic Methods of NPL Excitation

As was noted in Sect. 1.2 of Chap. 1, the nuclear reactions that can be used for pumping gas NPLs in experiments with pulsed reactors are:

When reaction (7.1) is used, the laser medium is excited by protons and tritium ions. In this case, 3He plays the role of a buffer component of the laser mixture and is simultaneously a volume pumping source. Calculations of the specific energy deposition to the gas medium when the reaction (7.1) is used were carried out in the studies [1, 2]. The gas pressure and the transverse dimension of the laser cell were varied (it was assumed that the length of the cell greatly exceeded its transverse dimension). It was shown that for each pressure value there is an optimal transverse dimension of the cell. When the dimensions are less than optimal, the losses of reaction (7.1) products on the walls of the cell become great. When the dimensions are greater than optimal, the decrease of the neutron flux in the direction from the cell boundary to the depth of the gas volume starts to have an effect. Some results of calculations are provided in Figs. 7.1 and 7.2. It should be noted that the model [2] makes it possible to perform calculations for different neutron spectra when more precise deceleration principles of charged particles are used. The results of the calculations in studies [1, 2] for the case of a mono-energetic flux of thermal neutrons differ by 10-15 % (Fig. 7.1).

The gaseous compound 235UF6 can be used in the capacity of a volume pumping source along with 3He. Calculations of the energy deposition for the mixture Не — UF6 for different pressures and concentrations of UF6 are provided in

© Springer Science+Business Media New York 2015 S. P. Melnikov et al., Lasers with Nuclear Pumping, DOI 10.1007/978-3-319-08882-2_7

Fig. 7.2 Total absorbed energy Ei per unit of length of an infinite cylindrical tube depending on its radius r1 with a thermal neutron fluence of 1 cm-2 [1]. Numbers next to the curves indicate the 3He pressure in atmospheres

study [3]. However, the search for nuclear-pumped gas media based on UF6 has not yielded a positive result in experiments to date. The issue involved is attributed to the high quenching rates of excited atoms by UF6 molecules [4, 5].

Thin layers of materials containing the isotopes 10B or 235U (surface sources) are used to excite the active mixtures by means of the reactions (7.2) and (7.3).

However, in this case the efficiency of energy transfer to the gas mixture by the nuclear reaction products is limited to <50 % (for an ultra-thin layer) owing to the

isotropy of their recession. For uranium layers, when conditions are optimal, this efficiency is 10-20 %. To our knowledge, the first calculations of the energy deposition from the surface source according to the reaction of (7.2) were carried out in study [6].

From the energy standpoint, the use of 3He as the volume pumping source is as efficient as the use of thin uranium layers. Indeed, as an example let us examine two cylindrical cells of identical geometry. Let metallic 235U be applied to the internal surface of one of them with an optimal thickness <5u, the average fission fragment [7] in the uranium, that is, 5U « cell is filled with a laser mixture that does not contain 3He. Then the total energy transmitted to the gas by the fission fragment in a unit of time is

W = 2лr18ULcOfNUФE0 є, (7-4)

where E0 is the total kinetic energy of the fission fragment of the uranium nuclei; Lc is the length of the cell; NU is the density of the uranium nuclei in the active layer; r1 is the internal radius of the cell; є is the efficiency of energy transmission to the gas by fission fragment (ratio of the energy transmitted to the gas by fragments to the total kinetic energy of the fragments released in the uranium layer); cf is the cross section of uranium fission by the neutrons; and Ф is the neutron flux density.

Let the second cell not have a uranium coating, but be filled with a gas mixture whose basic component is 3He. The total energy released in the gas volume of this cell per unit of time is

WHe = ПГ^сСНє^єФЕнєЄНє, (7-5)

where NHe is the density of the 3He; EHe is the kinetic energy of the reaction (7.1) products; єНє is the efficiency of energy transmission by the products of reaction (7.1) to the gas; cHe is the cross section of the reaction. From Eqs. (7.4) and (7.5) we have

WHe_ r 1 cHe NHe EHe £He

W 2SU cf NU E0 є

To conduct a comparative assessment, we assume that the basic component of the gas mixture in the first cell is 4He and the gas pressure in both cells is «2 atm; the transverse dimension of the cell r1 ~ 1 cm. With such correlations between pressure (which at T0 = 300 K corresponds to NHe « 5 x 1019 cm~3) and transverse dimen­sion of the cell, the decrease of the neutron flux owing to the interaction with 3He is not great, and it can be ignored in estimates. On the other hand, the accepted correlation of the radius and pressure of a gas laser excited by fission fragment and not containing 3He is close to optimal [7]; at the same time, є « 0.16. The value of є№ for these parameters is in the limits of 0.7 < є№ < 1 and is determined by the influence of the cell walls on the energy deposition of the reaction products. Assuming for thermal neutrons cf = 582 x 10~24 cm2, cHe = 5,327 x 10~24 cm2,

and considering that NU = 4.8 x 1022 cm-3 (metallic uranium), E0 = 168 MeV, EHe = 0.76 MeV, from the last equality we find WHe/W ~0.5 r1eHe~0.5 r1 cm-1. Consequently, in both cases, the energy depositions can be comparable in amount. However a serious restriction for pumping the gas medium with reaction (7.1) is necessary to use component 3He in the capacity of the buffer component, greatly limiting the choice of gas mixtures for the laser media.

This limitation is eliminated when the reactions (7.2) and (7.3) are used. The total energy transferred to the gas per unit of time by products of the reaction (7.2) is

Wb = 2лrl8вLcaвNвФEaЄв■ (7.6)

Here Ea is the kinetic energy of the a-particles; NB is the density of the nuclei of 10B in the active layer; £B is the thickness of the boron layer; eB is the efficiency of energy transition to the gas by a-particles; aB is the cross section of the reaction (7.2). From expressions (7.4) and (7.6) we find:

W 8UeofNUE0

W в £b£b°bnbeb

An a-particle that appears as a result of a reaction (7.2) can travel from the boron layer into the gas mixture only if it was born at a distance from the coating-gas interface that is less than its stopping range in the coating (source) material. Therefore, the maximal effective thickness of the boron layer should not exceed the stopping range of an a-particle with an energy Ea = 2.38 MeV which is about £B « 0.6 x 10-4 cm with a density of boron nuclei of NB = 0.136 x 1024 cm-3. The maximal value of efficiency of the energy deposition, as already mentioned, cannot exceed 50 %, that is, eB < 0.5. Assuming laser cells with uranium or boron layers are fully identical to the above cell in regard to geometry and gas mixture compo­sition, then with allowance for the fact that for thermal neutrons, ffB « 3,840 x 10-24 cm2, we find W/WB > 5.6. The advantage of using uranium layers with their larger energy release is evident.

Because the absorption cross section for thermal neutrons by the isotopes 3He and 10B is quite large, the placement of laser cells with such isotopes in the core of the nuclear reactor can lead to a perceptible reduction in its core reactivity margin, and if the number of cells becomes too large, this may also bring the reactor to a subcritical state. But cells with 235U layers can simultaneously play the role of fuel elements in the reactor core, avoiding a reduction in criticality. Furthermore, if the configuration is optimal, an autonomous reactor-laser may be formed with an adequate super-criticality margin, using a core entirely assembled from such laser cells (channels).