**2. Methods and sources of gas excitation by nuclear reaction products**

NPLs include active media that are excited directly using nuclear radiation, or with the use of intermediate nuclear-optical converters. There are three basic sources of nuclear radiation, which can be used to pump NPLs or convert nuclear energy into light energy on transitions of atoms and molecules:


Using the γ-ray radiation from nuclear explosion as the pump source was apparently done for the first time in VNIIEF (All-Union Scientific Research Institute of Experimental Physics) in 1971 [3]. Xenon emitted as Xe2\* excimer molecules was used as an active media. Experiments on xenon excimer laser pumping (λ ~ 170 nm) were done in a testing area in Nevada in 1973 [4]. Experiments to develop NPLs using nuclear explosive devices were carried out up until 1987 when underground test ban was introduced.

Optical radiation of gases excited by radioactive nucleus decay products (210Po, 238Pu, 239Pu, 241Am, etc.) was studied before to create gas scintillators [5, 6]. Radioactive isotopes usage for laser pumping [7] or pre-ionization in electric discharge lasers [8, 9] is limited by lower power density deposited in gas (up to 0.6 W/cm3 ). The main volume of works on NPL active media search and study of their parameters was performed on stationary and pulsed nuclearreactors. Nuclearreactors are the source of neutron and γ-radiation. Neutrons were usedto pump lasers, as in this case the energy input to laser medium is several times higher than due to γ-radia‐ tion. Direct pumping of active media is usually carried out not by neutron radiation but by nuclear reaction products with thermal neutrons (**Table 1**).


**Table 1.** Nuclear reactions [10] for NPL pumping.

Nuclear-pumped lasers have potential in a wide range of applications, especially in cases requiring high-power and compact lasers to be placed on autonomous remote facilities. The most promising areas of nuclear-pumped laser application are as follows: laser thermonu‐ clearfusion, long-distance transmission ofradiant energy and information, rocketlaser engine, laser isotope separation and photochemistry, stratospheric ozone layer recovery, and space junk removal. Considerable interest in this area research is also associated with significant difference between the mechanisms of level population during nuclear pumping and population processes in conventional gas-discharge lasers. Application of nuclear energy for active laser medium pumping can be considered not as the way to create high-power laser, but as the way to obtain energy from nuclear reactor. This necessitates consideration of fundamentally new equipment—a reactor laser designed to spatially combine nuclear laser active medium and nuclear reactor core. This approach opens up opportunity to generate

Attempts to achieve laser action during pumping of condensed media with nuclear radia‐ tion did not yield positive results. The main obstacle on the road of creating condensed media NPLs is their radiation damage: radiation defects of crystal lattice in solid-state laser, radiol‐ ysis, and gas bulb generation on the tracks of nuclear particles in liquid lasers. Presently known gas NPLs [3] radiate in spectral range 391–5600 nm in about 50 atomic transitions of Xe, Ar,

**2. Methods and sources of gas excitation by nuclear reaction products**

NPLs include active media that are excited directly using nuclear radiation, or with the use of intermediate nuclear-optical converters. There are three basic sources of nuclear radiation, which can be used to pump NPLs or convert nuclear energy into light energy on transitions

Using the γ-ray radiation from nuclear explosion as the pump source was apparently done for the first time in VNIIEF (All-Union Scientific Research Institute of Experimental Physics) in 1971 [3]. Xenon emitted as Xe2\* excimer molecules was used as an active media. Experiments on xenon excimer laser pumping (λ ~ 170 nm) were done in a testing area in Nevada in 1973 [4]. Experiments to develop NPLs using nuclear explosive devices were carried out up until

Optical radiation of gases excited by radioactive nucleus decay products (210Po, 238Pu, 239Pu, 241Am, etc.) was studied before to create gas scintillators [5, 6]. Radioactive isotopes usage for laser pumping [7] or pre-ionization in electric discharge lasers [8, 9] is limited by lower power

ions, CO molecules, and N2

+

). The main volume of works on NPL active media

molecular ion.

qualitatively new energy.

162 164High Energy and Short Pulse Lasers

Kr, Ne, C, N, Cl, O, I, Hg; Cd+

of atoms and molecules: **1.** nuclear explosions,

**2.** radioactive isotopes,

**3.** neutron radiation of nuclear reactors.

density deposited in gas (up to 0.6 W/cm3

1987 when underground test ban was introduced.

, Zn+

, Hg+

When nuclear reactors are used as neutron sources, two basic types of laser-medium excita‐ tion are utilized:


In the case of volumetric source of pumping using <sup>3</sup> He, the non-uniformity of pumping comes from the absorption of slow neutrons in 3 He and from the reduction of energy contribution in area near the wall, owing to removal of reaction products to the walls of the cell, in case of 235UF6—fission fragments energy loss on the walls of the cell. Results of computation of the total energy deposition and spatial distribution of the deposited energy depending on <sup>3</sup> He pressure and diameter of cylindrical cell are given in [11, 12], while [13] shows the results of computation for 235UF6-He at different pressure of mixture and content. In Ref. [14], authors show summary of results of experimental and theoretical studies dedicated to definition of energy contribution in NPL cells. Three experimental methods were considered: pressure shock method, interferometric method, and string calorimeter method. The cell size and path length of nuclear reaction products in the gas mixture determines spatial non-uniformity at surface pumping source use. Various calculation models for spatial distribution of energy deposition, influence of non-uniformity of uranium-containing layers, and analysis of experimental data on determination of uranium fission fragments energy loss in gas medi‐ um are given in [3].

Due to high 3 He and 10B neutron-absorption cross-section, loading laser devices using 3 He or 10B on the walls can significantly affect reactivity charge of nuclear reactor and even lead it to subcritical state. A laser cell containing 235U also serves as a fuel element of reactor. This has

triggered an idea of laser reactor, which must spatially combine active laser medium and nuclear reactor core [2, 15]. Initially considered option included uranium-235 hexafluoride serving as uranium-containing medium, the only uranium compound existing in gas phase at moderate temperatures. However, the use of 235UF6 complicated due to the chemical aggres‐ siveness of uranium hexafluoride and products of radiolysis. Laser radiation absorption by UF6 molecules, high speed of quenching of excited atoms and molecules in collisions with UF6, electron attachment to molecules of UF6 also prevents the use of uranium hexafluoride as a component of the laser mixture [16]. At present, the most realistic designs are heterogeneous reactor lasers using thin-film uranium fuel [17, 18]. The core of this reactor laser is a specific quantity oflaser cells with uranium layers appropriately placed in a neutron moderator matrix. With appropriate selection of components, the conditions for the reactor-laser operation are provided without utilizing additional fuel (uranium). The number of laser cells may vary from a few 100 to 1000, the total weight of uranium from 5 to 70 kg, and characteristic linear dimensions are 2–5 m [3, 18].

Studies in recent years were set out to explore opportunity to load uranium in active laser mixture in the form of fine dust with a particle size of 100–500 nm substantially lower than the path length of fragments [19]. In this case, it is possible to minimize energy loss in the fuel and substantially improve the efficiency and uniformity of pumping. It is necessary to ensure the transparency of such a mixture at the lasing wavelength. Theoretically, it is possible by arranging the dust particles in the form of periodic structure with a mutual distance compa‐ rable to laser wavelength, that is, in the form of dust crystals.

#### **2.1. Basic processes of formation and relaxation of nuclear-induced plasmas of gas mixtures**

Currently, direct nuclear pumping is implemented in gas media in which the populating of lasing levels occurs in a low-temperature plasma formed by ionizing radiation, in nuclearinduced plasmas. This section describes the basic processes of formation and relaxation of such plasma, in relation to active media of lasers with nuclear pumping and conversion of nucle‐ ar energy into the energy of spontaneous gas emission. The most complete information about the processes in plasma of active media of gas lasers with nuclear pumping is contained in monographs [3, 20].

*Initial stage of ionization processes in gas media*. Gas medium ionization occurs by various types of radiation: uranium fission fragments and transuranium elements, fast electrons, protons and tritons, lithium nuclei, α-particles, γ-quanta. Ionization in γ-radiation of gas is induced by fast electrons produced in the process of Compton scattering, photoeffect, and effect of electron-positron pair formation. In the initial stage of ionization process, primary ionization during the immediate interaction of charged particles and secondary ionization in interac‐ tion of media atoms with electrons formed as a result of primary ionization.

The process of ionization of an atom may be viewed as a binary collision of oncoming charged particle and one of the electrons of the atom's shell [21]. Due to the large difference in masses of heavy charged particles and the electron, only a comparatively small percentage of fragment energy can be transferred to the orbital electron. The spectrum of electrons produced by ionization of heavy particles is softer compared with the spectrum produced by ionization of gas by fast electrons [22, 23]. The average energy of electrons formed in neon as a result of ionization by fission fragments is 40 eV and fast electrons 150 eV [22]. In the case of fission fragments, the secondary electron may provide additional one or two acts of ionization on average, while in the case of fast electrons, it is from 5 to 10 [23].

triggered an idea of laser reactor, which must spatially combine active laser medium and nuclear reactor core [2, 15]. Initially considered option included uranium-235 hexafluoride serving as uranium-containing medium, the only uranium compound existing in gas phase at moderate temperatures. However, the use of 235UF6 complicated due to the chemical aggres‐ siveness of uranium hexafluoride and products of radiolysis. Laser radiation absorption by UF6 molecules, high speed of quenching of excited atoms and molecules in collisions with UF6, electron attachment to molecules of UF6 also prevents the use of uranium hexafluoride as a component of the laser mixture [16]. At present, the most realistic designs are heterogeneous reactor lasers using thin-film uranium fuel [17, 18]. The core of this reactor laser is a specific quantity oflaser cells with uranium layers appropriately placed in a neutron moderator matrix. With appropriate selection of components, the conditions for the reactor-laser operation are provided without utilizing additional fuel (uranium). The number of laser cells may vary from a few 100 to 1000, the total weight of uranium from 5 to 70 kg, and characteristic linear

Studies in recent years were set out to explore opportunity to load uranium in active laser mixture in the form of fine dust with a particle size of 100–500 nm substantially lower than the path length of fragments [19]. In this case, it is possible to minimize energy loss in the fuel and substantially improve the efficiency and uniformity of pumping. It is necessary to ensure the transparency of such a mixture at the lasing wavelength. Theoretically, it is possible by arranging the dust particles in the form of periodic structure with a mutual distance compa‐

**2.1. Basic processes of formation and relaxation of nuclear-induced plasmas of gas mixtures** Currently, direct nuclear pumping is implemented in gas media in which the populating of lasing levels occurs in a low-temperature plasma formed by ionizing radiation, in nuclearinduced plasmas. This section describes the basic processes of formation and relaxation of such plasma, in relation to active media of lasers with nuclear pumping and conversion of nucle‐ ar energy into the energy of spontaneous gas emission. The most complete information about the processes in plasma of active media of gas lasers with nuclear pumping is contained in

*Initial stage of ionization processes in gas media*. Gas medium ionization occurs by various types of radiation: uranium fission fragments and transuranium elements, fast electrons, protons and tritons, lithium nuclei, α-particles, γ-quanta. Ionization in γ-radiation of gas is induced by fast electrons produced in the process of Compton scattering, photoeffect, and effect of electron-positron pair formation. In the initial stage of ionization process, primary ionization during the immediate interaction of charged particles and secondary ionization in interac‐

The process of ionization of an atom may be viewed as a binary collision of oncoming charged particle and one of the electrons of the atom's shell [21]. Due to the large difference in masses of heavy charged particles and the electron, only a comparatively small percentage of fragment energy can be transferred to the orbital electron. The spectrum of electrons produced by ionization of heavy particles is softer compared with the spectrum produced by ionization of

tion of media atoms with electrons formed as a result of primary ionization.

rable to laser wavelength, that is, in the form of dust crystals.

dimensions are 2–5 m [3, 18].

164 166High Energy and Short Pulse Lasers

monographs [3, 20].

However, differences in the effects on gas media by different types of ionizing particles are not substantial, because the ultimate result is a combined effect of primary and secondary ionization. It follows from calculations in that the electron energy distribution and energy formation of electron-ion pair in the gas does not depend on the type of charged particles [24, 25]. The same conclusion can be drawn from the luminescent properties of plasma and gas NPLs output parameters, which do not depend on the type of charged particles, but depend on the power and duration of pumping [18].

**Figure 1.** Electron energy distribution in the ionized gas. 1—primary electrons of source; 2—electrons of ionization cascade; 3—electrons in the inelastic excitation region; 4—thermal and subthreshold electrons.

Usually, the calculation and analysis of parameters of nuclear-induced plasmas, develop‐ ment of laser action and NPLs radiation, ionization of gas medium are assumed to be homogeneous. One of the features of nuclear-induced plasma is associated with the forma‐ tion of tracks when passing through dense gas of heavy charged particles [26, 27]. Depend‐ ing on parameters of gas media, transverse dimensions of the tracks are 1–10 μm, and the track lifetime or the time to establish uniform ionization through diffusion is 0.1–1 μs [28]. Nonuniformity of ionization associated with track structure of plasma will be most noticeable in the following cases:


Fluctuations of plasma component concentrations induced by the track structure may have some effect on NPL characteristics excited by fission fragments, if population of upper levels is due to the fast charge process, for example, in laser on a mixture of He-Cd [28]. The influence of plasma track structure on recombination processes will be insignificant, as the track lifetime is much less than the characteristic time of recombination processes [29, 30]. Tracks' overlap‐ ping occurs at high pumping power densities; therefore, the track structure of plasma disappears. Estimates show that the overlap of tracks in atmospheric pressure helium occurs at excitation power densities ~2 W/cm3 [29].

Ionization of gas at the initial stage is carried out directly by charged particles and secon‐ dary electrons. A picture of the electron energy distribution in the gas is shown in **Figure 1**, where fe is the energy distribution function of the electrons, and Ee is the electron energy.

The entire electron energy range can be divided into three regions [24]:


At Ee > Im, the function of electron energy distribution differs greatly from Maxwell distribu‐ tion. Electrons in this region do not participate in recombination processes and this region supplies electrons in the subthreshold region. The electrons of inelastic region excitation and ionization cascade possibly play a major role in population of 3p levels of neon [30, 31].

#### **2.2. Kinetics of plasma processes at nuclear pumping of gas mixtures**

In quantum system, the gain (absorption) factor of the medium is described by [32]:

$$
\alpha = \sigma (N\_2 - N\_1 \frac{\mathbf{g}\_2}{\mathbf{g}\_1}) \tag{1}
$$

where indexes (1, 2) refer to upper level 2 and lower 1, *N*—level population, *g*—statistical weight of levels. The cross section of stimulated transition:

Nuclear-Induced Plasmas of Gas Mixtures and Nuclear-Pumped Lasers http://dx.doi.org/10.5772/63823 167 169

$$
\sigma = \frac{\mathcal{X}^2}{2\pi} \frac{A}{\Lambda \alpha \rho} \tag{2}
$$

where λ—transition wavelength, *Δω*—line width, *A*—transition probability. The amplifying medium (α > 0) requires maintaining population inversion: Population of the upperlevel must exceed population of the lower level (adjusted to degeneracy multiplicity). Formation of population inversion requires selectivity of population of the upper or lower level. The inversion can be provided not only by the predominant population of upper laser level, but also through selective cleaning of the lower level.

The active media of gas NPLs are often the double mixtures A-B (A—a buffer gas with a high ionization and excitation potential, B—a gas with a lower ionization potential, and lasing occurs in its transitions) or triple A-B-C. In triple mixtures, the third C gas usually plays the role of deactivator of lowerlevel and is not involved in upperlevel population, but can quench it to some extent. Therefore, we consider the kinetics of processes in plasma in the example a two-component mixture.

The first stage includes ionization and excitation of the buffer gas atoms A (formation of A<sup>+</sup> ions and excited atoms A\*), in some cases, direct excitation of active gas B [31, 33]. The main channels of energy transfer from A<sup>+</sup> and A\* to particles B are as follows:

**1.** Charge exchange processes

Fluctuations of plasma component concentrations induced by the track structure may have some effect on NPL characteristics excited by fission fragments, if population of upper levels is due to the fast charge process, for example, in laser on a mixture of He-Cd [28]. The influence of plasma track structure on recombination processes will be insignificant, as the track lifetime is much less than the characteristic time of recombination processes [29, 30]. Tracks' overlap‐ ping occurs at high pumping power densities; therefore, the track structure of plasma disappears. Estimates show that the overlap of tracks in atmospheric pressure helium occurs

Ionization of gas at the initial stage is carried out directly by charged particles and secon‐ dary electrons. A picture of the electron energy distribution in the gas is shown in **Figure 1**, where fe is the energy distribution function of the electrons, and Ee is the electron energy.

ionization potential of the gas atoms and molecules), in which the electrons energy is

**2.** In elastic excitation region Im < Ee < Vi (Im is the minimal threshold of electron or vibra‐ tional excitations), in which the energy of the electrons is reduced, primarily due to

**3.** In the subthreshold region (Ee < Im), electrons lose energy in small "portions" due to elastic collisions with gas particles, thus creating electron thermalization. In subthreshold region, the electrons are involved in the processes that are important in population kinetics and NPL levels' deactivation. These processes include the following: the electron-ion recom‐ bination, quenching of excited states, processes of attachment to electronegative gas,

At Ee > Im, the function of electron energy distribution differs greatly from Maxwell distribu‐ tion. Electrons in this region do not participate in recombination processes and this region supplies electrons in the subthreshold region. The electrons of inelastic region excitation and ionization cascade possibly play a major role in population of 3p levels of neon [30, 31].

2

= - (1)

1

excitation, and ionization in collisions with gas particles in excited states.

In quantum system, the gain (absorption) factor of the medium is described by [32]:

2 1

( ) *<sup>g</sup> N N g*

where indexes (1, 2) refer to upper level 2 and lower 1, *N*—level population, *g*—statistical

< Ee < E0 (E0 is the particle initial energy, Vi is the

[29].

The entire electron energy range can be divided into three regions [24]:

excitation of electron and vibrational states of the gas particles.

**2.2. Kinetics of plasma processes at nuclear pumping of gas mixtures**

a s

weight of levels. The cross section of stimulated transition:

at excitation power densities ~2 W/cm3

166 168High Energy and Short Pulse Lasers

**1.** The region of ionization cascade Vi

sufficient for ionization of gas particles.

$$A^\* + B \to (B^\*)^\* + A \tag{3}$$

$$A\_2^+ + B \to (B^+)^\* + 2A \tag{4}$$

**2.** Penning process (if A\* energy is higher than B ionization potential)

$$A^\* + B \to (B^\*)^\* + A + e \tag{5}$$

#### **3.** Excitation transfer

$$A \,\, ^\ast + B \to B \,\, ^\ast + A \,\, \tag{6}$$

The main type of ions in high-pressure plasma is molecular ions A<sup>2</sup> + , B<sup>2</sup> + , (AB)<sup>+</sup> which are formed in triple processes:

$$A^\* + A + M \to A\_2^\* + M \tag{7}$$

$$B^\* + B + M \to B\_2^\* + M \tag{8}$$

$$A^\* + B + M \to (AB)^\* + M \tag{9}$$

where M—third particle (A or B). Plasma neutralization occurs as a result of recombination processes, which, depending on specific conditions, may prevail or have dissociative recom‐ bination.

$$A\_2^{\;+} + e \to A^{\;+} + A \tag{10}$$

$$B\_2^{\ \ \ \cdot \ \ \cdot} + e \to B^{\ \ \ast} + B \tag{11}$$

$$(AB)^{+} + e \to B^{\*} + A \tag{12}$$

triple or shock-radiative recombination

$$A^\*(B^\*) + 2e \to A^\*(B^\*) + e \tag{13}$$

$$A^\*(B^\*) + e + M \to A^\*(B^\*) + M \tag{14}$$

Population of laser levels occurs during processes (3–5) for B+ atomic ions, (6, 11–14) for B neutral particles, as well as in cascade transitions from B\*\* high levels. It was previously considered [34, 35] that the processes of dissociative recombination of molecular ions are predominantly populated by p-states of atoms, but in recent years this conclusion was questioned [3].

In [36] based on radioluminescence intensity dependence from mercury vapor pressure (10−3– 10−7 Torr) in <sup>3</sup> He-Hg mixture was made a conclusion that in the process of Hg+ three-body recombination mostly populated d-states of mercury atoms. Thus, D-levels may also be populated in the processes of dissociative recombination of molecularions (at a higher density of mercury atoms).

In low-pressure gas discharge laser, the lower laser level is usually deactivated in optical transitions to lower levels, and in lasers with nuclear pumping of atmospheric pressure, the deactivation occurs in collisions with media atoms or plasma electrons, and in Penning reaction with the particles of additional gas. In excimer lasers, where at photon emission the excimer molecule passes in the lower dissociated or weakly coupled state, the lower level deactiva‐ tion is feasible.

Characteristics of laser radiation at pumping by hard ionizer depend on the power and duration of energy input into active medium, but do not depend on the type of ionizer [18]. This means that kinetics of processes in active media of lasers excited by an electron or ion beam and for nuclear-pumped lasers will be identical [37, 38]. Calculation of plasma param‐ eters and laser characteristics implies for kinetic models, representing the balance of rates of formation and decay of individual components in plasma. Kinetic equations are supplement‐ ed by the equations of electron energy balance. In some kinetic models, the number of plasma chemical reactions reaches several hundred (see, for example, [29, 39]). Typically, the rele‐ vant description of plasma and laser parameters' calculation suffices it to include 10–15 basic reactions. In this regard, it is sometimes advisable to use the so-called small models for calculation, which includes only the basic plasma processes, examples of such models [40, 41]. It should be noted that in many cases the basic level population process is either not defined (e.g., lasers with Xe, Kr, Ar IR transitions), or under discussion (e.g., laser on the 3p–3s transitions of neon [30, 31]). In other cases, relevant calculation is hindered by a large uncer‐ tainty in the values of (or even the order of magnitude) processes rate constants [42], the uncertainty in coefficient of light absorption by active medium particles [43].

*A B M AB M* ( ) + + ++ ® + (9)

<sup>2</sup> *A eA A* \* <sup>+</sup> +® + (10)

<sup>2</sup> *B eB B* \* <sup>+</sup> +® + (11)

( ) *AB e B A* \* <sup>+</sup> +® + (12)

*AB e A B e* ) 2 \*( \*( ) + + +® + (13)

*AB eM A B M* ( ) \* \*( ) + + ++ ® + (14)

where M—third particle (A or B). Plasma neutralization occurs as a result of recombination processes, which, depending on specific conditions, may prevail or have dissociative recom‐

Population of laser levels occurs during processes (3–5) for B+ atomic ions, (6, 11–14) for B neutral particles, as well as in cascade transitions from B\*\* high levels. It was previously considered [34, 35] that the processes of dissociative recombination of molecular ions are predominantly populated by p-states of atoms, but in recent years this conclusion was

In [36] based on radioluminescence intensity dependence from mercury vapor pressure (10−3–

recombination mostly populated d-states of mercury atoms. Thus, D-levels may also be populated in the processes of dissociative recombination of molecularions (at a higher density

In low-pressure gas discharge laser, the lower laser level is usually deactivated in optical transitions to lower levels, and in lasers with nuclear pumping of atmospheric pressure, the deactivation occurs in collisions with media atoms or plasma electrons, and in Penning reaction with the particles of additional gas. In excimer lasers, where at photon emission the excimer molecule passes in the lower dissociated or weakly coupled state, the lower level deactiva‐

Characteristics of laser radiation at pumping by hard ionizer depend on the power and duration of energy input into active medium, but do not depend on the type of ionizer [18]. This means that kinetics of processes in active media of lasers excited by an electron or ion beam and for nuclear-pumped lasers will be identical [37, 38]. Calculation of plasma param‐

He-Hg mixture was made a conclusion that in the process of Hg+ three-body

bination.

168 170High Energy and Short Pulse Lasers

questioned [3].

10−7 Torr) in <sup>3</sup>

of mercury atoms).

tion is feasible.

triple or shock-radiative recombination

#### **2.3. Design and development of experimental methods for nuclear-induced plasma research**

Pulsed nuclear reactors were used as a source of neutron radiation for NPLs research [3, 27, 44–47]: in Russia—VIR-1, VIR-2, TIBR-1M, BR-1, BIGR (VNIIEF), EBR-L (VNIITF, All-Russian Scientific Research Institute of Technical Physics), BARS-6 (FEI, Institute of Physics and Power Engineering), IIN-3 (Kurchatov Institute of Atomic Energy); in USA—TRIGA Mark-II (University of Illinois), SPR-III (Sandia Labs), APRF (NASA), Godiva-IV (Los Alamos National Laboratory). Relatively recent reports were issued about experimental NPL investigations in China on the CFBR-II [48]. First, NPL works in China were carried out on a stationary INPC nuclear reactor [49].

Thermal neutron flux density at stationary nuclear reactor reaches 1013 to 1014 n/cm2 s, and gas mixtures' pumping power does not exceed a few W/cm3 . Therefore, research on stationary reactors (IRT-2000 in Moscow Engineering Physics Institute (MEPhI), our works on WWR-K reactor) was mainly associated with the study of the spectral characteristics of plasma [50, 51], as well as the development oflasers with non-self-maintained discharge (WWR-K reactor) [52].

WWR-K reactor (**Figure 2**) is a heterogeneous unit of water-cooled type, operating on thermal neutrons. Desalinated water serves as moderator, reflector, and coolant. Uranium is used as reactor fuel enriched by uranium-235 isotope to 36%. WWR-K reactor is a powerful source of neutrons and gamma rays. The maximum thermal neutron flux in the central channel of the reactor reaches 2°1014 n/cm2 s. The reactor core is placed in an aluminum tank filled with water and is designed as hexagonal lattice containing fuel elements, control and protection system channels and experimental channels. The water temperature is kept constant and does not exceed 40°C. The core has a shape similar to cylindrical, with diameter of 645 mm and height of 600 mm. Central vertical experimental channels with diameter of 96 mm, which was used for nuclear-pumped laser works, pass through the core center. Biological protection of staff from the reactor radiation in horizontal plane is provided by layers of water of 850 mm wide, cast iron—210 mm and limonite concrete—2250 mm. Biological protection in vertical direc‐ tion is formed of 3700 mm of water layer and removable cast iron lids of 800 mm wide.

**Figure 2.** General view of WWR-K reactor.

The upper part of reactor tank includes rotating cast-iron lid, through which experimental channels are loaded. The left side includes the base made of plates on which vacuum pumps for pumping mixtures from laser devices under the reactor lid were placed.

Three designs ofreactor core laser systems [53] were developed and tested on WWR-K reactor. One design was intended for testing mixtures of xenon laser pumped by uranium fission fragments, the second for lasers on inert gas mixtures excited by the products of <sup>3</sup> He(n,p)3 H reactions and the third to run the laser on transitions of mercury triplets [54]. **Figure 3** shows the design intended for lasers on mixtures of inert gases excited by the products of 3 He(n,p)T reactions. The laser cell is designed as electropolished pipe of 36 mm in diameter with flanges for mirrors on the edges. The distance between the mirrors is 2.1 m, and the mirrors are on a quartz substrate with a dielectric multilayer coating. Mirrors are distanced for 0.5 m (non‐ transmitting) and 1.0 m (half-transmitting) from the reactor core. Laser channels cutting after a 6-month settling showed that KU-1 quartz substrate has sufficiently high radiation resist‐ ance. De-gassing and gas puffing were also conducted through waveguide pipe Ø36 mm. The laser light from this tube was extracted through the window of LiF or CaF2 and analyzed by registration system. IR radiation was simultaneously recorded by calorimeter located above the exit window and reflected on the LiF plate portion through the matrix of five PD-7G photodiodes. Radiation within the visible range was recorded by photodiodes matrix and the system for measuring the luminescence spectra based on the SPM-2 monochromator and FEU-106 photomultiplier operating in photon counting mode.

**Figure 3.** Scheme of laser cell of inert gases excited by <sup>3</sup> He(n,p)T reaction products. 1—laser pipe, 2—mirrors, 3—alu‐ minum lid of reactor, 4—waveguide pipe, 5—window, 6—LiF plate, 7—propellant, 8—cast iron lid of reactor, 9—pho‐ todiodes matrix.

**Figure 2.** General view of WWR-K reactor.

170 172High Energy and Short Pulse Lasers

The upper part of reactor tank includes rotating cast-iron lid, through which experimental channels are loaded. The left side includes the base made of plates on which vacuum pumps

Three designs ofreactor core laser systems [53] were developed and tested on WWR-K reactor. One design was intended for testing mixtures of xenon laser pumped by uranium fission

reactions and the third to run the laser on transitions of mercury triplets [54]. **Figure 3** shows

reactions. The laser cell is designed as electropolished pipe of 36 mm in diameter with flanges for mirrors on the edges. The distance between the mirrors is 2.1 m, and the mirrors are on a quartz substrate with a dielectric multilayer coating. Mirrors are distanced for 0.5 m (non‐ transmitting) and 1.0 m (half-transmitting) from the reactor core. Laser channels cutting after a 6-month settling showed that KU-1 quartz substrate has sufficiently high radiation resist‐ ance. De-gassing and gas puffing were also conducted through waveguide pipe Ø36 mm. The laser light from this tube was extracted through the window of LiF or CaF2 and analyzed by registration system. IR radiation was simultaneously recorded by calorimeter located above the exit window and reflected on the LiF plate portion through the matrix of five PD-7G photodiodes. Radiation within the visible range was recorded by photodiodes matrix and the system for measuring the luminescence spectra based on the SPM-2 monochromator and

He(n,p)3

He(n,p)T

H

for pumping mixtures from laser devices under the reactor lid were placed.

FEU-106 photomultiplier operating in photon counting mode.

fragments, the second for lasers on inert gas mixtures excited by the products of <sup>3</sup>

the design intended for lasers on mixtures of inert gases excited by the products of 3

Although we were not able to create a continuous laser with a direct nuclear pumping, the use of continuous ionizing radiation sources: stationary nuclear reactors and radioactive sources provide for detailed study of plasma properties of gas mixtures, and stationary state of pumping simplifies the analysis of processes kinetics in plasma [53].

Currently, the research is being conducted on DC-60 heavy ion accelerator [55]. The main parameters of the accelerated ion beam: ion type—from lithium to xenon, and ion energy from 0.5 to 1.75 MeV/nucleon. The intensity of ion beam is from 1012 to 1014 particles/s depending on type and energy of ions. Ion impulse duration is several nanoseconds and repetition rate of ions is 1.84–4.22 MHz. Mainly, the ions of argon were used as a source of ionization and excitation. The accelerated ion beam passes from evacuated transportation channel through 3-mm hole in the flange to irradiation chamber (**Figure 4**). The hole in the flange is closed by membrane of 2-μm titanium foil or 2.5-μm-thick havarfoil. The gas pressure in the cell is measured by capacitance diaphragm gauge mounted at the top of the chamber. The ions passed through the foil ionize and excite the gas mixture in irradiation chamber (**Figure 5**). Argon ion energy after separation foil is about 50 MeV. The emerging lightradiation passes through quartz window and condenser and focused on optical fiber. The beam falls on a compact spectrometer through the fiber, and the recorded spectrum is displayed on a computer.

**Figure 4.** View of the experimental setup on the DC-60 accelerator.

**Figure 5.** 400 Torr helium (a) and 600 Torr neon (b) luminescence induced by argon ions.

Beam from the narrow ion-excited region is reflected on the separating flange (a, b) and on the top of the chamber (b).

Studies on luminescence spectra of gaseous media were also conducted with the use of radioactive isotopes [36, 56–58] and pulsed nuclear reactors [59–61].
