The efficiency of transformation of deposited power (energy) into laser radiation is the most important parameter for all lasers. In the case of NPLs, the maximal efficiency of transformation of deposited nuclear energy into laser radiation belongs to the lasers operating on transitions of rare gas atoms, and the highest n > 1 % were registered for lasers operating on transitions of the Xe atom (1.73, 2.03, and 2.65 pm) and the Ar atom (1.27 and 1.79 pm). Table 3.6 compares the results obtained by various laboratories, and also provides the maximal efficiencies (nmax) constituting the ratio of the laser photon energy to the energy consumption for forming one primary active particle of plasma (ion or excited atom). This question is discussed in greater detail in the first section of Chap. 5. For comparison, Table 3.6 also includes information on efficiencies obtained from excitation of gas media by electron beams.

It follows from Table 3.6 that in some cases the real efficiency (n) approaches maximal, which testifies to minor energy losses in plasmochemical reactions of energy transfer and high selectivity of the final process of populating of the upper laser levels. Possibly the high efficiency of NPLs is explained by the noticeable

contribution to populating the upper laser levels from the processes of associative ionization with the participation of metastable states 6s of the Xe atom or 4s of the Ar atom [65].

For quasi-CW NPLs with a laser duration of >100 ps, which is substantially greater than the characteristic times of plasma processes, the efficiency usually is defined as the ratio of the output power to the instantaneous power absorbed in the laser’s active volume. As a rule the active volume is understood to be the entire excited volume of the laser cell. In particular, this method was used to determine the efficiency in the studies by VNIIEF and VNIITF. However, sometimes qI was determined with respect to the deposition of power to the mode (radiating) volume, which was less than the total cell volume. Naturally, in this case the value of nI will be greater. For example, in study [40], the efficiencies for NPLs using the mixture 3He-Ar (A = 1.79 pm), calculated by two methods, differ by roughly a factor of 100.

Another reason for the discrepancy in the efficiency data obtained in the various laboratories is the use of two different methods to determine the energy deposition to the laser medium: from the results of measurement of the pressure jump in the gas medium, or by calculation. The results of investigations in different laboratories showed that the energy deposition measured in experiments is less than the com­puted value by roughly a factor of 1.5-2. The reasons for the discrepancy between the computed and experimental data are considered in Sect. 7.4 of Chap. 7. It cannot be excluded that the values of nI obtained in the Sandia experiments, which exceed maximal values, may be explained by errors in determining the energy deposition.

Mixtures of rare gases with metal vapors are of interest for NPLs in connection with the possibility of obtaining lasing in the visible and UV ranges, while the ultimate efficiency of such lasers can be 10-20 % because of the high energy of the laser quantum and/or the possibility of using Ar, Kr, and even Xe as the buffer gases, with low energy costs of formation of ion-electron pairs. In addition, populating of upper laser levels of ions of certain metals can occur even at the first phase of relaxation processes, directly as a result of charge-transfer and Penning reactions. This circumstance makes it possible to lower the energy losses in comparison with schemes in which the populating of laser levels is implemented through longer sequences of plasma processes.

This section briefly examines the laser mechanisms and kinetic models both of operating NPLs and of certain active media on which lasing has not yet been obtained. One can become more closely acquainted with these questions in the survey and original studies cited below. Nearly all of the theoretical studies were dedicated to the kinetics of the active media of NPLs based on the transitions of ions and atoms of group II metals on the Periodic Table. Among these studies, the first of which was monograph [1], one can distinguish a cycle of theoretical investigations by IOFAN associates (see reviews [4, 106]), dedicated to analysis and calculations of the kinetics of active media of NPLs based on the transitions of Cd, Zn, Hg, Mg, Sr, Ca, Ba and Be ions. The kinetic models took into account from 100 to 160 plasmochemical reactions. The basic results of the calculations performed for the experimental conditions of the pulsed EBR-L reactor (see Chap. 2, Sect. 2.5) are shown in Table 5.10.

Among the metal vapor NPLs, active media based on mixtures of helium with Cg, Zn, and Hg vapors have elicited the greatest interest.

All the calculations of the characteristics of the alteration of cavity stability, S(t), described below were performed using data from experiments for He-Xe and Ar-Xe mixtures (the LUNA-2 M setup with planar uranium layers and the VIR-2 M reactor), as well as Ar-Xe and Kr-Xe mixtures (a cell with a cylindrical uranium layer and the TIBR-1 M reactor). The laser cell characteristics are presented in

b

0. 8

^ (t) 0 6 0.4

0.2

Sect. 7.4 of Chap. 7. Optical inhomogeneity evolution was calculated using the procedure described in the second section of this chapter.

The calculations revealed that, for the cell designs examined under standard pumping conditions, the cavities of lasers with active media that consist of helium — based mixtures do not leave the stable state. For mixtures of this type over the course of an entire pulse, S(t) = 1. This result is in full agreement with the experimental data, because the pulse shape therein after the lasing threshold is exceeded virtually duplicates the shape of the pumping pulse. However, when mixtures are used in which the buffer gases have high atomic numbers, as exem­plified by Ar or Kr, loss of stability is observed, during which the stability loss

effect is more clearly manifested as the mixture pressure is increased. However, if the pressure remains unchanged, this effect is intensified with an increase in the atomic number of the buffer gas.

The experimental dependences of the output laser power of a mixture with a value of Ar:Xe = 2,000:1 at P0 = 0.5 atm in a laser with planar uranium layers (I) and a neutron pumping pulse (n) upon time, normalized to their maximum values, are shown in Fig. 8.19а. The calculated temporal Sy(t) and Sx(t) stability character­istics are shown in Fig. 8.19b. The only small trough on the Sy(t) temporal diagram is in agreement with the trough at the experimental laser pulse peak. The results of the Sx(t) calculations demonstrate that a stability decrease for the x-component

clearly occurs in the vicinity of the laser pulse peak. Under experimental condi­tions, this decrease apparently also leads to the earlier decrease of the laser pulse as compared to the neutron pulse, which creates the outward appearance of a lasing pulse shift relative to the pumping pulse. The slight nonsynchronism between the troughs at the laser peak and on the Sy(t) diagram, as well as between the start of laser pulse decrease and the start of the dip on the Sx(t) diagram, can be explained by the shortcomings of the calculation procedure used, in particular, an escalating error in the calculation of density distributions past the pumping pulse peak. These same shortcomings should apparently also explain the fact that Sx(t) instability develops just as consistently, but not as intensely as the experimentally obtained the behavior of trailing edge of laser pulse.

The similar dependences obtained for a mixture Ar:Xe = 600:1 at P0 = 1.5 atm when using the very same laser cell are presented in Fig. 8.20. Here, as in the previous example, a some correlation is observed between kinks on the laser pulse front and the Sy(t) characteristics. So, the first three laser pulse kinks can be compared to the first three steps on the Sy(t) curves and to the dual dip in the vicinity of the peak—two closely positioned dips at Sy(t) immediately past t = 8 ms. Starting at roughly this moment in time, instability develops for Sx(t) and the aggregate effects of both the instabilities lead to the cessation of lasing.

Calculations of Sy(t) and Sx(t) characteristics revealed that the development of a positive gas lens (8.38) in a laser with planar uranium layers leads to cavity stability fluctuations in time, whereas a negative gas lens (8.41), starting at a specific moment in time, produces a consistent stability decrease.

The calculation results for a laser with a cylindrical uranium layer are presented in Figs. 8.21 and 8.22. In this instance, only a positive gas lens originates and cavity stability is described by correlations (8.38)-(8.40), in which the y coordinate must be replaced with the radial coordinate r. A relatively good correlation was observed between the laser pulse and the S(t) characteristics for a mixture Kr:Xe = 200:3 at P0 = 1 atm. The worst correlation between the laser pulse and the calculated S(t) characteristics was obtained for a mixture Ar:Kr = 100:1 at P0 = 1 atm.

Thus, NPLs using condensed media have for the present given rather modest results: pumping has only been done on glass neodymium lasers with optical radiations from NOCs. Moreover, there is no detailed information on the charac­teristics of these lasers. Despite this, the literature has proposed some schemes for powerful nuclear-laser devices based on condensed laser media. We will review some proposals without discussing issues related to feasibility.

Colloidal Nuclear Reactor! Optical Thermal Emission! Liquid Laser Medium

An article [61] reviews one of the first designs for a nuclear laser device using condensed laser media. In a simplified form, this device consists of two coaxial cylinders, wherein the liquid laser medium POCl3-ZrCl4:Nd3+ is placed in the inner cylinder with a transparent cover, and the reactor core occupies the space between the two cylindrical covers. A colloidal reactor core operating in stationary mode consists of a suspension of 0.5-5 ^m uranium carbide particles (total mass of

4.5 kg) in a vortex flow of helium. The power density that will be released inside the reactor core, according to estimates, is 20 kW/cm3. It is supposed that the laser media is pumped with the optical emission of fuel particles, which are heated to a temperature of 3,000K. The geometric dimensions and the energy characteristics of this device are not given in the work [61].

The majority of the credit for this book on nuclear-pumped lasers (NPLs) goes to its original Russian authors. I (George H. Miley) became the contributing American co-author when I learned of the Russian version of the book and proposed to develop a version in English. After correspondence with the Russian authors, I obtained permission from them to work on the English version. Subsequently, I managed to get a translation into English, which was not a simple task in view of the unique technical terminology and equations. While I made some minor changes and clarifications, these chapters generally follow the original Russian version. My additional contribution has been to add Chap. 13, a brief summary discussing work on this subject in the United States. Chapter 13 is somewhat similar to the Russian view of American work provided in the early chapters, but brings in some new perspectives.

The field of NPLs was born before the Cold War and the “Star Wars” program in the United States, when Karl Thom at NASA headquarters took an interest in NPLs for space power beaming. That led to research programs at NASA’s Langley Research Center, and the University of Florida. About this time I obtained support from the DOE for research at the University of Illinois, and later received some additional support from NASA. Other laboratories with programs at the time included Sandia National Laboratory (where David McArthur achieved the first NPL in the United States, using a cooled CO lasing medium and the pulsed nuclear reactor located at Sandia), and also Los Alamos National Laboratory where the pulsed Godiva reactor was employed. These programs were relatively small, however, and focused on basic research about radiation-induced plasmas as well as NPLs. But NPLs became entangled in “Star Wars” and became a part of the cat and mouse game between Russia and the United States. This provided greatly expanded funding for classified programs in several of our National Laboratories (such as Lawrence Livermore National Laboratory, Sandia National Laboratory, and the Idaho National Engineering Laboratory), intended to compete with the classified work in the “secret science cities” in Russia. This competition led to great strides forward in both countries. However, the end of the Cold War era also abruptly ended the flow of money to NPL research in the United States. Thus very few American researchers work in this area at the present, and the national laboratory programs on NPLs have all stopped. In contrast, the Russian laboratories have managed to maintain a reasonably vigorous program, as is discussed in this book.

Because I had one of the few unclassified early programs on NPLs in the United States, I was one of the few American scientists free to discuss NPLs with Russian researchers. These conversations started in September of 1981 when I met several of their scientists during Tenth European Conference on Controlled Fusion and Plasma Physics in Moscow, where I spoke about radiation-induced plasmas. Alex Filyukov, a Russian scientist, approached me during the meeting and struck up a conversation about my NPL work. This was during the height of the Cold War, so these conversations were often vague and guarded on their part. I could speak more freely because I knew little of the classified work occurring within the United States. Alex’s questions made it clear that he had studied all of my papers on the subject in great detail—he even knew some details that I had forgotten! It became obvious to me that he had been asked by their KGB or some official agency to interrogate me. After several discussions over the period of the conference, Alex stated that I was on “the right track” with my NPL research, but that he and Russian colleagues had some great concepts they could not yet disclose. This led to continued discussions and invitations for me to visit and give seminars at several places such as Moscow State University. Later after the fall of the Iron Curtain, I was able to visit the Russian “secret science cities” where the classified NPL research was going on. During those trips I met many more NPL researchers, including the co-authors of this book.

On those trips to Russia, I visited five specially built research reactors designed for NPL research in the “secret” laboratories. Shown above (reactor VIR-2M, Sarov) is a pulsed system with large laser pump regions located on top of the structure standing on posts in the foreground. The laboratory room containing the NPL and reactor incorporated special openings through a shielding wall with diagnostics behind. This allowed good detector sensitivity with minimum radiation interference. These facilities and NPL research on them are discussed in detail in this book.

I made four trips to the USSR prior to the fall of the Berlin Wall. These trips were for discussions of NPLs, at the invitation of the Russians, hosted by Alex Filyukov and scientists from the Lebedev Physics Institute in Moscow. At the time I learned that Alex Miskevich had been doing leading experiments at a laboratory somewhere north of Moscow, but I was not permitted to visit it. A few months after the fall of the Berlin Wall, I was invited to come again, and was hosted by Alex Miskevich (second from left—I am to his left, wearing glasses), and observed his NPL facilities at the Moscow Engineering and Physics Institute (MOT). He is shown holding one of his laser cells. During this visit I learned that there were major programs in the Russian Laboratories in the “secret cities” of Obninsk, Chelyabinsk-70, and Arzamas-16. I was invited to visit them, and shortly after that I made trips to the laboratories in these secret cities (where my present co-authors are located).

This photo shows U. S. scientists from the NASA Langley Research Center preparing a NPL experiment for the Fast Burst Reactor at the Army Aberdeen Test Laboratory in Maryland. Jack Fryer (technician), Frank Hohl, Nelson Jalufka, and Russell De Young are shown with the experimental setup.

Russell De Young at the Aberdeen Fast Burst reactor where nuclear laser experiments were carried out. The reactor is shown between two test laser setups. One is the Box Laser and the other is polyethylene (neutron moderator) covered cylindrical gas tube laser.

Much of the early NPL work in the United States was done by scientists at NASA Langley Research Laboratory, who made use of the excellent pulsed reactor facilities at the Army Aberdeen Maryland Laboratory. Dr. Frank Hohl, who was in charge of this work, and Dr. Russell De Young, who did his thesis on NPLs with me at the University of Illinois, were major contributors to the research. The photo above shows Frank Hohl and Russ De Young along with another NPL scientist Nelson Jalufka. The pulsed reactor at Aberdeen is shown in the second photo.

Another University of Illinois graduate, Dr. Mark Prelas (a Professor at the University of Missouri Columbia), along with Dr. Fred Boody (MS at University of Illinois and PhD from University of Missouri) were heavily involved in early NPL work. In particular, they focused on the concept of using nuclear pumped “flash — lamps” to pump the laser medium. Following the fall of the Berlin Wall, they made several visits to the Russian NPL laboratories and participated in meetings there, much as I did.

These photographs are intended to provide the reader some feeling for the flavor of the early NPL research. They certainly do not provide nor are they intended to provide a full picture of all the people and facilities that have been utilized in this work.

The association of NPLs with “Star Wars” was unfortunate and deleterious to NPL research in the United States. As the reader will discover, there are many very important civilian applications for NPLs that have been overlooked due to the associate with military applications. An additional problem was that just as the technology began to emerge, nuclear reactors fell into disfavor due to fears initiated by the Chernobyl and Three Mile Island reactor accidents. Practical NPLs would require the design of special types of nuclear reactors (both for terrestrial and space applications) and as a result of public concerns such reactors were “not in the cards.” However, the situation is now slowly changing, as many feel nuclear reactors should play an important role in the world’s future energy economy. The most unfortunate recent accident is the Fukushima Daiichi nuclear disaster in Japan represents a new setback in acceptance. But many people believe this will pass as lessons learned improve reactor safety and the Fukushima event was an extreme example. Consequently, the vision of a reactor-based laser system for various applications cannot be ruled out. This is particularly true in view of the importance of potential applications such as power beaming, inertial confinement fusion, chemical and materials processing, and deflection of asteroids and other space debris. Hopefully this book, the first on the subject, will introduce a new generation of scientists and engineers to this exciting and very important field of science. The ultimate applications for NPLs may well be something not yet envisioned, and the basic science remains challenging and intriguing. My Russian co-authors have, in my view, provided a unique insight into the status of the field and the potential now open to move into important applications. Thus I am extremely pleased to have played a role in helping bring this information to a much wider scientific audience.

I want to thank Autumn West, Robyn Bachar, and Maria Lipson for their diligent work on the translation and proofreading. However, I must take ultimate responsi­bility for this version of the Russian book, including any mistakes or omissions that have crept in. I also wish to thank my many colleagues and students who contributed to the advancement of NPLs in the United States and whose work I discuss here.

The first 12 chapters in this book are derived from the original version in Russian. Rather than attempting to discuss them myself, I felt it best to use the Russians’ description. Thus, the following introduction is adapted from the original Russian edition. In this section, “authors” refers to my Russian co-authors.

The use of nuclear radiation to pump active media and create nuclear-pumped lasers (NPL) on this basis is a comparatively new scientific-technical area. It is at the interface of two disciplines—quantum electronics and nuclear physics. This area has rapidly evolved over the last 40 years, from the first proposals on the use of nuclear energy sources for pumping lasers to the creation of diverse NPLs. Now NPL research has reached the point where engineering design development of continuous and pulsed nuclear laser units for various applications—integrated devices based on the achievements of nuclear physics and technology, quantum electronics, gas dynamics, optics, etc.—has become possible.

The authors of this book have participated in studies into NPL-related issues since the late 1960s. At that time, NPL research was in its incipient phase. In a number of laboratories (primarily in the United States), attempts were made to pump various active media with nuclear radiation and thus prove the fundamental possibility of direct conversion of nuclear energy into laser radiation. To search for NPL active media and study their characteristics, the All-Union Scientific Research Institute of Experimental Physics (VNIIEF) formed a science team consisting primarily of graduates of the Moscow Engineering and Physics Institute (L. Ye. Dovbysh, V. M. Karyuk, M. F. Kostenko, V. N. Krivonosov, S. P. Melnikov, A. N. Sizov, A. A. Sinyanskiy), which initially was directed by A. M. Voinov and A. T. Kazakevich, and later on by A. A. Sinyanskiy. Academicians Yu. B. Khariton and A. I. Pavlovskiy devoted great attention to the development of these studies. In the first phase of the exploratory research, significant assistance was provided by I. V. Podmoshenskiy (S. I. Vavilov State Optical Institute [GOI])—a virtual scien­tific consultant of the team of young researchers. The decision on organization of the work to study the problems of NPLs specifically at VNIIEF was a natural one, since in that period (and at present), VNIIEF was one of the few organizations with an inventory of diverse, powerful pulsed reactors. It was this circumstance that made it possible to implement the first successful experience in laser pumping with nuclear radiation in 1972, and then to continue research into various NPL problems.

The authors have striven to show the most important results and phases of NPL development not only at VNIIEF, but also at other Russian and foreign laboratories. The book may not be entirely free of subjectivity, because as a rule a scientific publication is the result of a compromise of an author (or authors) desiring to present the modern state of research in a given field with adequate completeness while displaying the achievements of his or her own laboratory.

The basic reason for writing this book was the lack of a scientific study that reflected in adequate detail the status of the work on NPL problems and prospects for their development. The authors worked to systematize information accumulated over decades, which had frequently been published in scientific publications or digests of conference materials that were available only in limited circulation.

This book may also be seen as a reference work, because it contains information about virtually all studies published in the last approximately 40 years related to NPLs.

This book consists of 13 chapters: Chaps. 1—6 (except for Sections 1 and 2 in Chap. 6), 11, and 12 were written by S. P. Melnikov; Chaps. 7—9 were written by A. N. Sizov; and Chap. 10 and Sections 1 and 2 in Chap. 6 were written by A. A. Sinyanskiy. Chapter 13 has been added by our American co-author, George H. Miley.

Chapter 1 provides the chronology of basic events associated with the advent and study of NPLs. The specifics of pumping lasers with nuclear radiation are also discussed.

Chapter 2 provides data on the characteristics of the pulsed reactors that were used to conduct the bulk of the studies of NPLs in Russia and abroad. It examines lasers and the experimental procedures, and discusses the specific features of experimentation with pulsed reactors associated with the problem of radiation resistance of optical materials and photodetectors.

Chapter 3 systematizes the extensive material on the results of experimental studies of spectral, energy, and threshold characteristics of various types of NPLs radiating at the transitions of Xe, Kr, Ar, Ne, C, N, O, Hg, Cd, and I atoms, the ions Cd+, Zn+, and Hg+, the CO molecule, and the molecular ion N2+. The results of studies of a number of active media for which no generation was detected during nuclear radiation pumping are discussed. And finally, Chap. 3 looks at a method of laser pumping with fast neutrons, and cites the results of studies of NPLs based on a He-Xe mixture in experiments with the BR-1 pulsed reactor.

Chapter 4 is dedicated to a discussion of processes that occur in a low-temperature nuclear-excited plasma: from the initial processes of ionization and excitation of the medium by nuclear particles to subsequent plasma-chemical reactions leading to the populating of laser levels. Data are cited on the energies of formation of primary particles of plasma (ions and excited atoms). In addition, Chap. 4 provides a survey of experimental work studying the parameters of nuclear — excited plasma and spectral luminescence characteristics of gas media.

Chapter 5 discusses the kinetics of plasma-chemical processes and lasing mech­anisms of all currently known gas NPLs, and looks at NPL kinetic models published in the literature and the results of computations of their characteristics.

Chapter 6 contains information on the results of investigations of the more complex nuclear laser devices, which are structural elements of multichannel reactor-lasers operating in fixed or pulsed modes. This chapter also gives the parameters of certain variations of designed pulsed reactor-lasers.

Chapter 7 briefly examines information about the methods of calculating the specific energy deposition of ionizing particles. The influence of non-uniformities of uranium-containing layers on the efficiency of the energy contribution of ura­nium nuclear fission fragments to the gas is studied. The dependence of the energy deposition and its effectiveness on the thickness of the layers and the density of the gas are considered. A dimensionless parameter of optimization of the energy deposition is introduced as a function of the reduced thickness of the uranium layers and the ratio of the transverse dimension of the excited volume to the length of the free path of the average fragment in the gas. It is demonstrated that the functional dependence of this parameter is in good accord with experimental data on the output power for lasers pumped by fission fragments. Experimental data to determine the energy deposition, the causes of their mutual discrepancy, and deviation from theoretical results are analyzed.

Chapter 8 is dedicated to studies of optical non-uniformities arising in hermetically sealed gas lasers excited by fission fragments. Formation and development of these non-uniformities are caused by the specifics of distribution of the specific energy deposition of fission fragments and relaxation thermal and gasdynamic processes. It is shown that owing to heat exchange of the excited gas with the walls and the substrates of laser cells, a wall zone is formed with large positive values of the refraction index gradient. These values are so great that lasing can be carried out only outside the zone limits. The size of the wall zone increases over time: for pumping pulse durations of ~ 0.1 s, it encompasses virtually the entire volume of the laser cell. Estimates are cited regarding the influence of optical non-uniformities on laser beam divergence, and results of calculations of the distribution of non-uniformities are compared with experimental results. The possibilities for improving optical characteristics of her­metic lasers by varying the initial parameters of the gas and the conditions of energy input are studied. The optical non-uniformities in the quasi-stationary and stationary (with external cooling of the cell) modes of irradiation are investigated.

Chapter 9 examines the specific features of flowing-gas lasers. The drawbacks of longitudinal circulation of gas mixtures are discussed. The influence of turbulent pulsations on laser optical characteristics, based on which the constraints on gas pumping speed are found, is evaluated. The advantages of transverse gas circula­tion, with subsequent release of excess heat downstream of each laser channel to radiators, are underscored. The derivation of equations describing the action of the radiator is provided, and their solution is cited. The structure of gas flow in the laser channel with transverse circulation is described. A model for forming optical non-uniformities in the flowing-gas channel, one that concurs with the experiment, is introduced. Methods are proposed for approximate calculation of gas density
distribution in such a channel. Possibilities of combined heat removal are discussed, in which the heat is removed simultaneously both by flowing the laser-active gas itself, and by surrounding the channel with an external coolant.

Chapter 10 is dedicated to a discussion of problem issues associated with development of multichannel stationary reactor lasers and their possible applications.

Chapter 11 provides a survey of the works investigating the possibility of pumping solid-state and liquid laser media with nuclear radiation and creating different versions of pulsed reactor lasers based on them.

Chapter 12 is a review of articles about the results of investigations of nanosec­ond pulsed NPLs. These lasers are usually pumped by radiation of the most powerful source of energy, a nuclear explosion. Such lasers can be used to resolve one of the problems of inertial thermonuclear fusion determination of the levels of energy that laser drivers must have to obtain specific amplification gains of the target.

Chapter 13, as noted earlier, has been added to the English version by George Miley to provide some added insight into research in the United States.

The authors would like to thank VNIIEF associates A. M. Voinov, L. Ye. Dovbysh, V. F. Kolesov, M. I. Kuvshinov, V. N. Krivonosov, B. V. Lazhintsev, V. A. Nor-Arevyan, and V. T. Punin for their many years of fruitful collaboration, as well as E. P. Magda (All-Russia Scientific Research Institute of Technical Physics [VNIITF]), A. I. Miskevich (Moscow Engineering and Physics Institute [MIFI]), and V. F. Tarasenko (Russian Academy of Sciences Institute of High — Current Electronics, Siberian Division [ISE SO RAN]). The authors thank A. P. Morovov for proofreading the book, and for valuable comments.

The authors are grateful to Yu. N. Deryugin, V. Yu. Matyev, and Ye. V. Prikhodko for participating in the calculations of energy deposition, spatial non-uniformities, and the stability dynamics of cavities of gas NPLs, as well as to A. A. Pikulev, V. M. Tsvetkov, S. L. Turutin, A. N. Korzenev, S. V. Patyanin, and P. V. Sosnin for investigations of various NPL characteristics.

Numerous investigations of NPLs performed at VNIIEF on the pulsed reactors VIR-2, VIR-2M, TIBR, BR-1, and BIGR would have been impossible without the help of the personnel of these units. The authors would like to express their gratitude to all associates of the reactor teams and especially note the great contribution to successful conduct of the experiments by V. N. Bogdanov, A. S. Koshelev, and S. F. Melnikov.

The authors would also like to express their gratitude to the associates of the scientific-technical library of the Institute of Nuclear and Radiation Physics at VNIIEF for assistance in finding information, and to S. Yu. Pikuleva for providing the figures in Chap. 4.

Updated to add: Finally, we are most grateful to our U. S. co-author, George H. Miley, who made this English version of the book possible.

Champaign, IL, USA

The interest in chemical lasers [173] may be explained by the fact that for them, in CW mode, high output powers of up to several megawatts have been obtained [174]. Various methods of acting on the active medium are used to initiate chemical lasers: gas discharge, photoinitiation, fast electron beams, у radiation from a nuclear explosion, etc.

Initiation of chemical lasers using HF(DF) molecules with ionizing radiation is one of the methods of creating powerful IR lasers in a spectral range of 2.7-4.4 qm. Marked successes in this area were achieved in pulsed mode using electron beams and the у radiation from nuclear explosions as the source of initiation. In the first case, the output energy of the HF laser using the mixture H2-F2-O2-NF3 with a chain reaction reached 4.5 kJ (n = 226 %) at the pulse duration of 50 ns [175], and in the second case, for the mixture SF6-H2 with a non-chain reaction, it reached 70 kJ (n ~5 %) in ~10 ns pulse (see Chap. 12).

When chemical lasers are initiated by nuclear radiation with a pulse duration >100 qs, which is typical of pulsed reactors, the specific power deposition is not great, so it is hard to ensure high rates of dissociation of the fluorine-containing substances, and accordingly, the rate of formation of the excited molecules HF*. As a result, HF molecules accumulate in their ground state, and effectively quench molecules of HF* in collisions. This circumstance leads to an increase in the laser threshold and a reduction in efficiency.

The possibility of creating chemical NPLs using pulsed reactors was examined in several studies. The first of them [176], carried out in 1970 and published in 1989, examined the mixtures 235UF6-H2 and 235UF6-H2-F2. It was concluded that for the first mixture, ФгА = 5 x 1014 cm-2 s-1, while for the second, ФгА « 1017 cm-2 s-1. Study [176] briefly discussed one version of chemical NPL at the mixture 235UF6-D2-F2-CO2 (A = 10.6 qm), with transfer of energy from the excited DF* molecules to the CO2 molecules. Study [177] examined several problems associated with development of the chemical laser DF-CO2, intended for experiments with the reactors Godiva-IV (t1/2~30 qs) and SKUA (T1/2 < 400 qs).

The 235UF6-H2 mixture is interesting in that at present it is perhaps the most realistic NPL active medium in which the “fissionable” material 235UF6 can be a component part of the laser medium. Study [178] calculated the gain for this mixture depending on the duration of the neutron pulse, the neutron flux density, the pressure and composition of the mixture, the temperature of the medium, and the dimensions of the laser cell. In contrast to other NPLs, which usually operate for an interval of time in which the thermal neutron flux density exceeds the threshold value, the characteristics of NPLs using the mixture 235UF6-H2 depend on the duration and shape of the neutron pulse. It was shown that for typical reactor pulse parameters (Фтах < 1017 cm-2 s-1, t1/2 = 30-200 ^s), the laser threshold is achieved at ФгА < 1014 cm-2 s-1, and the small-signal gain can be >5 x 10-3 cm-1.

When mixtures based on H2-F2 with a chain reaction are used, it is possible to obtain a laser efficiency of greater than 100 % owing to the chemical energy stored in the active medium [173]. The kinetics of one such laser based on the mixture He-F2-H2-O2, initiated by nuclear radiation, was examined in study [179] for experimental conditions of the EBR-L reactor. Calculations showed the possibility of achieving lasing at a number of vibrational transitions of the HF molecule. Lasing can occur at the leading edge of the reactor pulse, and for the optimal mixture He-F2-H2-O2 (12:2:1:2) at a pressure of 2 atm, the duration of the laser radiation is ~10 ^s with an efficiency of up to 100 %.

Unfortunately, the few experiments attempting to create chemical NPLs did not yield positive results. Such experiments were carried, for example, with the HPRR fast pulsed reactor (t1/2~50 ^s) using the mixtures 235UF6-H2 (Р = 0.07-0.8 atm) and 3He-F2-H2 (Р = 0.4 atm) [180]. The thermal-neutron flux density inside a laser cell 46 cm in length was >1017 cm-2 s-1. Measurements of the chemical compo­sition of mixtures showed that the concentration of HF molecules formed in the pulse ([HF]/[UF6] > 70 %) is entirely sufficient to achieve the laser threshold. In the opinion of the authors of [180], the lack of lasing may be explained by contamina­tion of the active medium with impurities due to the increased temperature of the laser cell or an insufficiently high appearance rate of atomic fluorine. Analogous experiments performed at VNIIEF in 1973 with the BIR-2-pulsed reactor (t1/2~100 ^s) with mixtures 3He-SF6-D2 and 3He-SF6-D2-CO2 also did not yield positive results.

In the United States, high-power NPL-based laser setups were developed in the scope of the FALCON program (Fission-Activated Laser CONcept) with Depart­ment of Energy support [37]. The basic executors of this program were the Sandia Laboratories with the support of the Idaho National Engineering Laboratory. Other participants included the Air Force Philips Laboratory, Los Alamos National Laboratory (LANL), Oak Ridge National Laboratory (ORNL), Mission Research Corporation, Babcock & Wilcox, W. J. Schafer Associates, Lockheed, Kaman, Science Research Labs, University of New Mexico, University of Illinois, and Texas A&M University. As is noted in study [37], the program examined two versions of high-power nuclear laser setups: a stationary RL with a laser radiation power of several megawatts, and a laser setup with a power of about 500 kW, which was pumped using a pool-type reactor. It is proposed that high-power NPL-based setups can be placed not only on land, but on ships as well.

Experimental investigations [38, 39] directed at modeling the pumping condi­tions of laser media in high-power nuclear-laser setups (such as large volumes of medium, long pumping pulses, effect of the medium-flowing and — heating on the quality of the laser beam) were carried out at Sandia Laboratories on an ACRR pool-type pulsed reactor. The description and characteristics of the ACRR reactor are provided, for example, in monograph [7]. The cylindrical core of the ACRR reactor, with a diameter of 80 cm and height of about 51 cm, is made of a mixture of UO2-BeO oxides placed on the bottom of a tank of water at a depth of 7 m (Fig. 6.23). The duration of single pulses varied from 7 to 250 ms. At maximal energy release in the reactor core of 300 MJ, the neutron flux density in the central channel was 8.7 x 1017 cm-2 s-1 [38].

The first experiments with NPLs on the ACRR reactor were carried out in 1987, with irradiation of a 1.5 x 7 x 60 cm laser cell in the central channel. An Ar-Xe mixture was used as the active medium. Placement of a laser cell with large dimensions in the central channel was difficult, because the channel has a diameter of just 22.8 cm. Therefore, in subsequent experiments, an ALEC (Advanced Laser External Cavity) was placed close to the surface of the core in a rectangular cavity with a cross-section of 20 x 132 cm2 (Fig. 6.23).

Successful experiments with the ALEC device made it possible to obtain baseline data for study of stationary modes of NPL operation with flowing of the gas medium through the active volume. For this purpose, experiments were carried with FLE-1 and FLE-2 (Flowing Laser Experiments) devices, which were irradi­ated in the cavity of the FREC-II subcritical assembly [7], located close to the ACRR reactor core. Centrifugal air compressors were used to pump the gas. The active volume of the laser was ~5l, with specific power depositions of up to several tens of W/cm3. The next phase of the investigations at Sandia involved experiments with the VSE (Volume Scaling Experiment) device, with an active volume of 50l, specific pumping power of 5 W/cm3, and operating time of 0.2 s [39].

Fig. 6.23 ACRR reactor with ALEC laser device [38]: (1) ALEC device; (2) laser cell; (3) ACRR reactor core

Fig. 6.24 Oscillograms of laser radiation for Ar-Xe mixture (2 = 1.73 pm) at argon pressures of 0.44 atm (1), 0.88 atm (2), and 1.25 atm (3) (specific power deposition 7 W/cm3; scale division 2 ms) [40]

Unfortunately, studies [38, 39] provided only a schematic description of the conditions of experiments on the ACRR reactor, and there are virtually no data on the characteristics of the laser radiation. Only in study [40] is there some informa­tion on the results of the first experiments with the Ar-Xe mixture (2 = 1.73 pm) at various pressures and specific power depositions in the case of placement of the laser cell in the central channel of the reactor (Fig. 6.24).

The distribution of the radiation intensity, shape, and area of a light spot on the cavity mirror surface of a sealed laser with plane-parallel arranged uranium layers was studied in ref. [52]. At the moment in time that lasing originates, the radiative zone is concentrated near the laser cell axis, then it begins to enlarge and its shape becomes close to rectangular at the lasing pulse peak. Radiation intensity distribution is symmetrical relative to the planes of symmetry, y — const and x — const, that passes through the optical axis. The lasing volume reaches ~60 % of the laser cell’s volume.

This behavior is determined by the dynamics of the development of gas density inhomogeneities and the spatial redistribution of specific energy deposition in the laser’s sealed cell over the course of the pumping pulse. It is natural that the dynamics for a laser with transverse gas pumping have a different, more complex nature. Consequently, the pattern of radiation intensity distribution throughout the laser channel cross-section should change as well, which was also experimentally proven in ref. [53].

During experimentation, an Ne-Ar-Xe mixture (300:100:1) at a pressure of 1 atm was used, which was flowed through the laser channel of an LM-4 module

at a velocity of ~7.2 m/s. The average thickness of the metallic uranium layers (90 % enriched by 235U) was ^ = 2 .67 pm. The uranium layers were deposited to aluminum substrates with a thickness of 4 mm and were covered with a thin (0.5 pm) aluminum film for the purpose of preventing 235U atom ejection into the gas [54, 55]. Just as during the experiments described in ref. [52], which involved a sealed channel, the distance between the layers was 2 cm and the channel dimen­sion along the gas flow direction was 6 cm. The length of the laser channel along the optical axis direction was 1 m. A high-reflectivity spherical dielectric mirror with a radius of curvature of 20 m and a semi-transparent planar dielectric mirror were used as the cavity mirrors. The distance between the mirrors was 1.4 m.

Excitation was accomplished by means of neutrons from the core of a BIGR reactor. Diagrams of the gas flow direction, as well as the mutual position of the laser module and the BIGR reactor, are presented in Figs. 9.1 and 9.15. The duration of the reactor’s exciting quasi-pulse was ~1.5 s. The average specific power deposition in the gas over the length of the laser channel at the exiting pulse peak reached ~6.2 W/cm3. The lasing threshold came to ~35 % of the maximum neutron flux. The maximum laser output power reached 12 W.

A charge-coupled camera operating in the infrared region of the spectrum recorded an image of the laser spot. An image of the laser beam’s transverse cross-section obtained from a single laser module channel was formed on the sensing element of a camera with a pixel size of ~0.1 mm. During neutron pumping, the camera operated in the burst recording mode and captured images of the laser spot at an interval of 30 ms. Over the course of the pulse, 25 frames were produced that depicted the laser beam’s development in chronological order.

FSA-G1 sensors with a photoresistor based on lead sulfide and IMO-2N optical radiation power meters were used to determine laser output power.

The experimental dependences of neutron flux density and laser output power upon time are shown in Fig. 9.27. Lasing begins at the moment in time that the

Fig. 9.27 Time dependencies of neutron flux density (1) and laser output (2) threshold is reached and thereafter its dependence upon time almost fully duplicates the similar dependence of neutron flux density.

Radiation intensity distributions at the surface of output mirror are presented in Fig. 9.28 for a number of successive moments in time.

The relative distribution of laser radiation intensity in the perpendicular and parallel directions relative to the gas flow’s motion is shown in Fig. 9.29.

At a velocity of U ~7 m/s and a channel dimension along the gas flow direction of b = 6 cm, the typical time that a portion of the gas resides in the channel is t0~bU~ 10~2 s, which is considerably shorter than the neutron pulse’s duration. This fact makes it possible to apply the procedure described in ref. [46] for calculating the gasdynamic characteristics of the stationary excitation mode to the calculation of the spatial distributions of gas density and specific power deposition in the laser channel at each given moment in time.

The cumulative results of the experiments and the calculations (Fig. 9.30) performed using the procedure described in ref. [46] under conditions of an excitation and a geometry that fully corresponds to the experimental conditions categorically evidences that:

• In flowing nuclear-pumped lasers, lasing is “spawned” in two regions that are symmetrical relative to the longitudinal plane of symmetry and that closely border the laser channel gas inlet, within which the specific power deposition and gas density approach the maximum values;

• In direct proximity to the uranium layer surface where power deposition reaches the maximum value, two narrow regions exist that do not take part in lasing, the presence of which can be explained by the origination of passive zones; and

• As the neutron flux density and subsequently power deposition increases at each given point in the gas volume, both of the symmetrical lasing zones merge into one, which expands in the gas flow direction, reaching a maximum size (~60 % of the laser channel’s volume) at a moment in time that corresponds to the exciting pulse peak.

A brief historic outline of NPL developments to date in the United States is given in Table 13.1. Clearly, parallel developments were achieved in Russia, and a history of these developments is discussed in previous chapters. Also, the history of early developments in the United States has been detailed in several review articles [1, 2, 10—13], so only the highlights of these earlier studies will be given on here before turning to more recent developments.

The first demonstration of an NPL was in 1974, when a y-ray-driven laser pumped by a thermonuclear explosion was reported [14, 15]. To distinguish the work focused on here from such experiments, some researchers have proposed the term reactor-pumped lasers (RPLs), but this discussion will continue to use the traditional NPL terminology. A similar concept using a fission reactor had been proposed as early as 1963 by Lloyd Herwig, who analyzed a CO2 laser approach [16, 17]. Despite attempts by several groups to demonstrate pumping of gases such as CO2, the first successful NPL was not achieved until 1975, when lasing of CO using a fast-burst reactor was reported at Sandia National Laboratories [7]. Since then, some 20 different experimental NPLs have been reported by groups at Sandia, Los Alamos National Laboratory (LANL), NASA Langley Research Center, and the Universities of Illinois, Missouri, and Florida.

Under the influence of reactor n, y-radiation on the photoreceivers, additional radiation currents arise, integral and spectral sensitivities are altered, the level of intrinsic noise increases, and the shape of the volt-amp characteristic is changed [40]. The degree of influence of reactor radiation depends first of all on the type of receiver and the absorbed dose rate. Some results for the radiation resistance of semiconductor photoreceivers are shown in study [44], while study [40] shows the results of measurements of radiation currents for certain types of semiconductor photoresistors and photodiodes, as well as photomultipliers and photocells.

In experiments with NPLs with pulsed reactors, various types of receivers are used, which make it possible to measure the energy, power, and the spectral and time characteristics of laser radiation. Some of them (thermal gauges of laser pulse

energy, photocells with an external photo effect) can be located at distances of 2­5 m from the core of the pulsed reactor with absorbed doses of 1-10 Gy (103­104 Gy/s), while others (photomultipliers and semiconductor photodiodes) are outside the boundaries of the reactor hall, where absorbed doses do not exceed ~0.01 Gy (~10 Gy/s). The possibility of using specific receivers to measure the characteristics of the NPLs must be tested in additional experiments.