Physics of Atomic Nuclei, Vol. 63, No. 12, 2000, pp. 2051–2066. Translated from Yadernaya Fizika, Vol. 63, No. 12, 2000, pp. 2147–2162. Original Russian Text Copyright © 2000 by Voronchev, Kukulin. REVIEW Nuclear-Physics Aspects of Controlled Thermonuclear Fusion: Analysis of Promising Fuels and Gamma-Ray Diagnostics of Hot Plasma V. T. Voronchev1) and V. I. Kukulin Institute of Nuclear Physics, Moscow State University, Vorob’evy gory, Moscow, 119899 Russia Received February 15, 2000 Abstract—A brief survey of nuclear-physics aspects of the problems of controlled thermonuclear fusion is given. Attention is paid primarily to choosing and analyzing an optimal composition of a nuclear fuel, reliably extrapolating the cross sections for nuclear reactions to the region of low energies, and exploring gamma-ray methods (as a matter of fact, very promising methods indeed) for diagnostics of hot plasmas (three aspects that are often thought to be the most important ones). In particular, a comparative nuclear-physics analysis of hydro- gen, DT, and DD thermonuclear fuels and of their alternatives in the form of D3He, D6Li, DT6Li, H6Li, H11B, and H9Be is performed. Their advantages and disadvantages are highlighted; a spin-polarized fuel is consid- ered; and the current status of nuclear data on the processes of interest is analyzed. A procedure for determining cross sections for nuclear reactions in the deep-subbarrier region is discussed. By considering the example of low-energy D + 6Li interactions, it is shown that, at ion temperatures below 100 keV, the inclusion of nuclear- structure factors leads to an additional enhancement of the rate parameters 〈σv〉 for the (d, pt) and (d, nτ) chan- nels by 10–40%. The possibility of using nuclear reactions that lead to photon emission as a means for deter- mining the ion temperature of a thermonuclear plasma is discussed. © 2000 MAIK “Nauka/Interperiodica”. 1. INTRODUCTION reactions for extracting required information about the dynamics of hot plasmas. In our opinion, attention Among important applications of nuclear physics, given to these important problems in review articles on that which is dealing with the problem of controlled controlled thermonuclear fusion and in original investi- thermonuclear fusion stands out in many respects. One gations into the allied range of problems is insufficient. of the main problems to be solved here is to evolve and It is shown in the present study, however, that such implement a large-scale thermonuclear reactor that issues are of nonnegligible importance for further would represent an economical source of energy and advancements in the problem of controlled thermonu- which would be safer than fission reactors. Searches for clear fusion and that they may even become crucial for an optimum composition of a nuclear fuel have so far solving some problems. We hope that the present sur- been one of the main lines in such investigations. Both vey, which is pioneering in these realms, will fill, at one- and multicomponent mixtures of light elements least partly, the gap between a vast body of currently have been considered. Despite many years of efforts in available information about the structure of light nuclei these realms, preference has not yet been given to a and specific investigations into the problem of con- unique fuel cycle. Of factors that are of prime impor- trolled thermonuclear fusion. tance for this, we would like to mention knowledge of the properties of light isotopes and the possibilities for Bearing in mind the technological potential of the their production, understanding of mechanisms that first experimental facilities of the tokamac type for govern nuclear reactions between light nuclei, and pre- heating and confining hot plasmas, researchers had cise information about cross sections for such pro- focused, for a long time, on two types of hydrogen ther- cesses. monuclear fuel, deuterium (DD) and deuterium–tritium (DT) fuels [1]. It should be recalled that large cross sec- The present survey, which is based in part on previ- tions for the process T(d, n)4He at low energies (E ≤ ous studies of the present authors, is devoted to nuclear- keV) and a considerable energy release in the reac- physics aspects of controlled thermonuclear fusion— 100 namely, to the role of nuclear-structure factors in reac- tion, Q = 17.6 MeV, seemed to give sufficient ground to tions between light nuclei, to radiative-capture pro- hope for experimentally implementing the ignition of cesses of the A(B, γ)C type in the deep-subbarrier the reaction as early as the 1970s at plasma tempera- region of energies, and to the possibilities of using such tures of a few keV, which were thought to be quite real- istic at that time. These were the reasons why, despite 1) Institute of Crystallography, Russian Academy of Sciences, Le- serious drawbacks of a DT mixture—a low efficiency ninskiœ pr. 59, Moscow, 117333 Russia. of the transformation of nuclear energy into electric 1063-7788/00/6312-2051 $20.00 © 2000 MAIK “Nauka/Interperiodica” 2052 VORONCHEV, KUKULIN energy because of the generation of an extremely fast particles, both charged particles and neutrons, on intense neutron radiation in the reaction being dis- the ions of the fuel. The latter process proceeds at small cussed and a need for producing and employing radio- values of the impact parameter, but it involves high active tritium, which is extremely hazardous—it was momentum transfers. In the literature, the above elas- justifiably thought that there was no serious alternatives tic-scattering processes are referred to as CES (Cou- to DT fuels. lomb elastic scattering) and NES (nuclear elastic scat- However, radical improvements of tokamacs, the tering) processes. Either leads to the heating of the advent of a new fusion reactor within the ITER interna- plasma and to an increase in its reactivity. Ion scattering tional project, and a vigorous development of different is a dominant mechanism of heating, but the role of reactor schemes (for example, open-type devices and a neutron scattering is nonnegligible in severely com- pressed laser targets characterized by large values of configuration featuring a so-called inverted magnetic ρ ρ field), as well as the emergence of the concept of iner- the parameter R, where and R are, respectively, the tial thermonuclear fusion, created preconditions for density and the radius of the target. considering initial plasma heating to a few tens of keV The second class of secondary phenomena accom- followed by the working range of burning temperatures panying the burning of a thermonuclear fuel includes reaching a few hundred keV. This gave impetus to those that lead to nuclear transformations. These are, in studying alternative, non-hydrogen, fuels like D3He, particular, catalytic nuclear reactions between fuel ions H6Li, D6Li, and H11B, which require higher ignition and active isotopes produced in the plasma. Catalytic temperatures (in relation to D + T fusion), but which processes proceed on both thermalized and nonther- possess a number of important ecological and econom- malized nuclei. In the latter case, the processes being ical advantages. For example, burning in the H + 6Li discussed are in-flight reactions—that is, some fast par- and H + 11B systems generates neutron fluxes and haz- ticles enter into nuclear reactions prior to undergoing ardous radionuclides only in negligible amounts. At the thermalization. It is necessary to take such phenomena same time, such fuels are not expensive, since they con- into account because particles produced in a plasma sist of stable light-element isotopes abundant on the have, on average, the mean energy of a few MeV, at Earth. which the cross sections for nuclear reactions are much larger than the corresponding cross sections at thermal At present, however, physical processes in the energies (10–50 keV). The set of in-flight processes is majority of non-hydrogen fuels have received much not exhausted by reactions on the products of nuclear less study than analogous processes in DT or DD fusion—reactions between fast fuel ions accelerated in cycles. Even the first investigations revealed that a the- elastic collisions are also of importance. The above oretical analysis of some alternative cycles is very suprathermal fusion reactions produce charged parti- involved because burning in hot plasmas is a highly cles and neutrons of energies as great as a few tens of ramified process involving a few tens of exothermic MeV [2]. Among other secondary processes of nonneg- reactions proceeding simultaneously with commensu- ligible importance, mention should be made of neu- rate probabilities. This type of process dynamics is tron-induced reactions leading to the production of markedly different from the character of the burning of active isotopes. The role of second-generation pro- a DT fuel, where the channel T d n 4 dominates over ( , ) He cesses is greater in non-hydrogen fuels, where a large all possible reactions. number of active-isotope species are produced owing to In studying thermonuclear fuels that show consider- the occurrence of a wide variety of nuclear reactions able promise, it is necessary to take into account two (see, for example, [3]). types of processes. Of these, the first is associated with Inputs necessary for a realistic analysis of the kinet- first-generation reactions representing direct channels ics of processes proceeding in thermonuclear-fusion of the burning of a nuclear fuel in reactions between reactors must include reliable data on the cross sections light elements originally present in the reactor region. for many reactions in a broad range of energies. Above As a rule, such processes provide a major part of the all, this concerns reactions involving light nuclei and energy released in the fusion process. There are also, proceeding in the low-energy region E < 500 keV, however, secondary phenomena that accompany the which is of prime importance for controlled thermonu- burning of any fuel and which play a very important clear fusion.