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by N. BASOV, director of the FIAN, USSR Academy of Sciences;
Yu. MIKHAILOV, Cand. Sc. (Phys. & Math.),
V. ROZANOV, Dr. Sc. (Phys. & Math.),
G. SKLIZKOV, Dr. Sc. (Phys. & Math.),
Staff members of the same institute
Laser thermonuclear fusion is one of the most consistent and advanced approaches to controlled nuclear fusion. The notion was advanced for the first time by Academician N. Basov, future Nobel laureate. Laser radiation can heat deuterium or a mixture of deuterium and tritium to such a high temperature and over such a short time that before thermal motion ensues, the fusion of hydrogen isotopes into helium nuclei, or nuclear fusion may set in. The energy released as a result will exceed the amount of energy expended in heating and the inevitable losses if plasma is heated to a temperature of 100 min C and the product of fuel density and the rate of the reaction known as Lowson's criterion exceeds a certain critical value of the order of 10 14 cm 3 /s. In an industrial reactor fusion must sustain itself and that is why it was thought originally that the fuel must be as dense as possible and that a hydrogen "ice" frozen at extremely low temperatures should be used instead of a gas.
An electric power station based upon that notion must comprise a laser system irradiating a target containing thermonuclear fuel. A glass or plastic pellet is used for such a target. The energy of neutrons released by nuclear fusion is converted into electricity by conventional methods employed at nuclear power stations.
In the 1960s laser nuclear fusion became the main venue for fusion studies at FIAN. The laboratory of quantum radiophysics developed lasers with an output of 10 10 W, a considerable achievement for the time, and carried out a variety of experiments concerned with interactions between laser radiation and matter at energy flux density at the target reaching 10 6 - 10 9 W per cm 2* , much brighter than the light of the most powerful floodlight. But that was not enough for a nuclear fusion reaction to commence. The density of the flux had to be increased by 4 to 5 orders of magnitude, but first a clear understanding had to be acquired of what "laser plasma" really is and what processes take place in it under the action of laser radiation, how it is spatially and temporally distributed after irradiation, and of what particles it consists.
During the experiments made at the time a solid target was exposed to laser radiation to find out how many electrons are lost as the atoms of a given element turn into ions. The radiation of multiple charged ions was investigated to determine the temperature, density, and other plasma characteristics important for target monitoring. Having studied the spectra of various elements in the periodic table in great detail scientists at FIAN began to investigate the process of interaction between laser radiation and extremely hot plasma.
In the initial experiments the temperature and density of hot plasma were considerably below those required for nuclear fusion. A breakthrough came in 1968 when thermonuclear neutrons were detected for the first time after a target of a compound of lithium and deuterium was exposed to laser radiation pulses of an extremely short duration of the order of 10 -11 s. The result fell short of
* One of the most important properties of laser radiation, showing the amount of energy falling on the target area unit in the time unit. Given short (10 -9 s) laser pulses and small size targets, this figure can be very big even for comparatively weak laser.- Ed.
Lowson's criterion, but now FIAN had the means to measure what should be measured...
For laser nuclear fusion to become a source of energy plasma must not only be heated, but also compressed to a density hundreds of times greater than solid state density. Bearing this in mind FIAN developed a technique for spherical plasma heating and subsequent target compression. By this technique the outer layers of the target are heated to tens of millions of degrees and evaporate. The process of that explosive evaporation is known as ablation. It results in a pressure momentum which compresses the internal unheated part of the plasma and the so-called compression by ablation. Heating must be uniform along the entire target surface. A theoretical and experimental method had to be devised for heating and compressing a ball of plasma.
FIAN pioneered the use of spherical targets in 1972 with the help of a 9-channel setup based on a neodymium laser and later named KALMAR. The experiments yielded record parameters and made it possible to achieve a breakthrough in fuel compression down to 8 g/cm3 , heating to temperatures of the order of 10 min 0 C, and neutron generation. The experiments came as an important incentive for laser fusion studies around the world.
The power of the laser beam should not be increased indefinitely for nuclear fusion initiation by spherical heating. After a certain limit has been reached a powerful X-ray radiation flux in the surface layer of the target appears and the emission of high-energy electrons begins. This wastes a considerable amount of laser energy, and to make things worse. X-ray quanta and "hot" electrons heat up the core of the target much too soon, which prevents the required degree of compression from being attained.
A high laser radiation flux density has been reached in plasma heating equipment used now; it is of the order of 10 16 - 10 17 W/cm 2 . This is several hundred to several thousand times the required value. Irradiation procedures are needed therefore for heating plasma to the required temperature at moderate flux densities of the order of hundreds of billions of watts per cm 2 of the target area.
There are two factors impeding plasma compression; the above- mentioned heating of the target at the irradiation stage and the deviation of the target's shape from the ideal sphere. The latter problem proved difficult, but nev-
ertheless soluble. One way to resolve it is to use a target as close to the ideal sphere as possible and irradiate by a greater number of beams.
There are different ways of tackling the former problem. The notion evolved in the early 1970s by FIAN and the Institute of Applied Mathematics of the USSR Academy of Sciences was based on entirely different ideas. The requirements to be met by laser pulse parameters were stringent, while the uniformity of compression and target heating were achieved by making target structure more sophisticated. Now the target comprised of a thin shell filled with fuel, which no longer had to be solid hydrogen. The shell consisted of several layers which ensured a high laser radiation absorption rate as well as a high compression rate. Besides, a separate layer insulated internal layers of fuel against overheating by penetrating radiation. That technique exposed a larger amount of fuel to heating and compression compared to pulse modulation. In principle such a target makes it possible to generate energy several hundreds or even thousands of times greater than energy expended in heating.
Since laser output was moderate, premature heating was effectively ruled out by a "multiple-shell" target. Practically no high- energy electrons were generated and hard X-ray radiation was kept at a low level. The method became known as "low entropy compression", but it, too, posed a number of problems. The most important thing was to ensure that the process proceeds symmetrically As soon as compression by ablation becomes nonuniform in one place of the target shell or another, plasma escapes from the laser "squeeze" before it is heated or compressed properly.
At the time many scientists subscribed to the view that a stable target compression is impossible if thin shells were used, but thick shells could not be used either, because they imploded too slowly and did not compress plasma sufficiently The idea had to be advanced both theoretically and experimentally to evolve an optimum solution. This work continued between 1975 and 1984.
In cooperation with a team of scientists of the Institute of Applied Mathematics of the USSR Academy of Sciences led by Academicians A. Tikhonov and A. Samarsky, FIAN theoreticians developed computer programs modeling target processes. Combined with experiments they gave an important insight into the physics of laser plasma and helped to understand what processes substantially influence target compression and heating and the course of fusion and thereby made it possible to disregard the unimportant ones.
The initial experiments with low-entropy compression were carried out with the help of the above-mentioned KALMAR equipment. At the same time a setup of the more advanced DELFIN series was on the drawing boards. It was considerably more powerful, had more channels, and was better adapted to compressing larger and more sophisticated targets.
KALMAR was also used to test entirely new techniques and devices for target monitoring. FIAN, for instance, developed instruments capable of measuring the parameters of plasma heated to fusion temperatures and compressed to 10 19 -10 25 particles to 1 cm 3 at a spatial resolution of 1-10 mu and a temporal resolution of 10 -10 -10 -9 s. Equipment for target monitoring elements of the laser system of KALMAR and DELFIN were developed and manufactured in cooperation with the Central Institute of Optics and Spectroscopy of the GDR Academy, the Schiller University in Jenna, the GDR, and the Polish Institute of Plasma Physics and Laser Microsynthesis.
Important work was underway at FIAN's laboratory of neutron physics to develop targets. The present-day laser fusion target consists of a spherical shell filled with fuel. The shell made of different materials has a radius of 100-1,000 mu, with the wall thickness amounting to 0.3-5 percent of the radius. To ensure stable compression allowances for nonuniformity and deviations from required wall thickness and sphericity are kept within one percent.
Research done at FIAN dispelled fears that it is impossible to achieve stable plasma compression as factors stabilizing the process were found. A highly heat-conductive target shell made it possible to distribute laser heat along target surface more evenly and thereby reduce heating nonuniformity. Another breakthrough came as gaseous deuterium with the initial pressure of 36 atm was compressed by a factor of 1,000 to a density of 8 g/cm 3 in the KALMAR machine. The confinement criterion was met by a wide margin; the product of plasma density and retention time was about 5x10 14 s/cm 3 . The breakthrough came, however, at a temperature that was lower than the required one by an order and at target dimensions smaller than those required for self-sustained nuclear fusion.
Theoreticians have suggested more sophisticated targets to increase stability FIAN has tested composite-shell targets (polystyrol-vacuum-glass-fuel). The transfer of energy through the impact of the external laser-irradiated shell against the internal one improves the spherical symmetry of heating and compression.
At the end of 1981 FIAN's laboratory of laser plasma put one of the world's largest laser fusion machines into operation, DELFIN I, and opened a new stage in the DELFIN program by developing powerful multichannel neodymium laser equipment. 108 laser beams, each 45 mm in diameter, formed six groups to ensure spherically symmetrical heating of targets via 6 channels with beam energy kept at several kJ. The objective of the study was to establish the parameters of low-entropy heating and compression of targets. Operating at laser energies of tens kJ will, in our view, make possible compression of the order of 30 g/cm 3 at plasma temperatures sufficient for a considerable energy yield (no less than 1010 neutrons per pulse).
In 1982-1984 DELFIN I experiments used laser energies up to 2 kJ and energy flux densities of 10 14 W per cm 2 . The so-called high-aspect targets were used with the aspect ratio (the ratio of target radius to shell thickness) upwards of 100. The most important thing was to establish whether
stable compression could be achieved by the low-entropy technique. Results of considerable importance have been achieved; targets with an aspect ratio of 150-250 imploded at up to 300 km/s compressing plasma by a factor of 3,500. It transpired that the degree of compression decreases with the reduction of laser pulse duration front. The same was observed at aspect ratios of more than 250.
DELFIN I reached a ratio of absorbed-to-incident energy of 50 percent and it was shown that the ratio, also known as the absorption coefficient, increases with laser energy in accordance with the so-called classical absorption concept predicted by theoreticians as the chief feasibility condition for a laser fusion reactor. Experimental results agree with theory and that makes it possible to forecast a nearly 100 percent absorption of laser radiation in a reactor.
It is impossible to describe all the results of the program in one article. It transpired, for instance, that target compression may generate ultrastrong magnetic fields of the order of 10 5 - 10 8 Gauss capable of influencing the course of nuclear fusion, and among other things, spinning alfa particles (helium nuclei produced by the fusion of hydrogen isotopes) and confining them to the target zone. As a result, the energy which otherwise might be carried away by the alfa particles can in certain circumstances be spent on sustaining the reaction, which greatly facilitates the task of the future reactor: to produce as much energy as possible at minimum consumption. Magnetic fields are generated within the spherical target as compression instabilities proliferate. An objectionable effect has come in very useful, indeed.
Another example: even in relatively modest experiments (laser energy of about 1 kJ) the evaporated target material is scattered at tremendous velocities of up to 1,000 km per second. And in a thermonuclear explosion one can expect superhigh pressures of about 10 14 atm and temperatures above one bin degrees. Before it was simply unthinkable to obtain, let alone study, matter with such parameters. Such studies would be of interest for specialists in various fields of science and technology As a powerful source of ions, neutrons and X-rays, laser plasma will certainly be an object and also a tool of future research. For example, laser compression of a target has been used to model early stages of development of the Earth.
Studies by FIAN researchers in this field enjoy international recognition. In the United States experiments of this kind were conducted two years later than in the USSR. Today the Soviet concept is generally accepted and most experiments in the world follow the FIAN pattern.
Quite a few problems have as yet to be resolved. The studies of the stability of compressing high aspect ratio targets must go on. It is not yet clear how the generation of high-energy particles can be dealt with. It interferes with compression and wastes considerable amounts of laser energy. Targets with a maximum neutron yield have not yet been developed. The physical threshold beyond which fusion becomes self-sustaining has not yet been reached, and finally, a number of important problems of fusion combustion have not yet been solved.
The next stage is the development of a reactor. Studies in the physics of reactor plasma will continue. Laser control is to be automated and lasers themselves must be improved to reach energies of 10 5 -10 6 J in an industrial reactor.
In the early 1980s FIAN researchers began assessing the parameters to which lasers capable of operating in a reactor must measure up. The requirements are very stringent indeed. Over 10- 20 nanoseconds the machine must emit between 3 and 5 MJ of energy The machine will consist of several dozen lasers (channels), each carrying upwards of 100 kJ. The entire energy must be focused on a target about 1 cm in diameter from a distance of 30-50 m and the machine must be capable of operating at a frequency of 10 Hz. It must also be capable of operating continuously for at least one year at a reasonable cost. Neodymium lasers available today measure up to these requirements, but their efficiency is of the order of 1 percent which is too low. The CO 2 laser is the most attractive of those available today and these include KrF-, HF-, and CO-lasers. Its chief drawback is that it operates on a long wavelength of 10.6 mu . The interaction of that sort of radiation with the target will generate high-energy electrons on its surface and it is not clear how these can be dealt with.
The presence of fast particles in laser plasma may overheat target core and interfere with compression, but according to some estimates their energy can be used in certain circumstances to heat and compress the fuel. It transpired that it is possible to select irradiation conditions and target shell structures that would ensure a uniform preheating within reasonable limits and enhance compression. It is, however, very difficult indeed to establish optimum conditions and intensive studies are under way to resolve the problem.
The ultimate aim of FIAN researchers is an industrial fusion reactor. In the early 1970s FIAN evolved the general notion of such a reactor for the first time in the USSR. In 1975 it was discussing three types of reactor, the fusion reactor, the hybrid reactor, and the reactor producing free hydrogen, a chemical fuel.
The hybrid reactor in very general terms consists of any fusion reactor surrounded by a blanket of fissionable material, for instance, uranium. Fusion neutrons induce a chain reaction in the blanket. In such a reactor demands on plasma temperature and plasma confinement parameters are not as stringent as in the fusion reactor.
Compared to other fusion machines the laser one is more attractive above all because of its small dimensions, or rather the small dimensions of the reactor since the lasers are not connected to it mechanically and can be located outside the reactor. The smaller the dimensions of the reactor the smaller is the blanket surface, and the higher is the density of the flux of fusion neutron incident upon it and the efficiency of the reactor.
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N. Basov, Yu. Mikhailov, V. Rozanov, G. Sklizkov, LASER FUSION, A REVIEW OF PROGRESS // London: Libmonster (LIBMONSTER.COM). Updated: 14.09.2018. URL: https://libmonster.com/m/articles/view/LASER-FUSION-A-REVIEW-OF-PROGRESS (date of access: 21.04.2021).
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