Novette Target-Physics Experiments

Energy and Technology Review Lawrence Livermore National Laboratory April 1985 Novette Target-Ph ysics Experiments We have used the Novette laser to explore physical effects vital to the attainment of ICF goals: those governing the transfer of energy from a laser beam to a target capsule and the peak compression reached in the fuel. The objectives of the LLNL laser to produce the targets for laser For further information contact fusion program are to understand and irradiation, and develop our modeling R. Paul Drake (415) 422-6706. develop the inertial-confinement fusion capability so that we can accurately (IeF) approach to controlled specify both the laser facility and the thermonuclear power for both military targets we will need to achieve our goal and civilian (i.e., power-production) of producing high-gain implosions. applications. In IeF, an intense energy Target-physics experiments serve source, such as a laser, is used to two purposes. First, they determine the implode a capsule containing constraints on lasers and targets intended thermonuclear fuel (e.g., a mixture of to produce high-gain implosions. Second, deuterium and tritium) to high densities they test our understanding of the low and temperatures. Weapon-physics gain implosions we can now produce. experiments have already been To do these novel experiments, we have conducted with the Novette laser, and had to develop many new diagnostics more are planned with the Nova laser and experimental techniques.} Figure 1 facility. However, full realization of the illustrates the variety of diagnostic military and civilian IeF applications instruments arrayed around a single reF requires, first, the attainment of well experiment. Our experiments with diagnosed, high-gain performance of Novette2 produced more information thermonuclear capsules. per laser shot than any previous reF To achieve such high-gain implosions experiments, helping us to make rapid (in which the energy released by progress in several areas of target implosion and fusion of the fuel is many physics. times the laser input energy), we must Our target-physics studies can be advance the state of the art in laser roughly categorized either as laser construction, target fabrication, computer plasma interaction experiments or as modeling, and target-physics implosion-physics experiments. In the experiments. We must learn to build laser-plasma interaction experiments, we increasingly powerful lasers at an explore the phenomena that occur as the affordable cost, develop new materials laser light interacts with the target and fabrication and assembly techniques plasma (e.g., absorption and hot-electron 35 production). This information, in turn, targets with one or two beams of green is used to define laser and target (0.53-.um) or ultraviolet (O .26-.um) light, specifications. For example, a poor choice frequency-converted from the laser's of laser and target can produce too many fundamental infrared (1.0S-.um) hot electrons, which prevent high-gain wavelength. At the end of the laser implosions. Laser-plasma interaction amplifier chain, each 74-cm-diameter experiments in which we study hot beam was focused by a large lens to electron production enable us to avert strike a spot on the target less than a few this problem by better target design. millimetres in diameter. The concentrated In implosion-physics experiments, we laser light had an intensity greater than focus on understanding the physics of 10 14 W/cm2 (more than a million billion imploding targets. The detailed modeling times the intensity of sunlight at the required in these experiments is done earth's surface). with the the LASNEX computer code.3 The high temperatures produced by Well-diagnosed implosion experiments such a beam cause the surface of the enable us to compare predicted and target to ablate, and the electrons in the measured performance. The models can heated material are torn from their atoms then be improved against the deficiencies to form a fully ionized gas called a exposed and further tested. These two plasma. The laser light absorbed by the types of experiments-laser-plasma plasma heats it to so high a temperature interaction and implosion physics-thus that it radiates soft x rays. The soft x rays provide a bootstrap process, in which strike the capsule and vaporize its progress in one area supports and surface. The vaporized material blows off Fig. 1 facilitates progress in other areas. equally in all directions at high velocity. The array of diagnostic instruments To understand why such experiments This violent outward flow of material used in Novette target-physics experi are necessary, we must look at the produces an equal and opposite reaction, ments. These instruments imaged sequence of events that produce a fusion causing a rocket-like inward thrust that the source and measured the timing reaction. In meaningful laser-fusion compresses the fuel to high density, and spectral composition of the light and x rays from the target, as well experiments, the laser pulse must strike simultaneously heating at least part of it as the neutron yield and the neutron an appropriate target at the right to the ignition temperature of the fusion activation. location. Our Novette laser irradiated reaction. If the inertia of the burning fuel confines it long enough (hence the term Radiochemistry ~ inertial confinement fusion), there will be D a net production of energy. o Arrays of photodiodes There are several villains in this piece, Neutron one of which is the high-energy yield electrons generated in the plasma by <' Raman scattering and other processes. o These hot electrons have enough energy to penetrate deep into the fuel, depositing energy and heating it. This is exactly what we want to avoid. Hot fuel is harder to compress than cold fuel, so ~ t ========:J prematurely heated targets never reach Ta ..etl t:J the densities they would otherwise theoretically attain. It has become clear that one of the major keys to the ) Light collectors development of high-gain implosions is ¢:) for optical to suppress these high-energy electrons. ~ spectroscopy We used the Novette laser to B investigate several critical aspects of this o o ~\~OPtical/x_ray complex process: the interaction of the streak camera Dante soft-x-ray j ~ laser light with the plasma, the efficiency spectrometers J X-ray microscope of soft-x-ray production, and the compression of the D-T fuel to high Cassegrain Leake. "y,tal X-<oy,peclromele< optical pyrometer spectrograph density. 36 INERTIAL FUSION ser-Plasma Interaction Raman scattering was insignificant Experiments because the scattered light wave and the UIn laser-plasma interaction plasma wave could not become very experiments, we try to determine what intense before escaping the resonant happens when a high-intensity laser region. Because the Novette laser was so beam penetrates and interacts with a much more powerful, we could spread plasma. The mission of such experiments its beam over a larger target area and for ICF is to identify the processes that still maintain the same intensity as in the affect high-gain targets and to discover Argus experiments. This produced larger, how they vary as laser and plasma more planar plasmas that gave these parameters change. waves room to grow more intense before This mission has three parts. First, escaping. The plasmas in fu ture, high we must produce and diagnose plasma gain experiments will be even larger. conditions that are relevant both to current target-physics experiments and Raman Scattering in Large to future high-gain targets. Second, we Plasmas must identify both those processes that To simulate these future, larger can scatter the laser light and keep it plasmas, we used thin foil targets made from being absorbed in the target and of plastic or gold. These targets exploded those that convert the laser light into hot early in the laser pulse and expanded electrons. Finally, we must determine the both toward and away from the laser to severity of adverse effects and, if produce a very large plasma. Figure 2 is necessary, mitigate them. The study of a time-resolved image of the ultraviolet Raman scattering illustrates all three of bremsstrahlung emission from such a these. target, color-coded by computer The most desired process for ICF by processing to show the variation in which laser light is absorbed in a plasma intensity of the emissions as a function is collisional absorption. In this process, of time (horizontal axis) and distance the incident light wave causes the from the target plane (vertical axis). electrons in the plasma to oscillate. The Figure 3 compares data from a oscillating electrons then collide with the large number of such shots with the ions in the plasma, absorbing the energy predictions of the LASNEX computer of the light. code. Figure 3a shows the density along In contrast, Raman scattering converts the axis of the plasma, at the center of the incident light into two waves. One the I-ns laser pulse. The gray area shows of these is an electron plasma wave, the behavior predicted by LASNEX, and in which undulations in the electron the solid circles show the density profiles density travel through the plasma. The energy in an electron plasma wave is often converted into (unwanted) hot electrons. The second kind of wave is a scattered light wave, which escapes and carries its energy out of the plasma. Raman scattering, thus, prevents the laser light from being absorbed gradually through electron collisional absorption Fig. 2 and also produces harmful hot electrons.4 A thin plastic film , irradiated with green Raman scattering, like several other (O.S3-jlm) light from the Novette laser, explodes to produce a large, uniform interaction processes, occurs only at a plasma. This streak-camera image was specific location in the plasma (a made with the ultraviolet light emitted resonant region) where the laser light by the plasma.

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