fusion energy foundation

_funds into specific programs for developing advancedlasers, light ion, heavy ion, and electron beam drivers, aswell as a reactor technology development program.In substance the report focused on the connectionsbetween inertial confinement fusion and hydrogen bombdesign and the progress in design of laser fusion targets(pellets containing fusion fuel). Although there have beenmajor scientific problems in terms of coupling thfe laserenergy to the target and projecting the resulting compressionand heating of the fusion fuel, the panel found thatprogress has been sufficient to ensure success. In particular,laboratory research combined with continuing progressin H-bomb development and testing has advanced tothe point that particular qualities of the inertial confinementfusion driver needed for scientific demonstrationcan be confidently projected.All the evidence was not available at the time that theFoster report was completed, but the panel did djiscoverthat the krypton fluoride laser appeared to be preciselythe tool to achieve scientific demonstration and providethe technology for power reactors. Developments sincethen have fully confirmed this initial evaluation.How Inertial Confinement WorksIn an H-bomb, the confining pressure is indirectlyprovided by the electromagnetic radiation output of anatom bomb. The majority of the initial energy spectrumof an atomic fission bomb consists of "soft" X+rays. Atarget consisting of hydrogen fusion fuel is placed! next tothe A-bomb. The soft X-rays traveling at the speed of lightimpinge on the fusion target before any blast effects arefelt. These X-rays are absorbed by the surface layer of thetarget. As a result, this material is vaporized and rapidlyexpands outward. This is called ablation. In the samefashion as a rocket engine, ablation generates an equaland oppositely directed force (in this case, an inwardlydirected force called an implosion). The great power ofthe atom bomb X-ray burst quickly transforms this inwardlydirected force into an inwardly directed shockwave. The shock wave acts as a giant compression cylinder,driving the interior of the fusion fuel target to higher andhigher densities.The geometry of the target is arranged so that thecompression shock will converge. When the shock wavefinally reaches the center of the target, the hydrogenfusion fuel has been compressed to densities greater thanthat of lead and hundreds to thousands of times greaterthan that of liquid hydrogen. At this point, the shock wavecollapses on itself, and its energy is transformed into heat.As a result, a minute amount of the core of the compressedfusion fuel is heated to the multi-million-degree temperaturesneeded to ignite thermonuclear reactions (about50 million degrees Celsius). The energy from these initialfusion reactions is absorbed by the "cold" outer layers ofthe compressed fuel, in this way igniting the cold fuel.In fact, this heating is so fast that a supersonic thermonuclear6urn wave is generated, which roars through thecompressed fuel so quickly that most of it reacts beforethe entire target blows up. For a typical H-bomb, all ofthis takes but a few millionths of a second, and the onlyforce confining the fuel while it burns is the inertia of itsown mass.Laboratory inertial confinement replaces the necessarilygigantic atom-bomb "match" with a tiny though just as(a) Laser light hitting fusion fuel target (b) Ablating plasma (c) Decoupled ablating plasmaFigure 1ABLATING IMPLOSION IN LASER FUSIONThe basic phases of ablative implosion are shown here schematically. In the first phase, a, an ablating plasma isformed by the laser light hitting the surface of the fusion target. In phase b, the laser light is absorbed within theablating plasma and the plasma then transports the resulting heat energy to the target surface so that ablation ismaintained. If this transport is interrupted, as /> shown in c, then the ablating plasma (also called corona) becomesdecoupled from the target surface.50 FUSION September 1980