Confinement of thermonuclear microexplosions in imploding vortices

Abstract

PRESENT efforts towards laser induced fusion envision spherical pellets consisting of the thermonuclear material in the centre, surrounded by a shell of some inert pusher material and an outer ablator shell. The same principles for the pellet design can be used if the implosion process is induced by a relativistic electron beam or intense ion beam (ref. 1, papers presented to Fifth IAEA Conference, Tokyo, 1974 and J. W. Shearer, unpublished). At least one other design of a non-spherical fuel pellet has been published². This is non-symmetric and would have the advantage of using just one laser beam for target bombardment. But in all of these designs there remains one principal difficulty: laser, electron or ion beams with a sufficiently large total energy output for high density pellet compression and efficient thermonuclear burn have a comparatively long pulse length. Therefore, in order to use these beams for the required high density pellet compression rather large aspect ratios r/Δr (r=pellet radius, Δr=thickness of pusher and ablator material) are dictated. These may result in rapidly growing Taylor-type instabilities. The growth rate for these instabilities could in principle be reduced below dangerous levels by making the pellet and implosion process highly symmetrical. But, since the symmetry requirements are extreme this may be difficult to achieve. The growth rate for the Taylor instabilities could also be reduced by smaller aspect ratios, but smaller aspect ratios need shorter and precisely shaped pulses, which again are difficult to achieve.