Plasma researchers at the University of Rochester have made the first use of a technique for improving a major form of laser-induced nuclear fusion known as "direct drive." In direct-drive fusion, lasers from many directions deposit energy directly on a shell containing fusion fuel; the light causes the shell to implode and trigger fusion reactions.

Traditionally, direct drive has suffered from serious limitations, mainly because non-uniformities in the laser light's intensity cause the shells to implode in a less than optimal fashion. At the University of Rochester's 60-beam OMEGA laser system, researchers have utilized a method, known as "polarization smoothing," for significantly improving the laser beam uniformity. In a large laser such as the ones at OMEGA, each beam typically has unavoidable spatial fluctuations in intensity. To reduce these intensity fluctuations, researchers split each beam into two parts, each containing complementary or "orthogonal" components of the beam's electric field. Each of the polarized beams fluctuates independently of the other, so overlapping them averages or smooths out such intensity modulations. When such beams were used to induce fusion reactions (with the fuel shell imploding to about 7% of its original radius or 1/3000 of its original volume) the primary neutron yield from deuterium or deuterium-tritium filled plastic shells increased by about 70% compared to similar implosions without polarization smoothing. The emission of neutrons is generally proportional to the fusion reaction rate. At the same time, the smoother beams increased the compressed shell's "areal density" (density times radius) by 40-70%. Maximizing the areal density is a major factor for eventually achieving self-sustaining fusion reactions with laser fusion because it increases the opportunity for alpha particles, created as a result of fusion reactions inside the shell, to deposit their energy and heat the plasma further. Theoretical models predict additional improvements. These results bode well for direct-drive implosions of targets on OMEGA and Livermore's planned National Ignition Facility.

For more information, contact David D. Meyerhofer, 716-275-0255, ddm@lle.rochester.edu