Laser-ablative acceleration of targets to near inertial-fusion conditions

NRL is investigating the physics issues involved in the direct illumination approach to laser fusion. Millimeter diameter planar targets are ablatively accelerated to speeds and conditions near those required of fusion pellet shells using 3 to 5 ns, 1.05 μm light from the PHAROS II laser. At 5 x 1013 W/cm2 targets were accelerated to 160 km/sec with a velocity uniformity of +- 7% through twenty times the initial target thickness and while remaining cool (< 10 eV). A joint NRL-LLNL experiment on SHIVA extended these results to beyond 1014 W/cm2; the uniformity of acceleration of dense mm-diameter targets improved to better than +- 3.5% and the interaction physics remained benign. Also, absorption was above 65% and fast electrons were a minor energy channel. Thus, the moderate-irradiance direct-illumination approach appears to be an attractive option for laser fusion. To further verify this conjecture, particular attention is paid to the factors affecting the uniformity of acceleration or targets such as laser beam and target uniformity requirements, and various hydrodynamic mechanisms such as Rayleigh-Taylor and Kelvin-Helmholtz instabilities. New diagnostic techniques are also discussed for measurements of plasma properties. These include: double-foil methods, x-ray backlighting, tracer-material techniques, Zeeman splitting measurements of magnetic fields, and others