Short-wavelength, direct-drive laser fusion experiments at the Laboratory for Laser Energetics

Measurements are reported of absorption, hot electron generation, preheat, thermal transport and irradiation uniformity in direct-drive, spherical target experiments conducted with six 351-nm wavelength beams of the OMEGA symmetric irradiation facility. On-target energy in excess of 400 J, pulses of 650 ps duration and uniform irradiation at an intensity in the range of 10/sup 13/ to 10/sup 15/ W/cm/sup 2/ characterize the laser conditions for these experiments. Absorption varied from 100%, for Ti targets at an intensity of 10/sup 13/ W/cm/sup 2/, to 60% at an intensity of 2 x 10/sup 15/ W/cm/sup 2/. The suprathermal electron fraction was found to be less than 3 x 10/sup -3/ of the absorbed energy in these experiments. Higher ablation pressures were measured than in comparable 1054-nm experiments. At a laser intensity of 10/sup 15/ W/cm/sup 2/ an ablation pressure of 100 Mbar was deduced from plasma blowoff measurements. Measurement and calculation of the irradiation uniformity indicate that the 24 beam OMEGA, 351-nm laser should be capable of producing the required drive uniformity to compress targets to 200 times liquid DT density with less than 2000 joules of laser energy.