A new technique is presented for obtaining highly smooth focused laser beams. This approach is consistent with the constraints on frequency tripling the light, and it will not produce any significant high-intensity spikes within the laser chain, making the technique attractive for the high-power glass lasers used in current fusion experiments. Smoothing is obtained by imposing a frequency-modulated bandwidth on the laser beam using an electro-optic crystal. A pair of gratings is used to disperse the frequencies across the beam, without distorting the temporal pulse shape. The beam is broken up into beamlets, using a phase plate, such that the beamlet diffraction-limited focal spot is the size of the target. The time-averaged interference between beamlets is greatly reduced because of the frequency differences between the beamlets, and the result is a relatively smooth diffraction-limited intensity pattern on target.
The performance of gas-filled, plastic-shell implosions has significantly improved with advances in on-target uniformity on the 60-beam OMEGA laser system ͓T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 ͑1997͔͒. Polarization smoothing ͑PS͒ with birefringent wedges and 1-THz-bandwidth smoothing by spectral dispersion ͑SSD͒ have been installed on OMEGA. The beam-to-beam power imbalance is р5% rms. Implosions of 20-m-thick CH shells ͑15 atm fill͒ using full beam smoothing ͑1-THz SSD and PS͒ have primary neutron yields and fuel areal densities that are ϳ70% larger than those driven with 0.35-THz SSD without PS. They also produce ϳ35% of the predicted one-dimensional neutron yield. The results described here suggest that individual-beam nonuniformity is no longer the primary cause of nonideal target performance. A highly constrained model of the core conditions and fuel-shell mix has been developed. It suggests that there is a ''clean'' fuel region, surrounded by a mixed region, that accounts for half of the fuel areal density.
OMEGA, a 60-beam, 351 nm, Nd:glass laser with an on-target energy capability of more than 40 kJ, is a flexible facility that can be used for both direct- and indirect-drive targets and is designed to ultimately achieve irradiation uniformity of 1% on direct-drive capsules with shaped laser pulses (dynamic range ≳400:1). The OMEGA program for the next five years includes plasma physics experiments to investigate laser–matter interaction physics at temperatures, densities, and scale lengths approaching those of direct-drive capsules designed for the 1.8 MJ National Ignition Facility (NIF); experiments to characterize and mitigate the deleterious effects of hydrodynamic instabilities; and implosion experiments with capsules that are hydrodynamically equivalent to high-gain, direct-drive capsules. Details are presented of the OMEGA direct-drive experimental program and initial data from direct-drive implosion experiments that have achieved the highest thermonuclear yield (1014 DT neutrons) and yield efficiency (1% of scientific breakeven) ever attained in laser-fusion experiments.
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