No abstract

Rapid heating of a compressed fusion fuel by a short-duration laser pulse is a promising route to generating energy by nuclear fusion, and has been demonstrated on an experimental scale using a novel fast-ignitor geometry. Here we describe a refinement of this system in which a much more powerful, pulsed petawatt (10(15) watts) laser creates a fast-heated core plasma that is scalable to full-scale ignition, significantly increasing the number of fusion events while still maintaining high heating efficiency at these substantially higher laser energies. Our findings bring us a step closer to realizing the production of relatively inexpensive, full-scale fast-ignition laser facilities.

Metal foil targets were irradiated with 1 mum wavelength (lambda) laser pulses of 5 ps duration and focused intensities (I) of up to 4x10;{19} W cm;{-2}, giving values of both Ilambda;{2} and pulse duration comparable to those required for fast ignition inertial fusion. The divergence of the electrons accelerated into the target was determined from spatially resolved measurements of x-ray K_{alpha} emission and from transverse probing of the plasma formed on the back of the foils. Comparison of the divergence with other published data shows that it increases with Ilambda;{2} and is independent of pulse duration. Two-dimensional particle-in-cell simulations reproduce these results, indicating that it is a fundamental property of the laser-plasma interaction.

Investigations of 7 Li(p,n) 7 Be reactions using Cu and CH primary and LiF secondary targets were performed using the VULCAN laser ͓C.N. Danson et al., J. Mod. Opt. 45, 1653 ͑1997͔͒ with intensities up to 3ϫ10 19 W cm Ϫ2. The neutron yield was measured using CR-39 plastic track detector and the yield was up to 3ϫ10 8 sr Ϫ1 for CH primary targets and up to 2ϫ10 8 sr Ϫ1 for Cu primary targets. The angular distribution of neutrons was measured at various angles and revealed a relatively anisotropic neutron distribution over 180°that was greater than the error of measurement. It may be possible to exploit such reactions on high repetition, table-top lasers for neutron radiography.

Protons accelerated by a picosecond laser pulse have been used to radiograph a 500 microm diameter capsule, imploded with 300 J of laser light in 6 symmetrically incident beams of wavelength 1.054 microm and pulse length 1 ns. Point projection proton backlighting was used to characterize the density gradients at discrete times through the implosion. Asymmetries were diagnosed both during the early and stagnation stages of the implosion. Comparison with analytic scattering theory and simple Monte Carlo simulations were consistent with a 3+/-1 g/cm3 core with diameter 85+/-10 microm. Scaling simulations show that protons>50 MeV are required to diagnose asymmetry in ignition scale conditions.