Abstract.  Fast Ignition holds the promise of improved efficiency and reduced laser energy requirements for Laser Fusion Energy systems. While the concept is simple, to heat a region of the compressed fuel pellet of scale size 40 microns to a temperature of 10keV, the coupling of energy from an ultraintense 10-20 ps laser pulse to this small hot spot is not so simple. The main approach proposed to date is by coupling a beam of 1 to 2 MeV electrons from the interaction laser spot to the fuel core using a metal cone insert to get close to the compressed core. However, multi-millijoule laser prepulses create extended preplasmas within the cone, effectively moving the electron generation source region far back from the cone tip and core [1]. In order to avoid these effects, a much reduced level of prepulse is required which can be obtained by efficiently frequency doubling the laser pulse, thus increasing the contrast by many orders of magnitude.

In the present report, we present preliminary results from high intensity second harmonic experiments carried out at the Titan Laser facility at the Lawrence Livermore National Laboratory. Intensities on target of up to 5 x 10¹⁹ W cm^-2 were obtained with prepulse levels below the measurement threshold of 10 microjoules. A comprehensive set of diagnostics including K-alpha imaging of buried copper layer targets, calibrated HOPG measurements of K-alpha emission, electron energy spectrometer measurements of electron temperature, and angularly resolved high energy Bremsstrahlung emission detectors allowed a quantitative measurement of electron distribution functions, both inside the target and escaping from the target. Measurements were also carried out of the spectral, spatial and temporal characteristics of the reflected light adding additional insight to the interaction process. An artificial prepulse of the order of 3mJ was injected onto the target in a number of shots to compare with the prepulse free shots. Both planar and cone geometries were studied and compared. A summary of the experimental measurements will be presented and discussed in the context of Fast ignition Fusion Energy.

Reference :
[1] A.G. MacPhee et al., Phys. Rev. Lett. 104, 055002 (2010)