Abstract

Femtosecond high power laser systems offer the posibility to create plasmas with high electron densities and short scale lengths at relatively moderate energies. One possible application of such lasers is the recently proposed ’fast ignitor’ [Tab94]. The basic idea of this scheme is to seperate the compression phase from fuel ignition that leads to lower requirements on the driver lasers. After compressing the fuel by a factor of several hundred the ignition is performed by a burst of electrons that are created by an intense subpicosecond laser pulse. Such suprathermal electrons can be produced by several mechanisms. For laser intensities where the quiver energy equals or even exceeds the electron rest energy m_ec², relativistic effects become important. Now magnetic fields are no longer negligible and the longitudinal component of the Lorentz force accelerates electrons in the longitudinal direction [Wil92], The experiments were performed with the frequency doubled P102 laser at CEA/L-V in Limeil. The laser delivers up to 8 J in about 300 to 400 fs at 528 nm. The conrast ratio is better than 10¹², therefore no preplasma, even at the highest intensity, is created on the target surface before the main pulse. The pulses were focused using an off-axis parabola leading to a minimum focal spot size of 5 μm. The central spot contained about 20 % of the full energy on target, the maximum intensity therefore was about 2 · 10¹⁹ W/cm². The remaining part of the energy is distributed over a large spot.

© 1997 Optical Society of America