Abstract

In the Fast Ignitor concept, hot electrons generated by short pulse interaction, are absorbed in the compressed core of an ICF capsule, creating a hotspot for ignition.¹ This scheme requires fundamental studies of the generation of fast electrons and ions and their transport through the plasma. Previous measurements, from the VULCAN laser, indicate that short pulse/solid target interactions at intensities of 2x10¹⁸ Wcm^-2 result in 10% of the incident laser energy being transferred to fast ions with a mean energy of 1.3 MeV.² In recent experiments, these measurements have been extended to intensities up to ~ 10¹⁹ Wcm^-2. One arm of the VULCAN laser is used to produce short (0.7 - 2 ps) laser pulses of 1.05 μm light, with an energy of up to 30 J. P-polarized light is focused onto the target using an off-axis f/4 parabola, resulting in a ~ 25 μm diam. spot size. In this set of experiments, the energy and number of fast electrons was measured by observing K-α x-ray emission from layered targets, typically 25 - 50 μm of Pd on ~ 50 μn Sn.³ The characteristic K-α emission (20-25 keV) from the two layers was detected with an x- ray CCD camera, operated as a nondispersive spectrometer. These measurements indicated that electrons were generated with energies ~0.5 - 1 MeV. In addition to these spectroscopic measurements, fast ion emission was measured by CR-39 plastic nuclear track detectors, and collimation of the plasma flow was observed by Shadowgraphy and Schlieren optical probing, indicating the existence of large magnetic fields.

© 1994 Optical Society of America