Skin depth plasma front interaction mechanism with prepulse suppression to avoid relativistic self-focusing for high-gain laser fusion F. OSMAN, 1 YU CANG, 1 H. HORA, 2 LI-HUA CAO, 3 HONG LIU, 3 XIANTU HE, 3 J. BADZIAK, 4 A.B. PARYS, 4 J. WOLOWSKI, 4 E. WORYNA, 4 K. JUNGWIRTH, 5 B. KRÁLIKOVA, 5 J. KRÁSA, 5 L. LÁSKA, 5 M. PFEIFER, 5 K. ROHLENA, 5 J. SKÁLA, 5 and J. ULLSCHMIED 5 1 School of Quantitative Methods & Mathematical Sciences, University of Western Sydney, Penrith South, Australia 2 Department of Theoretical Physics, University of New South Wales, Sydney, Australia 3 Institute of Applied Physics and Computational Mathematics, Beijing, China 4 Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland 5 Prague Advanced Laser Source ~ PALS! Institute of Physics and of Plasma Physics, Academy of Sciences, Czech Republic, Prague, Czech Republic ~Received 13 October 2003; Accepted 24 November 2003! Abstract Measurements of the ion emission from targets irradiated with neodymium glass and iodine lasers were analyzed and a very significant anomaly observed. The fastest ions with high charge number Z, which usually are of megaelectron volt energy following the relativistic self-focusing and nonlinear-force acceleration theory, were reduced to less than 50 times lower energies when 1.2 ps laser pulses of about 1 J were incident. We clarify this discrepancy by the model of skin depth plasma front interaction in contrast to the relativistic self-focusing with filament generation. This was indicated also from the unique fact that the ion number was independent of the laser intensity. The skin layer theory prescribes prepulse control and lower ~near relativistic threshold! laser intensities for nonlinear-force-driven plasma blocks for high-gain ignition similar to light ion beam fusion. Keywords: Iodine laser; Ion emission; Laser intensity; Force-driven plasma; Neodymium glass layer 1. INTRODUCTION Studying the ion emission from laser-produced plasmas re- sulted in new aspects for the application in laser fusion after the chirped pulse amplification ~CPA! of Mourou ~Strick- land & Mourou, 1985; Mourou & Tajima, 2002! provided laser pulses of picosecond duration and powers greater than terawatts. For longer pulses in the range of nanoseconds, it is well known ~ Begay et al., 1983; Haseroth & Hora, 1996! that the emission of ions with several hundred megaelectron volts are generated at laser irradiation of targets as expected from relativistic self-focusing ~ Hora, 1975; Jones et al., 1982; Osman et al., 1999; Hora, 1991, 2000a!, where how- ever the ions are emitted nearly isotropic or with some pref- erential direction perpendicular to the laser beam ~ Basov et al., 1987; Häuser et al., 1992!. What is new with the picosecond interaction is that very intense directed beams are produced into the direction of the laser beam. Light ion beams may be interesting for producing spark ignition in laser fusion ~ Roth et al., 2001!. In other cases, directed electron beams ~ Umstadter et al., 1996; Hora et al., 2000; Gahn et al., 2000! were observed. Such electron beams should ignite fusion detonation fronts in large amounts of low compressed ~10 times the solid state! DT where 10 kJ of laser pulses are expected to produce 100 MJ fusion energy ~ Nuckolls & Wood, 2002!. In contrast to these cases of beam generation of particle beams by relativistic effects, we present here a mechanism ~ Hora et al., 2002; Hora, 2003! derived from experiments ~ Badziak et al., 1999, 2003! where the picosecond–terawatt interaction produces plane plasma blocks and where relativ- istic self-focusing had to be avoided very carefully. We report here how the block moving in the direction of the laser light may act as a very intense light ion beam to pro- duce a fusion reaction front in uncompressed solid DT just as known from the earlier discussed scheme of light ion beam fusion ~ Hora, 1983! based on an extensive discussion Address correspondence and reprint requests to: F. Osman, University of Western Sydney, School of Quantitative Methods & Mathematical Sci- ences, Locked Bag 1797, Penrith South DC NSW 1797, Australia. E-mail: f.osman@uws.edu.au Laser and Particle Beams ~2004!, 22, 83– 87. Printed in the USA. Copyright © 2004 Cambridge University Press 0263-0346004 $16.00 DOI: 10.10170S0263034604221164 83