Generation of Superponderomotive Electrons in Multipicosecond Interactions of Kilojoule Laser Beams with Solid-Density Plasmas

• Published in: Physical review letters Vol. 116; no. 15
• Main Authors: Sorokovikova, A., Arefiev, A. V., McGuffey, C., Qiao, B., Robinson, A. P. L., Wei, M. S., McLean, H. S., Beg, F. N.
• Format: Journal Article
• Language: English
• Published: United States American Physical Society (APS) 12-04-2016
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY
• ISSN: 0031-9007; 1079-7114

The interaction of a multipicosecond, kilojoule laser pulse with a surface of a solid target has been demonstrated to produce electrons with energies far beyond the free-electron ponderomotive limit $m_{e}c^{2}a^{2}_{0}$/2. Particle-in-cell simulations suggest that an increase in the pulse duration from 1 to 10 ps leads to the formation of a low-density shelf (about 10% of the critical density). The shelf extends over 100 μm toward the vacuum side, with a nonstationary potential barrier forming in that area. Electrons reflected from the barrier gain superponderomotive energy from the potential. Some electrons experience an even greater energy gain due to ponderomotive acceleration when their “dephasing rate” $R = γ – p_{x}/m_{e}c$ drops well below unity, thus increasing acceleration by a factor of 1/$R$. Furthermore, both 1D and 2D simulations indicate that these mechanisms are responsible for the generation of extensive thermal distributions with $T_{e}$ > 10 MeV and a high-energy cutoff of hundreds of MeV.