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

Investigation of laser pulse length and pre-plasma scale length impact on hot electron generation on OMEGA-EP

“…Upon extracting the propagating electrons 30 μm in the target, and calculating their angular direction with respect to time, several conclusions can be drawn. Integrating the forward going electron spectra in the 5 ps case demonstrates that the super-ponderomotive electron generation mechanism seen in 1D simulations is preserved in the 2D simulations ( figure 11 (left)), which was also seen in work performed by Sorokovikova et al [12]. The measured hot tail is less energetic than in the 1D case, but still present.…”

“…After 5 ps the density profile of the target is remarkably similar to a 10 μm scale length pre-plasma. The plasma that expands functions much as an initial pre-plasma does, allowing a potential well to form and accelerate electrons as shown in previous work [11][12][13].…”

“…First, why are super-ponderomotive electrons rarely generated with an initial pre-plasma? Prior experiments [6,7,16] and simulations [11][12][13] consistently generate hot electron tails with pre-plasma. Second, why are super-ponderomotive electrons generated more often, or at all without pre-plasma?…”

“…Another shortcoming found in the first experiment was that the BMXS has a very limited range in examining hot electrons. To examine super-ponderomotive electrons such as those seen in many previous simulations [11][12][13][14][15][16] the EPPS diagnostic was introduced. The electrons measured by the EPPS are those that have enough energy to escape the target, which supplements the BMXS perfectly.…”

“…The source of these electrons was investigated via 1 and 2D PIC simulations, which showed that an electrostatic potential well develops in the pre-plasma which traps electrons. Simulations performed by Robinson et al [11], Sorokovikova et al [12], Krygier [13] and Arefiev et al [14] show these electrons gain large amounts of energy from direct laser acceleration after being trapped and pre-accelerated inside the potential. Simulations performed by Paradkar et al [15] show that stochastic heating of trapped electrons in the potential well is also a source of these electrons.…”