Direct Electron Acceleration in Multi-Kilojoule, Multi-Picosecond Laser Pulses

The physics mechanism behind the acceleration of electrons to energies much higher than the laser ponderomotive potential is reported on. Such electrons have been observed for the last twenty years [Perry], but they were small in number, about 1 percent of MeV electrons. Extended pulse durations, large focal spots and high intensities of current high-power lasers all favor generation of `super-ponderomotive' electrons. Recent PIC simulations [Sorokovikova] give twenty times larger MeV electron doses than NOVA-PW. Such an enhancement, if realized in experiments, would boost all applications of short laser pulses (e.g., proton/ion acceleration, X-ray generation, and positron production). Evidence is provided that the dominant acceleration mechanism of super-pondermotive electrons is direct acceleration by transverse electromagnetic fields as they co-propagate in under-dense plasma; the most energetic electrons are initially Raman-scattered in the opposite direction of the laser pulse and then reflected by the electrostatic potential at the front of the plasma expansion into vacuum. In the classical figure-of-eight motion in a plane wave, electrons gain only a small average forward drift with momentum proportional to a$_{\mathrm{0}}^{\mathrm{2}}$/2 while oscillating in the laser field; in contrast, electrons injected at relativistic energy remain in phase with the laser field longer gain several times more energy, running in- and out of phase with the light; this continues until the laser is reflected by over-critical plasma, while fast electrons propagate into the target. Direct acceleration requires tens of plasma wavelengths of under-dense plasma, as well as pulse durations of tens of plasma periods, conditions that are found in large focal spot multi-picosecond laser pulses. References: Sorokovikova, A., Arefiev et al., Phys.Rev.Lett. 116, 155001 (2016). Perry, et al., Rev.Sci. Instr. 70, 265 (1999).