Proton acceleration from microdroplet spray by weakly relativistic femtosecond laser pulses

Angular distribution of protons is measured from ethanol droplet spray irradiated by linearly polarized 150 fs laser pulses at an intensity of 1.1×10¹⁶ W∕cm². Fast protons (with energies >16 keV) with an anisotropic distribution can be observed only in or near the polarization plane of the laser fields, while the slow protons (with energies ≦16 keV) emit with nearly an isotropic distribution. Two-dimensional particle-in-cell simulations suggest that three groups of protons originate from different acceleration regimes in the laser-droplet interaction. The first group with the highest energies is accelerated backwards by the anisotropic charge-separation field near the front surface (laser-droplet interaction side) due to the resonance absorption; the second group (forward emission) is generated by the target-normal sheath acceleration mechanism; and the third group, with the lowest energies, is accelerated by the hydrodynamic expansion of the droplet plasmas.