Energetic electrons and protons generated from the interaction of ultrashort laser pulses with microdroplet plasmas

Generation of fast electrons and protons from ultrashort laser interactions with microdroplet plasmas has been simulated by two-dimensional particle-in-cell simulations. At weakly relativistic laser intensities, two jets of hot electrons are found to emit symmetrically in the backward direction when there is preplasma formed around the droplet surface. These electron jets are associated with resonance absorption around the target surface with a spherical geometry. At relativistic laser intensities, electron bunches generated by the laser ponderomotive force separated by a laser period are superimposed to the resonant hot-electron jets. Relevant with the hot-electron emission, energetic protons can be separated into two groups: one with higher energies mainly emits in certain angles within the laser polarization plane and another with lower energies emits nearly isotropically in all directions. The protons in the first group are accelerated by the electrostatic fields induced by the hot-electron jets due to the resonance absorption, while those in the second group are generated by the ambipolar expansion of the microdroplet plasma. (C) 2005 American Institute of Physics.