Electron acceleration by laser pulses with high repetition rate can be used for technical applications. To reach conditions for the wake-field laser acceleration, it was demonstrated recently in experiments that it is beneficial to use near single cycle laser drive pulses with sub-4 fs duration, with narrow waists. To explore possible electron density ramp-up injection as an alternative to ramp-down and ionization injections, we performed numerical simulations of electron bunches generation in the ramp-up region. The PIC code Epoch2D and input parameters near to experiments were used. We assumed thin plasma slabs with super Gaussian density profiles of order 4-80, FWHM about 30 µm. We found that density ramp-up injected bunches can have charges several times higher than those obtained by ionization injection. There can be created a group of up to ten bunches in a sequence of bubbles, with not too mutually different maximum energy and charges. At oblique incidence of drive pulses on steep ramp up profiles, we find significant enhancement of the first bunch charge. For large slant angles -45 or 45 degrees, the bunch charge enhancement is about twenty times. We conclude that the ramp-up injection can be a useful alternative injection on steep enough density profiles.
Electron acceleration with optical injection by a perpendicularly propagating and orthogonally polarized low intensity laser pulse into a nonlinear plasma wave driven by a short intense laser pulse was explored by particle- in-cell simulations. The scheme presented here provides an energetic electron bunch in the first ion cavity with a low energy spread. The electron bunch short and compact, with the mean energy about 400 MeV and a low energy spread about 10 MeV in time of 6 ps of acceleration. The injected charge is several tens of pC for the low intensity of the injection pulse. Initial positions of electrons forming the energetic bunch are shown and then these electrons are followed during the simulation in order to understand the injection process and determine electron bunch properties.
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