A method of using intense Laguerre-Gaussian (LG) laser pulse is proposed to generate ultrarelativistic (multi-GeV) electron beams with controllable helical structures based on a hybrid electron acceleration regime in underdense plasmas, where both the longitudinal charge-separation electric field and transverse laser electric field play the role of accelerating the electrons. By directly interacting with the LG laser pulse, the topological structure of the accelerated electron beam is manipulated and it is spatially separated into multi-slice helical bunches. These results are clearly demonstrated by our three-dimensional particle-in-cell simulations and explained by a theoretical model based on electron phase-space dynamics. This novel regime offers a new degree of freedom for manipulating ultrashort and ultrarelativistic electrons, and it provides an efficient way for generating high-energy highangular-momentum helical electron beams, which may find applications in wide-ranging areas.
Target-normal sheath acceleration (TNSA) of protons from a solid-density plasma target consisting of a thin foil, with a thin hydrogen layer behind it and a plasma-filled tube with a parabolic density profile at its front, is investigated using two-dimensional particle-in-cell simulation. It is found that the targetback sheath field induced by the laser driven hot electrons is double peaked, so that the protons are additionally accelerated. The hot sheath electrons, and thus the TNSA protons, depend strongly on the tube plasma, which unlike the preplasma caused by the laser prepulse can be easily controlled. It is also found that the most energetic and best collimated TNSA protons are produced when the tube plasma is of near-critical density.
A scheme for controlling the direction of energetic proton beam driven by intense laser pulse is proposed. Simulations show that a precisely directed and collimated proton bunch can be produced by a sub-picosecond laser pulse interacting with a target consisting of a thin solid-density disk foil with a solenoid coil attached to its back at the desired angle. It is found that two partially overlapping sheath fields are induced. As a result, the accelerated protons are directed parallel to the axis of the solenoid, and their spread angle is also reduced by the overlapping sheath fields. The proton properties can thus be controlled by manipulating the solenoid parameters. Such highly directional and collimated energetic protons are useful in the high-energy-density as well as medical sciences.
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