Fast electrons generated in ultra-intense laser interaction with a solid target can produce multi-MeV ions from laser-induced plasmas. These fast ions can have different applications ranging from ion implantation to nuclear reactions. The most important parameter is the efficiency of fast ion production. An analytical model and particle-in-cell simulations were employed to examine acceleration mechanisms that can provide an optimal plasma density distribution due to a laser prepulse. We considered the acceleration of ions leaving a plasma layer with different density gradients, from a step-like overdense plasma to an underdense plasma with a smooth density gradient. The effects of the plasma initial scale length and density on the ion acceleration were analysed, and we found that the optimal case should have some plasma parameters. It is shown that overdense plasmas provide a higher density of accelerated ion energy than underdense plasmas at intensities below 10 19 W cm −2 .
By utilizing a pulsed laser beam of TEM(1,0)+TEM(0,1) mode, it was found numerically for the first time that an electron bunch can be effectively trapped by the transverse ponderomotive force in the transverse direction and at the same time accelerated by the longitudinal ponderomotive force to about 378 MeV at the laser peak intensity of I∼5.48×1018 W/cm2. In addition, the electron bunch size is preferably small: at this laser intensity the electron bunch thickness is ∼10λ in the longitudinal direction and the bunch radius is about 625λ in the transverse direction.
A focused short-pulse laser of TEM (1,0)+TEM (0,1) mode has two intensity peaks in the radial direction, so that the transverse ponderomotive force may trap electrons between the two peaks. At the same time the longitudinal ponderomotive force may accelerate electrons at the head of the laser pulse, when the laser is focused. When the electrons move to the laser tail, the laser may diverge and the electron deceleration becomes relatively weak. Our numerical analyses demonstrate that electrons are trapped well by the laser potential well, and that at the same time the acceleration by the longitudinal ponderomotive force induces the electron bunch compression. This trapping and compression mechanism is unique: the electron bunch can be compressed to the scale of the laser pulse length.
When electrons are accelerated by the ponderomotive force of an intense short-pulse laser, the electrons are accelerated at the head of the laser and they lose their energy gained at the tail of the laser. Therefore the electrons cannot finally obtain the laser energy. In this research, an overdense slab plasma separator is introduced in order to separate the accelerated electrons from the laser, just before they enter the deceleration phase. The laser is reflected by the plasma separator, so the electrons pass through the thin plasma separator without a significant influence and are successfully accelerated.
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