|
When a plasma foil is irradiated by an intense laser pulse at a grazing angle, the pre-pulse of the laser evaporates the solid plasma and generates an expanding near-critical-density (NCD) layer. We consider the situation where the plasma density profile in this layer is strongly nonuniform (high-contrast laser), i.e., the density scale length is comparable to the laser spot size. In this case a relativistic electron vortex (EV) is excited in the NCD layer after the laser pulse depletion. It drifts perpendicular to the density gradient at a constant velocity, typically 0.2 - 0.3 times the speed of the light. The strong coupling between laser and NCD plasma gives rise to intense current density that generates significant magnetic field. The magnetic pressure leads to charge separation in the vortex, which can be used to accelerate protons.
The basic mechanism of acceleration is different from the magnetic vortex acceleration (MVA), where a magnetic dipole vortex is generated in a uniform NCD target and acceleration only happens when it reaches the rear side of the target. In our work, the EV, or magnetic monopolar vortex, serves as a stable slow-moving structure in which protons can be captured and gain considerable kinetic energy from the charge separation field associated with the EV. Similarly to collisionless shock acceleration, protons initially at rest can be reflected to twice the EV drift velocity.
The mechanism of EV drift in the strongly nonuniform plasma is different from the weakly nonuniform plasma that are mostly studied in the previous works. In order to obtain a deeper insight into the dynamics of EV drift, we perform 2-dimensional (2D) particle-in-cell (PIC) simulations and track the trajectories of a number of electrons that are trapped in the EV. Our results suggest the laser-driven electrons are subject to an E x B drift which drives the collective motion of the vortex structure. Based on this, an analytical model for the vortex velocity is derived in terms of laser and plasma parameters, the prediction of the model agrees well with the PIC simulations.
It demonstrates that drift velocity of EV, and therefore the maximum proton energy, can be effectively controlled by the incidence angle of the laser and the plasma density gradient.
A representative scenario is studied with 2D PIC simulations—with laser intensity at 10^21 W/cm^2 and incident at 10 degrees—is discussed, in which a quasi-monoenergetic proton beam is obtained with a mean energy 140 MeV and an energy spread of 10%. 3D effects are discussed briefly towards the end of the presentation.
|
