Low Emittance Proton Beam Generation in Laser Foil Interaction

Summary form only given. In this paper, we focused on an enhancement of proton beam quality in laser-foil interactions. Recently, a generation of a high-energy proton beam has been explored, and many interesting results have been reported. It is important to enhance the beam energy and also to improve the beam quality. Our research shows a suppression mechanism of a transverse proton divergence by a localization of electrons. When an ultra-intense laser illuminates a foil target, foil electrons are accelerated strongly by the ponderomotive pressure and high energy electron bunches are locally generated in the foil target. The electron bunches accelerated propagate to the opposite side of the laser illumination and produce a local negative potential. Consequently the transverse proton divergence is suppressed by the local potential and the high energy and the low emittance proton beam is generated. In our simulations, we calculated six combinations of the laser intensity and pulse duration for a fixed laser input energy by using 2.5-dimensional PIC (particle-in-cell) simulations. The simulation results show that the average energy of the protons accelerated is about 3 MeV for all the cases. However, a remarkable difference appears in the transverse beam emittance. For the laser intensity of I=10²⁰ W/cm² and the pulse duration of tau = 25 fast, the normalized transverse ms emittance reaches 0.92times10^-2 Pi mm mrad and it is 45.2% compared with that in the case of I=10¹⁹ W/cm² and tau = 250 fs