Dynamic focusing of laser driven positron jets by self-generated fields

Abstract

Focusing effect of laser-driven positron jets by self-generated target sheath fields has been observed for the first time experimentally and the results are supported by the computational studies. In the experiment, OMEGA EP short-pulse (0.7 ps, 500 J) irradiates mm-size gold targets with a concave back surface and reference flat-surface targets. Both targets exhibited positrons with quasi-monoenergetic energy peaks while targets with concave curvature also showed increased number of positrons at the detector. The data is consistent with hybrid-PIC simulations confirming that the time-varying electric fields driven by electrons escaping from the target significantly change the trajectories of positrons. These simulations show a small radius of curvature on the rear side increases the relative focusing effect and the positrons to electrons ratio in the escaping plasma. For the smallest radius of curvature, positron jets that are up to 10 times denser can be achieved.

Collimation, compression and acceleration of isotropic hot positrons by an intense vortex laser

“…Laser-driven positron sources have attracted widespread attention for their diverse applications in various areas, such as electron-positron collider [1,2], positron annihilation spectroscopy (PAS) [3,4], laboratory astrophysics [5,6], and injection into plasma-based positron accelerators [7,8]. In these applications, high-quality positron pulses with high energy, high density, short duration and low divergence angle are highly desired.…”

“…If positrons are used as injectors for plasma accelerators, small beam size and short pulse duration are also demanded to match millimeter-scale acceleration length in the wakefield [7]. Moreover, to enable stable wakefield acceleration, the positron properties such as energy, emittance, and charge also should be carefully optimized [7,8,11,12].…”

“…However, only a small amount of expanding positrons within a narrow band of energies can be focused and collimated when applying external magnetic fields. Kim et al [8] experimentally demonstrated that laser-driven relativistic positron beams could be focused and accelerated by self-generated target sheath electric fields from curved target rear sides. Further simulations showed that the beam density and peak energy were effectively manipulated by the radius of target curvature.…”

Inhibition of electron refluxing in laser-gas interactions for enhanced positron generation

“…This can be achieved by using low-density pre-formed plasma at the target surface, since this ablated pre-plasma can significantly improve both the laser absorption and fast electron generation [20][21][22]. Other methods, such as using either a Helmholtz coil or curved targets [23,24], have also been proposed to increase the positron density due to the focusing of the positron by an external magnetic field or self-generated fields.…”