Spin-polarized particle beams are widely used in modern physics. Polarized electron beam plays an important role in electron-positron colliders. In general, energetic polarized electron beam is generated via conventional accelerators, either from storage rings or linear accelerators, which is large in scale and budget. With rapid development of laser techniques, the laser driven wakefield acceleration (LWFA) becomes accessible.

Ultra-intense and ultra-short laser pulses can drive plasma wakefield that would trap and accelerate electrons at acceleration gradients almost four orders of magnitudes higher than the one in traditional accelerators. Thus it promises a compact and cost-efficiency approach for particle acceleration. However, several challenges should be addressed to realize a LWFA for polarized electron beam.

In a study published in New Journal of Physics, the researchers from Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, and international collaborators, has proposed an all-optical approach to obtain energetic polarized electron beam based on LWFA driven by a vortex laser beam (also known as vortex laser) and verified the feasibility via full three dimension (3D) particle-in-cell simulations incorporating spin dynamics.

In this scenario, they utilized four lasers with good synchronization and a density tailing target.

The first 1064 nm infrared (IR) laser aligned the bonds of the HCl molecules, and then a ultraviolet (UV) light was used to photodissociate the HCl molecules. A 234.62 nm UV light was used to ionize the Cl atoms through resonance-enhanced multiphoton ionization. Thermal expansion of the electrons created large Coulomb field that expels the Cl ions. A fully polarized electron target was therefore produced for sequential acceleration by the driven LG laser. With a density ramp, the electrons were injected to bubble as soon as the LG laser traversed the density peak and was accelerated steadily by the bubble field.

To investigate whether the polarization preserved during injection phase and acceleration phase, they carried out full 3D particle-in-cell (PIC) simulations incorporating spin dynamics.

According to the simulations, the peak flux was limited for Gaussian mode driven laser in order to preserve the polarization during injection, which was in line with theoretical analysis. Nevertheless, thanks to novel topology of the vortex LG laser, the restriction on the electron beam current was released. 20kA peak flux electron beam of polarization over 80% was obtained via LG cases.

This study paves the way for all-optical setup to generate polarized energetic electron beams based on the accessible facilities.