Spin-polarized particle beams are widely used in modern physics including nuclear physics, particle physics as well as material science. In particular, polarized electron beam plays an important role in electron-positron colliders, which enhances the annihilation across section and serves as a source to generate polarized positrons and photons.
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 4 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: (i) the preparation of the pre-polarized target, the preservation of the polarization during (ii) injection phase and (iii) steady acceleration phase.
A collaborative research team led by State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences (CAS), Heinrich-Heine-Universit？t Düsseldorf, Forschungszentrum Jülich and University of Crete, 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. Their study was published in New Journal of Physics.
In this scenario, four lasers with good synchronization and a density tailing target were utilized. 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, full three dimension(3D) particle-in-cell(PIC) simulations incorporating spin dynamics were carried out. According to 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 research paves a new path with all-optical setup to generate polarized energetic electron beams based on the accessible state-of-the-art facilities.
This work was supported by the Strategic Priority Research Program of Chinese Academy of Sciences, the National Science Foundation of China and the Recruitment Program for Young Professionals.
Sketch of the all-optical vortex laser-driven polarized electron acceleration (Image by SIOM)
Mr. Cao Yong
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS