Gamma radiation vortex burst in the nonlinear Thomson scattering with refocusing spiral plasma mirror

The high energy radiation that carry the orbit angular momentum has become an extremely important tool to investigate the angular momentum related physics in microworld. But the generation and characterization of high energy radiation vortex remain the urgent issues to be solved. To address this issue, the group of Profs.Liming Chen and Wenchao Yan from the Key Laboratory for Laser Plasmas of Shanghai Jiao Tong University came up with the configuration by using the special refocusing spiral plasma mirror to combine the laser wakefield acceleration with the nonlinear Thomson scattering. 

Currently, there has been two achievable routes to generate the radiation vortex in short wavelength range. One based on the free electron laser, in which the additional helical undulator was added behind the pre-undulators to generate the vortex soft x-ray. The another more effective way based on the Compton/Thomson scattering when the relativistic electron bunch collide with the vortex laser. Compared with scheme that based on the traditional electron accelerator, the radiation while using the laser wakefield accelerated electron beam shows the advantages of much higher time resolution and peak brightness. And the common dual laser scattering scheme need the additional vortex laser, which has to deal with the drift of space-time synchronization between the electron beam and scattering laser.

To overcome these difficulties, the group of Prof.Liming Chen and Wenchao Yan  put forward the configuration, as shown in Fig.1. When the Gaussian laser that drive the laser wakefield acceleration impinges on the refocusing spiral plasma mirror, the reflected laser would be compulsively multiplied with the spiral phase  and turn into the Laguerre-Gaussian mode. In the meantime, the paraboloid surface will refocus the Laguerre-Gaussian laser into a small spot with an extremely strong field strength. The gamma radiation vortex would burst when the accelerated electron beam collide with the refocusing Laguerre-Gaussian laser.

As shown in Fig.2, the reflected laser was refocused right at the focal plane of the paraboloid . The normalized amplitude of refocused laser has increased to , which makes the subsequent scattering process enter the nonlinear regime.

By the assumption of neglecting the thickness of the sample electron slice, the radiation distribution on a fixed observation plane are as shown in Fig.3. The radiation distribution present the identical topology structure with the scattering laser, but there emerges the unusual phenomenon of radiation distortion due to the radiation deflection by the intense laser field. But this distortion phenomenon does not break the angular momentum conservation of the radiation vortex. 

The transverse radiation divergence angles are within and  in the direction of laser polarization and its perpendicular, respectively. The spectra comparison between the flat and refocusing spiral plasma mirrors indicated the significant enhancement on gamma photon yield by the refocusing effect. The peak radiation brightness was estimated to be .

Conclusion
The paraboloid plasma mirror has the natural alignment advantage on space-time synchronization between the scattering laser and electron bunch, which guarantees the collision precision. This scheme will significantly simplify the experiment setup for the generation of radiation vortex, and the refocusing structure could effectively increase the yield of high energy photons, which is achievable with current nano fabrication technology. The generated ultrafast bright gamma radiation vortex would pave the way for fundamental researches on the nuclear physics, quantum electrodynamics and ultra-resolution image.