Scientists Obtain High Quality Electron and Bright Betatron Radiation from Laser-plasma Accelerator Simultaneously

May 04, 2014     Email"> PrintText Size

Ultrashort hard x-ray emission can be generated from femtosecond (fs) laser pulse interacting with plasma. Due to the fs time-scale, laser-driven x-ray source can be used in time-resolved dynamic probe in biological and condensed matter applications. However, the existing laser-driven sources suffer the limitation of x-ray conversion efficiency and the spectrum contrast ratio to the background.

The team lead by Prof. CHEN Liming from Group of Laser High Energy Density Physics of the Institute of Physics, Chinese Academy of Sciences (IOP) made a series of progress in this field. Firstly, they greatly improved the Kα conversion efficiency and spectrum contrast using high contrast laser-solid interaction [PRL. 100, 045004 (2008)]; then they used fs laser pulse interact with nanometer-size clusters, enhanced the photon yield to a record ~ 10¹¹/J [PRL. 104, 215004(2010)], which is suitable for single-shot applications. After that, Prof. CHEN's team collaborated with Academician ZHANG Jie’s team in Shanghai Jiao Tong University to study the collimated betatron radiation with fs duration when a relativistic laser driven plasma wakefield accelerator. They use only 3 TW laser radiate the clustering-gas successfully evoke the Direct Laser Acceleration (DLA), which result in a ten-fold enhancement comparing to the emission flux produced by using gas target with the same parameters [Sci. Reports 3,1912(2013)].

Usually one cannot obtain bright betatron X-rays and high quality electron beams with low emittance and small energy spread simultaneously in the same accelerating wave. Prof. CHEN’s team proposed a method to solve the problem, which consecutively applied and obtained two years experiments in Callisto laser system in Lawrence Livermore National Laboratory, USA. The experimental results overcome the bottle-neck mentioned above, and drastically enhanced the betatron radiation with high quality electron beams. The experiment observed two distinct electron bunches in a single laser shot, one featured with quasi-monoenergetic spectrum and another with continuous spectrum. The latter is able to generate high flux betatron X-rays. Numerical simulation reveals that two bunches of electrons are injected at different stages due to the bubble evolution. The first bunch is injected at the beginning to form a stable quasi-monoenergetic electron beam, while the second one is injected later due to the oscillation of the bubble size as a result of the change of the laser spot size during the propagation. Due to the inherent temporal synchronization, this unique electron-photon source can be ideal for pump-probe applications with fs time resolution.

This work was published recently on Proceedings of the National Academy of Sciences [PNAS 111(16), 5825-5830(2014)].

This work was supported by the National Basic Research Program of China (Grant No. 2013CBA01501), National key Scientific Instrument and Equipment Development Project (No. 2012YQ120047) and the NSFC (Grant No.11334013).

Fig.1. Two electron bunches appear as channel length increasing. The 1st bunch is mono-energetic and the appearance of intense betatron radiation is related with the 2nd electron bunch. (Image by IOP)

Fig.2. simulation shows the double injection and the 2nd electron beam achieved high charge, large oscillation amplitude and higher acceleration energy, which is suitable for enhancing betatron radiation. (Image by IOP)