One of the important applications of the inherently few fs long electron bunches is currently the generation of ultrashort XUV/x-ray bursts with narrow or broad bandwidth via undulator radiation, Thomson backscattering, or betatron radiation. Especially for narrowband x rays, a precise control of the electrons’ peak energy Epeakand a low full width at half maximum (FWHM) energyspread E are important.

Researchers at Max-Planck-Insitut fu¨r Quantenoptik and Shanghai Institute of Optics and Fines Mechanics (SIOM/China)  report the generation of stable and tunable electron bunches with very low absolute energy spread(E5 MeV) accelerated in laser wake fields via injection and trapping at a sharp downward density jump produced by a shock front in a supersonic gas flow. [Physical Review Letters, 2013; 110 (18)：185006]

In conclusion, we have shown that shock-front injection,i.e., electron trapping at sharp density jumps produced by a shock front in supersonic flows, offers a very powerful, yet simple method to inject electrons into laser-driven wakefields. The results demonstrate that not only the reproducibility,stability, and tunability is improved strongly compared to self-injection experiments, but also that the absolute energy spread of E5 MeVis significantly lower compared to other injection methods producing highly relativistic electron bunches. The charge per energy interval, an important number for all future applications such as undulator radiation, was increased strongly due to the narrow bandwidth and the bunch charge being on the order of 1–100 pC. The maximum electron energy was limited around 150–200 MeV due to limitations of the setup in the present experiment; however, several 100 MeV as already shown in other experiments with similar laser parameters are expected with improvements being currently under way. Thus, relative energy spreads below 1% FWHM are within reach.