Stable femtosecond X-rays

Technology based on high peak power lasers has the potential to provide compact and intense radiation sources for a wide range of innovative applications. In particular, electrons accelerated in the wakefield of an intense laser pulse will oscillate around the propagation axis and emit X-rays. This betatron source, which essentially reproduces the principle of synchrotron on a millimeter scale, provides bright radiation with femtosecond duration and high spatial coherence. But despite its unique features, the usability of the source has been constrained by its poor control and stability. In this letter, we demonstrate reliable production of X-ray beams with tunable polarization. Using ionization-induced injection in a gas mixture, the orbits of the relativistic electrons emitting the radiation are reproducible and controlled. We observe that both signal and beam profile fluctuations are significantly reduced, and the beam pointing varies less than a tenth of the beam divergence. The polarization ratio reaches up to 80 percent and the polarization axis can be easily rotated. We anticipate a broad impact of the source as its unprecedented performance opens the way to new applications.

These results have been published in LIGHT, november 2017

Caption: Betatron radiation beam profiles in the classical (transverse injection) and recently studied (ionization injection) regimes.