Stable femtosecond X-rays with tunable polarization from a laser-driven accelerator

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 that are accelerated in the wakefield of an intense laser pulse oscillate around the propagation axis and emit X-rays. This betatron source, which essentially reproduces the principle of a synchrotron at the millimeter scale, provides bright radiation with femtosecond duration and high spatial coherence. However, despite its unique features, the usability of the betatron source has been constrained by its poor control and stability. In this article, we demonstrate the 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 the signal and beam profile fluctuations are significantly reduced and that the beam pointing varies by less than a tenth of the beam divergence. The polarization ratio reaches 80%, and the polarization axis can easily be rotated. We anticipate a broad impact of the source, as its unprecedented performance opens the way for new applications. A laser-accelerator-based source of ultrashort X-ray pulses with unprecedented stability has been demonstrated by scientists in Europe. Ultrashort X-ray pulses are invaluable for exploring the dynamic properties of matter at very short time scales. High-peak-power lasers are promising for producing such beams, but their usefulness has been limited by the poor control and stability of X-ray beams they generate. Andreas Döpp from the Ecole Polytechnique in France and co-workers tested a new, tunneling ionization scheme for injecting electrons longitudinally into a laser-wakefield accelerator. They generated a kiloelectron-volt-scale X-ray beam with far superior stability than that obtained using the usual injection approach. In addition, the X-ray emission was highly polarized, and the polarization could be tuned by tilting the polarization axis. Such a source could be useful for applications requiring broadband, femtosecond X-ray pulses.