When a high energy ultra-short laser pulse propagates through an under dense plasma (gas target), the ponderomotive force of the laser pulse expels electrons away creating a wakefield with strong electric fields behind the laser pulse. Electrons with enough initial energy get trap inside and start transversely oscillating while being accelerated to relativistic energies. This oscillatory motion of the electrons in the wakefield results in so-called Betatron radiation.
This radiation features high flux broadband energy distribution, with typical energies in the range of 10 - 100 keV, and a μm-source source. The latter provide a high degree of spatial coherence that can be exploited in applications such as phase contrast imaging. Furthermore, the betatron source inherits the drive laser's ultra-short duration of tens of femtosecond, making the source also an excellent probe for ultrafast dynamics.
- J. Wenz et al., Quantitative X-ray phase-contrast microtomography from a compact laser-driven betatron source, Nature Communications volume 6, Article number: 7568 (2015)