Streaking of betatron X-rays in a curved laser wakefield accelerator

Experiments conducted on the Advanced Laser Light Source at INRS demonstrated, for the first time, the streaking of betatron X-rays in a curved laser wakefield accelerator via controlling of the plasma density gradient. In a plasma with a transverse density gradient, laser wavefront tilt develops gradually due to phase velocity differences in different plasma densities. The wavefront tilt leads to a parabolic trajectory of the plasma wakefield and hence the accelerated electron beam, which leads to an angular streaking of the emitted betatron radiation. In this way, the temporal evolution of the betatron X-ray spectra will be converted into angular "streak," i.e., having a critical energy-angle correlation. By controlling the plasma density and the density gradient with multi-stage 3D printed gas targets, we realized the steering of the laser driver, electron beam and betatron X-rays simultaneously. The experimental results were verified with a theoretical model and Particle-in-Cell simulations. Angularly resolved X-ray spectra from such a curved laser wakefield accelerator open an avenue towards single-shot diagnostic techniques for reconstructing the temporal evolution of the electron acceleration process. Moreover, our work could also find applications in advanced control of laser beam and particle propagation, such as generating curved plasma channel for coupling multiple wakefield accelerators as well as enabling compact synchrotron sources.