Photon Acceleration by Laser Wakefields

In recent times, theory and simulation have been able to reproduce photon acceleration from wakefields where light is up- shifted in frequency by a moving refractive index gradient. The effect has been considered as a possible method of diagnosis of a wakefield such as the one postulated for a compact electron accelerator. Here we present further observations of spectral modulation caused by a laser-produced wakefield. In our experiments, the Astra laser at the Rutherford Appleton Laboratory was focused into jet of helium. Between 300mJ and 600mJ of laser energy was delivered into a 25 micron focal spot in pulses between 50fs and 200fs. The spectrum of the transmitted light was measured. The lower intensity laser pulse allowed a linear regime to be investigated. A photon kinetic code has been implemented to model the experiment. The feasibility of using this effect as a wakefield diagnostic will be discussed.