Photon acceleration as the laser wakefield diagnostic for future plasma accelerators
In the near future, laser-plasma particle accelerators will be able to sustain electric fields in excess of 100 GeV/m, along plasma channels several Rayleigh lengths long. For these extreme conditions, present day laser wakefield diagnostics such as Frequency Domain Interferometry will not be able to resolve the wakefield structure and determine the magnitude of the electric field. In this paper, we present a detailed comparison between frequency-domain interferometry and a photon acceleration based wakefield diagnostic. We determine the experimental parameters for which photon acceleration becomes the only viable diagnostic technique. Dispersion effects on the probe beam and the implications of an arbitrary phase velocity of the plasma wave are discussed for both diagnostic techniques. We also propose an experimental set-up for a photon acceleration diagnostic allowing for the simultaneous measurement of the electric field structure and the laser wakefield phase velocity. Comparison with results from photon acceleration experiments by ionization fronts are also presented.