Photon Acceleration from Optical to XUV

This paper is about phase-matched photon acceleration, a scheme for dephasingless photon acceleration in a particle-beam-driven wake. Electromagnetic radiation seeing a time-varying plasma density shifts in frequency. To achieve unlimited photon acceleration, a laser pulse is situated in the wake behind a relativistic electron bunch so that the laser pulse sees a frequency-upshifting plasma density gradient as observed in the coordinate frame translating with the electron bunch.
Using a tapered density profile to keep witness laser pulse at the phase in the wake where the frequency shifts up, simulations suggest that the frequency of the witness pulse can be shifted up arbitrarily, limited only by the energy in the drive electron bunch.