Echo-Enabled X-Ray Vortex Generation (Echo-v)
At NLCTA, we are examining new concepts and new regimes of operation for advanced electron beam-based light sources. In a recently proposed scheme, a method to generate high-brightness electromagnetic vortices with tunable topological charge at extreme ultraviolet and x-ray wavelengths was proposed. Based on a modified version of echo-enabled harmonic generation (EEHG) for free-electron lasers, the technique uses two lasers and two chicanes to produce high-harmonic microbunching of a relativistic electron beam. Here however, the microbunching exhibits a corkscrew distribution that matches the instantaneous helical phase structure of the desired x-ray vortex. THis is referred to a "Echo-v". The strongly correlated electron distribution emerges from an efficient three-dimensional recoherence effect in the EEHG transport line and can emit fully coherent vortices in a downstream radiator for access to new research in x-ray science.
The concept is similar to EEHG, except the energy modulation in the e-beam from the lasers has a 3D structure that carries a correlated longitudinal and azimuthal phase. This can occur one of two ways; either the laser field can be a higher order Orbital Angular Momentum (OAM) mode with an azimuthal phase, or the interaction can occur at a harmonic in a helical undulator. The latter has the advantage that a simple Gaussian laser profile can be used to drive the interaction, which yields a coiled spring-like energy distribution of electrons. This helical microbunching geometry can occur in either or both of the undulator sections, such that the Echo-v process can be used to tune the frequency and OAM mode content of the output light. If the helical structure is imposed only in the first stage, the Echo-v process is predicted to generate a low-order OAM mode at a high harmonic frequency. If the helical structure is imposed only in the second stage, the OAM mode number can be as large as the harmonic number (shown in simulation here).