Echo-Enabled X-Ray Vortex Generation (Echo-v)

Echo-V layout.jpg

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.


Echo-V high el.jpg

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).  


References

PhysRevLett.109.224801.pdfPhysRevLett.109.224801.pdf