Superradiant x-ray emission in nonlinear plasma wakefields

Plasma accelerator based betatron radiation stands as a promising alternative towards a compact x-ray source. The production of temporally coherent betatron x-rays, however, is yet to be observed experimentally since it requires very high quality electron bunches in order to enable the onset of the microbunching instability. 

Here, we show that temporally coherent, superradiant x-ray emission in nonlinear plasma wakefields can be achieved without such stringent conditions. To achieve this, we rely on Generalized Superradiance (J. Vieira et al., Nat Phys, 2020), an extension of Dicke’s superradiance that can be achieved by using particle bunches with specific spatiotemporal modulations. This effect has been described in the context of particle bunches under the effect external magnetic fields and intense lasers in vaccum.  

Through theory and Particle in Cell simulations, we show that low-intensity lasers (I < 10^18 W/cm2) are able to seed generalized superradiance in nonlinear plasma wakefields by using an initially un-modulated particle bunch. By suitably beam-loading a nonlinear plasma wakefield, we find that the onset of generalized superradiance can enhance the radiated intensity and energy by several orders of magnitude in comparison to standard incoherent emission.