Coherent radiation from superluminal plasma wakefields

Light sources based on plasma accelerators rely on the oscillations of relativistic electron bunches in nonlinear plasma waves (e.g., betatron radiation). However, the plasma wakes themselves can also radiate if their amplitude is sufficiently high. This suggests a broader radiation concept based on emission from collective excitations. Collective excitations (often referred to as quasiparticles) are a broadly adopted concept in condensed matter physics. Here, we use it to describe any collective dynamics that exhibit wave-like properties, such as plasma wakefields. We show that the trajectory of the collective excitation defines the coherence properties of the radiation just as if it were a point-like charge. We demonstrate with theory and simulations, using the radiation diagnostic for OSIRIS (RaDiO), that a superluminal nonlinear wakefield generates an optical shock at the Cherenkov angle, just like a superluminal particle. We find that the radiated intensity scales with the number of electrons squared for frequencies of up to a few hundred times the plasma frequency, which shows that the emission is superradiant, making this a promising plasma-based source of temporally coherent radiation.