Acceleration of electron beam micro-bunching via plasma optics

Abstract Free electron lasers (FEL) are technological marvels that offer high power, coherent and tunable radiation down to x-rays, making them essential for the probing of materials and biological processes among other things. The infrastructure size and cost limit these facilities to a few in the world. Therefore, there is a real need to reduce these parameters of the FEL process. The main point here is the long undulator sections necessary to micro-bunch the beam to achieve coherence. In this work we show the capabilities of a low density plasma optic to substantially accelerate the micro-bunching process in mere cm before entering the undulator. The chosen low-density plasma avoids significant changes and gives some transverse parameters improvements in most of the electrons ( $$\\approx 68\\%$$ ) at the exchange of some energy spread for non-chirped beams. Through particle in cell simulations, two longitudinal density modulation processes have been observed inside the plasma. One based on the local beam focusing due to wakefield (long period) and another one caused by the interaction between part of the beam and an internal plasma structure created after wavebreaking (short period). The second one acts as a seed that during the passage of the beam in the undulator accelerates the micro-bunching apparition by $$\\approx 8$$ times, and further PIC extrapolation and GENESIS4 qualitative calculations show a possible saturation speedup of up to $$\\approx 1.8$$ times.