Commissioning of a photocathode and interaction laser system at RadiaBeam compact inverse Compton light source

Abstract

In the context of building a compact Inverse Compton Scattering X-ray source, an Yb:YAG laser was used to generate both the electron emission from a photocathode and act as the interaction laser on a 100 MeV inverse Compton scattering experiment. The laser generates 25 mJ pulses at 1030 nm, 1.5 ps long, up to 120 Hz. 10 % of the energy is sent into a Fourth Harmonic Generation (FHG) module where frequency doubling happens twice. Up to 200 μJ of adjustable UV laser can be exploited and sent towards the photocathode. The rest of the energy, 90 % of the initial IR beam, is propagated to the interaction region. The goal is to match a 1 mm beam diameter (flat-top, 1/e²) on the photocathode and 40 μm (1/e²) at the interaction region with high stability. To reach it, significant effort was put into optimization using state of the art laser propagation software and various tools like low aberrations lenses, truncated Gaussian beam, vacuum transport, relay of images, and closed loop stabilization system. In the end, this project pairs strong optical and mechanical constraints. A significant part of it was built and commissioned, showing exciting results. We will describe the whole laser system together with the various steps to reach the accelerator's needs, and the current achievements.