Radiation Physics commissioning of LCLS-II superconducting Linac. Gun and cryomodules commissioning

The Linac Coherent Light Source II (LCLS-II) at SLAC is a X-ray free-electron laser based on a 4 GeV superconducting linear accelerator capable of continuous-wave operation. While the machine has not yet reached its full design power, an initial phase of commissioning has focused on characterizing radiation produced by field emission, unintended electron discharge from RF cavities, which can impact beam quality, cause component damage, and create radiation hazards.

Using in situ diagnostics from the radiation safety system, including ionization chambers, synthetic diamond detectors, and Cherenkov fibers, we investigated field emissions originating from both the laser gun and cryomodules. For the gun, we characterized the captured current at the cathode and quantified the transmitted dark current through the beamline. By applying a dedicated methodology involving downstream collimators, we showed that a large fraction of the dark current can be intercepted by the collimation system.

For the cryomodules, we used radiation monitors and FLUKA simulations to estimate captured current and its evolution over time. Radiation was observed in several cavities, with field emission detected at gradients as low as 8 MV/m. Detailed analysis of some individual cryomodules revealed trends in backward and forward emission, enabling us to identify specific cavities contributing most to field-emitted current. These results support further mitigation strategies and provide a transferable approach to characterizing field emission in superconducting linacs.