Wakefields in superconducting rf cavities and the impact on vacuum ultraviolet free-electron laser oscillator performance

The Fermilab Accelerator Science and Technology facility is currently in operation with its linac based on TESLA-type superconducting rf cavities. Using a 3-MHz micropulse repetition rate with a long macropulse composed of up to 3000 micropulses, and with beam energies demonstrated at 300 MeV and projected to reach 800 MeV with two additional cryomodules, the feasibilities for a vacuum ultraviolet (VUV) and an extreme ultraviolet (EUV) free-electron laser oscillator (FELO) with the two energies are evaluated. We have used both the ginger code with an oscillator module and the minerva/opc code to assess FELO saturation prospects at 120 nm with a 5.0-cm-period undulator of 4.5-m length and the minerva/opc code to assess the FELO at 13.4 nm with adjusted parameters. The simulation results support saturation at both of these wavelengths which are much shorter than the demonstrated shortest wavelength record of 168.6 nm from a storage-ring-based FELO. This indicates superconducting rf linac-driven FELOs can be extended into this VUV-EUV wavelength regime previously only reached with single-pass FEL configurations. In addition, emittance-dilution effects due to wakefields in the cavities and the resulting submacropulse centroid slew effects on FELO performance are addressed using minerva/opc simulations for the first time.