Realizing A Hard X-Ray Storage Ring Free Electron Laser Oscillator at the APS-U

We show that the APS-U could support a hard X-ray storage ring free electron laser oscillator, providing a promising avenue toward high repetition rate, narrow bandwidth coherent light sources. The results of our numerical simulations demonstrate that a transverse gradient undulator yields ~8% and ~6% single-pass gain at 8.05 keV and 10 keV respectively. We further identify a configuration at 5 keV that does not require a TGU but still exceeds a 5% gain threshold despite the relatively short 5-meter long insertion device. All cases presented retain spectral purity on the order of meV and reach a steady-state output whose equilibrium is consistent with the Renieri Saturation Limit. We have calculated the 5 keV case to have an average brightness of ~10^26 photons/(s * mm2 * mrad2 * 0.1% BW), representing an increase in more than four orders of magnitude from the standard APS-U undulator. These results indicate that a storage ring free electron laser oscillator at multi-keV photon energies is feasible with nominal APS-U parameters and standard x-ray cavity optics.