Very High Efficiency X-ray FEL Oscillator

X-ray Free Electron Laser in an Oscillator configuration (XFELO) offers significant advantages over single pass machines, including an enhanced spectral brightness and potentially improved output stability. However, a poor outcoupling efficiency of the X-ray optical resonators (< 4%), represents a significant practical limitation of the XFELO output in a low gain regime. In response to this problem, RadiaBeam Technologies and UCLA are proposing to develop an XFELO operating in a high gain regime, where a single pass FEL gain is sufficiently high to overcome optical cavity losses (a regime, similar to a regenerative amplifier FEL). A particularly promising approach is TESSA (Tapering Enhanced Stimulated Superradiant Amplification) oscillator configuration, where a strongly tapered undulator is embedded into the optical cavity, to combine high repetition rate, with the very high single pass efficiency and peak power. The main result of the Phase I activities is the development of a reliable time-dependent 3D simulation model for the planned experimental demonstration of the high frequency oscillator. In addition, we carried out a detailed planning for experimental proof-of-concept investigation of the TESSA oscillator at the UV wavelength, to be conducted in the Phase II. The Phase II project will be carried out at the Argonne National Laboratory LEA facility, where the existing 4 meters TESSA undulator, will be embedded inside the optical cavity in an oscillator configuration. In Year 1, a LEA photoinjector will be upgraded to achieve pulse train operations, and in Year 2 the entire oscillator system will be commissioned and optimized. Commercial Applications and Other Benefits: The FEL technology, hitherto developed for the research needs only, could also find numerous industrial applications, with improved cost efficiency, reliability and versatility. Towards that end, a successful implementation of the high gain TESSA oscillator, will open a path to high impact industrial FEL applications in lithography, nanoengineering, and directed energy.