Interferometric Solutions for Optics Stabilization at Superconducting Linac Driven X-ray Cavity Free Electron Laser Systems

Abstract The development of next generation X-ray laser sources with enhanced peak and average spectral brightness have been actively investigating concepts mimicking the laser regenerative amplifiers. These systems circulate X-ray photons through an undulator system using an optical cavity operating at X-ray wavelengths. The repeated interaction between the recirculating X-ray pulses and the successive fresh relativistic electron bunches leads to power build up and gain saturation, generating Fourier-limited, ultra-bright pulses. Compared to self-amplified spontaneous emission processes used in most currently operating free-electron laserss and the single-pass self-seeded methods, this regenerative amplification approach is another step forward in x-ray laser technology. However, as the regenerative systems are expected to be driving by a superconducting linac operating at MHz repetition rate such as the Linac Coherent Light Source (LCLS)-II HE linac currently under construction, the expected cavity round trip time dictates extended cavity lengths on the order of 100s of meters. Achieving and maintaining the optimal cavity optics alignment, thus the maximal interaction between the recirculating photons and the electron bunches, is one of the most critical challenges, considering in particular relative ground motion between optical benches separated by large distances across broad timescales. To address the stringent stability requirement, we propose the integration of a continuous wave laser guide system for the stabilization of the cavity, the accelerator, and the X-ray components. This paper outlines the concept and its potential implementation using the LCLS-II HE linac, at SLAC National Accelerator Laboratory.