Population Inversion Hard X-ray Lasing

X-ray Laser Oscillator (XLO) is an ongoing project to harness the power of hard x-ray fluorescence. A population-inversion gain medium pumped with a focused XFEL pulse is placed in the center of one of the two parallel arms of a back-scattering Bragg crystal cavity. Additional x-ray focusing optics is placed in the cavity to ensure good spatial overlap with the pump pulse. XLO uses SASE XFEL pump pulses generated at 120 Hz repetition rate by the SLAC copper linac with a temporal pulse spacing of 35 ns, matching the 10 m cavity length; XLO-II will use a larger pulse spacing of 1000 ns (corresponding to 1 MHz repetition rate of LCLS-II-HE), and, depending on the cavity length, each pulse will be recycled between 10 and 30 times through the cavity before the next pulse arrives.