  X-ray free electron laser oscillators (XFELO) is future light source to
produce fully coherent hard X-ray pulses. The X-rays circulate in an optical
cavity built from multiple Bragg reflecting mirrors, which has a high
reflectance in a bandwidth of ten meV level. The X-ray crystal mirrors exposed
to intense X-ray beams in the cavity are subjects to thermal deformations that
shift and distort the Bragg reflection. Therefore, the stability of the XFELO
operation relies on the abilities of mirrors to preserve the Bragg reflection
under such heat load. A new approach was used to analyze the heat load of
mirrors and the XFELO operation. The essential light-matter interaction is
simulated by the GEANT4 with a dedicated Bragg-reflection physical process to
obtain the precise absorption information of the XFELO pulse in the crystals.
The transient thermal conduction is analyzed by the finite-element analysis
software upon the energy absorption information extract from GEANT4 simulation.
A simplified heat-load model is then developed to integrate the heat load in
the XFELO. With the help of the heat-load model, the analysis of XFELO
operating with several cryogenically cooled diamond mirrors is conducted. The
results indicate that the heat load would induce an oscillation when XFELO
operates without enough cooling.
