University of Hamburg, Institut für Experimentalphysik, Hamburg, Germany
XFEL. EU, Hamburg, Germany
DESY, Hamburg, Germany
This work presents an pump-probe setup for measuring the thermal evolution of diamond crystals at cryogenic temperatures under the heat load conditions of an X-ray free electron laser oscillator (XFELO). As the diamond Bragg reflectors of an XFELO are subjected to intense heat loads during operation, the correct understanding of the thermal evolution in diamond plays a major role in the correct modeling of an XFELO. Stoupin et al.* did a room temperature x-ray diffraction measurement on the nanosecond transient thermal response of diamond to an optical pulse. The measurements presented in this paper for the first time incorporate effects due to the very short penetration depth of only a few μm of an XFELO pulse in combination with the high mean free path in diamond at cryogenic temperatures. While at room temperature the heat equation based on Fourier's law accurately fits the measured results, this vastly changes due to the onset of ballistic processes at cryogenic temperatures. These changes, which are hard to predict theoretically, show the necessity of measurements of the thermal evolution in diamond with special regard to a correct mimicking of the heat load in an XFELO.
*S. Stoupin et al., Phys. Rev. B, vol. 86, p. 054301, 2012.
University of Hamburg, Institut für Experimentalphysik, Hamburg, Germany
XFEL. EU, Hamburg, Germany
DESY, Hamburg, Germany
Free-electron laser(FEL) x-ray radiation can deliver pulses with a huge amount of energy in short time duration. X-ray optics like Bragg reflectors therefore must be chosen in a way that they can withstand radiation-material interaction without getting damaged so that they can maintain their technical functionality. Therefore thermal and mechanical reactions of Bragg reflectors to the radiation induced thermal strain and force (radiation pressure) have been considered in this study. The theory of thermoelasticity has been used to simulate the strain conditions at saturation of the amplifying process in an X-ray free-electron laser oscillator(XFELO). One aim of this study was to investigate, if the radiation pressure could be an effect that gives a considerable contribution to the strain propagation. The results of the simulations have shown that, if Bragg backscattering of the X-ray pulse by a diamond crystal with 99% reflectivity and 1% absorptivity is assumed, the value of the thermally induced strain is about two magnitudes higher than the radiation pressure induced strain.