بحث بعنوان Numerical Modeling of Thermal Loading of Diamond Crystal in X-ray FEL Oscillators

بحث بعنوان
Numerical Modeling of Thermal Loading of Diamond Crystal in X-ray FEL Oscillators
SONG Mei-Qi , ZHANG Qing-Min , GUO Yu-Hang , LI Kai , DENG Hai-Xiao
Abstract:
Due to high reflectivity and high resolution of X-ray pulses, diamond is one of the most popular Bragg crystals serving as the reflecting mirror and mono−chromator in the next generation of free electrons lasers (FELs). The energy deposition of X-rays will result in thermal heating, and thus lattice expansion of the diamond crystal, which may degrade the performance of X-ray FELs. In this paper, the thermal loading effect of diamond crystal for X-ray FEL oscillators has been systematically studied by combined simulation with Geant4 and ANSYS, and its dependence on the environmental temperature, crystal size, X-ray pulse repetition rate and pulse energy are presented. Our results show that taking the thermal loading effects into account, X-ray FEL oscillators are still robust and promising with an optimized design.
Conclusions
Diamond crystal is commonly accepted as the material for fabricating mirrors for X-ray FEL oscillators. One of the most critical questions still open is how the thermal loading of diamond crystal degrades the performance of the X-ray source. In this paper, Geant4 and ANSYS are jointly used to simulate the interaction between the X-ray photon pulse and diamond crystal, and the crystal temperature shift due to thermal loading.
Our results indicate that a diamond crystal with large transverse cross section and small thickness, and a low environmental temperature, are helpful to reduce the thermal loading effects in XFELO. For the XFELO example operated at 1 MHz repetition rate in this paper, in order to ensure the bandwidth of 1 meV, the X-ray pulse energy in the Bragg cavity should be lower than 45 µJ under 100 K environmental temperature.
It is expected that this result is useful for the manufacture and establishment of a feasible crystal mirror in X-ray FEL oscillators. It is worth stressing that this study is preliminary and there are still several practical physical effects that are not included, such as accurate calculation of crystal reflectivity from dynamic theory [21], the noisy start-up and coherent build-up of XFELO, and the thermal expansion of the crystal lattice.
These and other effects will be left for our subsequent reports. The numerical method used here can be easily extended to model the mono−chromator in hard X-ray FEL selfseeding schemes.
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