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Photo-Thermo-Mechanical Analysis and Control for High-brightness and High-repetition-rate X-ray Optics


X-ray optics has been serving the exploration of nature in synchrotron radiation for many decades. In recent years, X-ray free-electron lasers (XFELs) open a new era of X-ray based research by generating extremely intense X-ray flashes. The outstanding properties including extremely high brightness enable unprecedent discoveries in many research fields, such as structural biology, photochemistry, atomic, molecular and cluster physics. To assist research, the X-ray optics used in synchrotron radiation are either directly employed or modified for their utilizations in XFEL. One emerging challenge for the implementation of X-ray optics in XFEL is that the extremely bright photon beam introduces highly localized and strong thermal load on the X-ray optics, especially at high repetition rates. The thermal load induces severe non-uniform thermal deformation of the optics, degrading their ability to preserve the superior XFEL beam quality, such as high brightness and outstanding coherence.

To understand the thermal load effects and provide design principles to minimize these effects, in this study, we carry out coupled photo-thermal-mechanical analyses. We first provide necessary background and review of current studies on thermal load in X-ray optics. Next, we derive an analytical model to yield qualitative understanding on thermal load. According to the analytical model, the thermal load effects can be decomposed into two parts: the tunable part (due to the overall strain increase) and untunable part (due to the strain and surface slope inhomogeneity). We then further investigate and quantitatively evaluate the performance degradation of the optics due to thermal load in practical operating modes: pump-probe mode, pulse train mode and quasi-steady state. In pump-probe mode, the tunable part is invisible due to the limited amount of energy deposited, while the untunable part results in significant performance reduction for short delay time between the two pulses. For beamline monochromator, additional effects such as laser footprint distortion and tranverse coherence degradation are also harmful for photon science experiments. In pulse train mode where more than two pulses are incident, thermal load accumulates pulse by pulse. Both tunable and untunable parts are observable. The optics performance degrades dynamically due to continuing thermal load accumulation. Accordingly, the tuning needs to follow this dynamic behavior to recover the central photon energy shift and align the spectrum if multiple monochromators are cooperating in the system. At quasi-steady state, the performance is constantly affected and necessary design is needed to refrain the thermal load. Three designs of monochromators are proposed and analyzed for different repetition rate at different cooling temperatures. Critical repetition rate is accordingly determined. One desirable way to improve the thermal load tolerance of the X-ray optics is to operate them at cryogenic temperature, as confirmed by simulation. However, at cryogenic temperature, the emerging unconventional nondiffusive heat transfer phenomena may result in the performance deviation from designed status. We introduce and propose simplified nondiffusive models to characterize these new features and prepare for the experimental analysis with cryogenic cooling. Based on this study, the thermal load effects are be better understood. Design insights including geometry design and operation condition are also provided, contributing to the preparation for practical experimental test and maintaining the X-ray optics performance at high-brightness and high-repetition-rates.

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