Abstract
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.
Highlights
In the hard x-ray regime, the operating free electron lasers (FELs) are based on the SASE mode [1,2,3,4,5], which can generate x-ray pulses with unique characteristics, such as ultrahigh peak power and ultrashort pulse duration
The stability of the X-ray free electron laser oscillators (XFELOs) operation depends on the ability of the mirrors to maintain the Bragg reflection under such thermal load
Following transient thermal behavior, including single pulse and multiple pulse inputs, was analyzed by finite element analysis software based on the energy absorption information extracted from the GEANT4 simulation
Summary
In the hard x-ray regime, the operating free electron lasers (FELs) are based on the SASE (self-amplified spontaneous emission) mode [1,2,3,4,5], which can generate x-ray pulses with unique characteristics, such as ultrahigh peak power and ultrashort pulse duration. With high-brightness electron bunches at MHz repetition rate, the intracavity x-ray pulses could have a pulses energy of about 800 μJ and beam radius of about 30 μm. Such intense x-ray pulses inevitably impose a high heat load on the mirrors, which result in lattice distortions in the crystal. The first one utilizes a high power conventional laser in the long-wavelength regime to model laser-mirror interactions The advantage of this method is the ability of precisely controlling the pulse duration and pulse energy, which is crucial for light-material interactions.
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