软X射线自由电子激光单色器优化方法

An Optimization Method for Soft X-Ray Free Electron Laser Monochromator

软X射线自由电子激光(FEL)光栅单色器的设计是个多目标优化任务,涉及到能量分辨能力、脉冲展宽、光斑投影大小、物距、像距、光栅线密度、光栅变线距等众多参数,优化起来十分复杂。本文提出了光栅单色器的统一优化方法模型,该模型把多目标优化任务转化为单目标(光栅焦点常数C;)优化任务。基于该模型,本文对比了三种软X射线FEL光栅单色器设计方案,并直观地对单色器能量分辨能力、脉冲展宽和光斑在光栅上投影大小进行评估。此外对这三种光栅单色器的设计方案进行了详细的计算,并绘制出了"C;-波长-优化参数"三元图。通过"C;-波长-优化参数"三元图,能够清晰地得到光栅单色器的能量分辨能力、脉冲展宽和光斑投影大小随着C;变化而变化的趋势。本项工作为光栅单色器的设计和优化提供指导作用。

The design of soft X-ray free electron laser(FEL) grating monochromator is a multi-objective optimization task, which involves many parameters such as energy resolving power, pulse stretching, footprint, source distance, image distance, grating line density, varied-line-spacing parameter and so on. This paper proposes a general method model to optimize the soft X-ray FEL grating monochromator. The model transforms multi-objective optimization into single one based on a single variable called blazed grating focus constant C;. By using this model, three types of soft X-ray FEL grating monochromators are compared and analyzed, and the energy resolving power, footprint on grating and pulse stretching are estimated conveniently. The design schemes of the three grating monochromators are calculated in detail, and a “C;-wavelength-optimization parameter” ternary diagram is drawn to optimize the monochromator. The ternary diagram can intuitively describe the tends of energy resolving power, pulse stretching and footprint on grating. This work provides a guidance for the design and optimization of soft X-ray FEL grating monochromators.