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摘要:
X射线自由电子激光器（FEL）由于其超高亮度、超短脉冲等特点，在世界范围内得到广泛应用。 基于尾流场理论，我们计算了上海X射线自由电子激光器（SXFEL）中从直线加速器出口到波荡器末端，束流在245 m不锈钢传输线和波荡器中的阻抗壁尾场。通过对两种不同的阻抗壁尾场的叠加，发现将导致束流纵向相空间的畸变。在SXFEL上进行束流物理的实验，并得到与理论预测非常吻合的实验结果。 结合之前对主要直线加速器部分的详细模拟和实验研究，为后续FEL整体束流优化提供了参考。
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关键词:
- 上海软X射线自由电子激光 /
- 纵向相空间 /
- 阻抗壁尾场 /
- 能量分布畸变
Abstract:X-ray free-electron laser (XFEL), due to its ultra-high brightness, ultra-short pulse and other characteristics, has been built worldwide. Based on the theory of wakefield, we calculate the resistive wall wakefield from the linear accelerator (linac) exit to the end of the undulator in Shanghai X-ray free electron laser (SXFEL) with bunch traveling through the 245 m stainless steel transfer line and copper beamline in undulator. Then we analyze the resistive wall wakefields which eventually lead to the distortion of the longitudinal phase space within the bunch. Finally, the theoretical predictions of influence of resistive wall wakefield are compared with experiment results on SXFEL, which shows great agreement. The detailed research provides a direction for subsequent FEL optimization.
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Figure 2. Total wakefield in the bypass-line 1 and bypass-line 2.
$ {\sigma }_{z} $ = 0 represents the head of the bunch, and the y-axis represents wakefield in MeV unit
Figure 3. Wakefield in the copper flat-plate pipe. The
$ {\sigma }_{z} $ = 0 represents the head of the bunch, and the y-axis represents wakefield in MeV unit
Figure 4. Total wakefield for bypass-line 1, bypass-line 2 and the copper flat-plate pipe.
$ {\sigma }_{z} $ = 0 represents the head of the bunch, and y-axis represents wakefield in MeV unit
Figure 6. (a) Distribution for bunch A, and (b) measured and calculated wakefields in bypass-line 1
Figure 7. Measured longitudinal phase space in position 1 and position 2. The figure is separated in 500×500 pixels, and x axis stands for bunch length (left is beam head), y stands for the energy (top is higher energy)
Figure 9. (a) Distribution for bunch B and (b) measured and calculated wakefields for all the pipes
Table 1. Parameters of three different types of pipes
material type length/m radius/mm bypass-line 1 stainless-steel round 125 17.50 bypass-line 2 stainless-steel round 120 8.00 undulator copper flat plate 40 2.15 
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Table 2. Initial bunch parameters in SXFEL
length/µm energy /GeV charge/pC bunch A 250 1.35 500 bunch B 120 1.35 500
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