能散对直线感应加速器焦斑分布的影响

The evaluation methods for focal spot size such as root mean square (RMS), full width at half maximum (FWHM), and 50% modulation transfer function (MTF) equivalent uniform distribution are introduced. For a linear induction accelerator, the focusing process of a long-pulse (about 100 ns) electron beam is studied by establishing a theoretical model and carrying out particle-in-cell (PIC) simulation of each beam slice. The effects of the pulse rising/falling edges with large differences in energy, current intensity, and flat-top region on the FWHM and MTF focal spot sizes are analyzed. The research results show that the rising/falling edges of the beam with a large energy spread will lead to a significant increase in the beam focal spot, especially the MTF focal spot. In the simulation results, the FWHM size increases by about 9%, while the MTF size increases by about 24%, which is one of the main factors currently causing the relatively large bottom width of the beam.

介绍了均方根(RMS)、半高全宽(FWHM)和50%调制传递函数(MTF)等效均匀分布等焦斑尺寸评价方法。针对一台直线感应加速器,通过建立理论模型和开展逐束片PIC模拟研究了长脉冲(约100 ns)电子束的聚焦过程,分析了能量、流强与平顶区差异较大的脉冲上升/下降沿对FWHM和MTF焦斑尺寸的影响。研究结果表明:能散度较大的束流上升/下降沿会导致束流焦斑,尤其是MTF焦斑显著增长,在模拟结果中FWHM尺寸增长约9%,而MTF尺寸增长达到约24%,是目前导致束流底宽偏大的主要因素之一。