Predicting relative energy spread in two-bunch plasma wakefield acceleration
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摘要: 针对空泡机制中的双束等离子体尾波电子加速设计,给出了能够快速得到被加速束流在最大加速距离下的相对能散的预测公式。通过加速初始时刻束流纵向分布以及束流所处位置的纵向尾波场可得到束流最终相对能散。该预测公式不仅可应用于驱动束流与被加速束流初始能量相同的情况,还可应用于两个束流初始能量不相同的情况。由该预测公式得到的束流相对能散与被加速束流和驱动束流的初始能量的比值有关,而与两个束流初始能量的数值无关。利用准静态近似的粒子网格模拟程序QuickPIC对理论进行了模拟验证,模拟结果与理论预期结果一致。Abstract: We present a formula to directly obtain the final relative energy spread of a trailing beam at the maximal acceleration distance. The formula works for electron beams in a two-bunch plasma wakefield acceleration stage in the so-called nonlinear bubble regime. It only requires the longitudinal profile of the trailing beam and the longitudinal wakefield within the trailing beam at the beginning of an acceleration. This formula not only works well for drive beams and trailing beams with the same initial energies, but is also available for those beams with different initial energies. We find that the relative energy spread of the trailing beam obtained from the formula is determined by the ratio of the trailing beam’s initial energy to the drive beam’s initial energy rather than the specific value of their initial energies. We perform several computational simulations using the quasi-static particle-in-cell code QuickPIC, and the results agree well with that calculated from the formula.
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表 1 PIC模拟和式(3)得到的相对能散
Table 1. Relative energy spread from PIC simulations and equation (3)
$ D $ drive beam $ d/{\text{k}}_{\text{p}}^{-1} $ trailing beam $ \vartriangle s/{\text{m}} $ $ {\delta _{Er}}/{\text{{\text{%}} }} $ $ {\delta _{Ep}}/{\text{{\text{%}} }} $ $ {W_{ds}}/{\text{GeV}} $ $ {\sigma _{ {\textit{z}} d}}/{\text{k}}_{\text{p}}^{-1} $ $ {n_{bd}}/{{\text{n}}_{\text{p}}} $ $ {\varepsilon _{Nd}}/{\text{k}}_{\text{p}}^{-1} $ $ {\sigma _{r{\text{d}}}}/{\text{k}}_{\text{p}}^{-1} $ $ {W_{ts}}/{\text{GeV}} $ $ {\sigma _{ {\textit{z}} t}}/{\text{k}}_{\text{p}}^{-1} $ $ {n_{bt}}/{{\text{n}}_{\text{p}}} $ $ {\varepsilon _{Nt}}/{\text{k}}_{\text{p}}^{-1} $ $ {\sigma _{rt}}/{\text{k}}_{\text{p}}^{-1} $ 1 10 1 80 0.25 0.05 4 10 0.25 76.36 0.25 0.05 5.8 2.33 2.36 2 10 1 80 0.25 0.05 4 20 0.25 76.36 0.35 0.05 5.8 1.72 1.74 2 5 1 80 0.17 0.05 4 10 0.25 76.36 0.25 0.05 2.9 1.72 1.74 -
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