Optimization of transformer ratio and beam loading in a plasma wakefield accelerator with a structure-exploiting algorithm
Optimization of transformer ratio and beam loading in a plasma wakefield accelerator with a structure-exploiting algorithm
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DOI：
10.1063/5.0142940
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发表时间：
2023-01
影响因子：
2.2
通讯作者：
Q. Su;J. Larson;T. Dalichaouch;F. Li;W. An;L. Hildebrand;Y. Zhao;V. Decyk;P. Alves;S. Wild;W. Mori
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文献类型：
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作者：
Q. Su;J. Larson;T. Dalichaouch;F. Li;W. An;L. Hildebrand;Y. Zhao;V. Decyk;P. Alves;S. Wild;W. Mori
基于等离子体的加速已成为未来线性对撞机或下一代光源的加速器技术的有前途的候选者。我们考虑等离子体尾场加速器（PWFA）概念，其中等离子体波尾流由粒子束激发，尾随束在尾流上冲浪。对于线性对撞机，从驱动束到尾流以及从尾流到尾束的能量传递效率必须很大，同时必须保持尾束的发射度和能量扩散。在加速电子时同时实现这一目标的一种方法是使用纵向形状的束和非线性尾流。在线性体系中，存在一种分析形式来获得最佳形状。然而，在非线性状态下，无法精确获得使能量传输效率最大化的驱动器的最佳形状，因为目前没有理论描述所有非线性程度的尾流结构和激励过程。此外，离子通道半径在尾流前部没有被很好地限定，其中等离子体电子没有被驱动束完全吹出。我们使用一种新颖的优化方法来有效地确定 PWFA 中驱动梁和尾梁的电流分布，从而提供低能量扩散、低发射率和高加速效率的结果。我们将纵梁电流分布参数化为分段线性函数并定义优化目标。对于尾束，该算法快速收敛到近似倒梯形的尾束电流分布，类似于非线性尾场理论的超相对论极限所预测的。对于驱动梁，通过在非线性区域中优化来最大化变压器比而发现的梁轮廓也类似于线性理论所预测的。与非线性理论相对论极限预测的线性斜坡相比，通过优化方法找到的电流分布提供了更高的变压器比。
Plasma-based acceleration has emerged as a promising candidate as an accelerator technology for a future linear collider or a next-generation light source. We consider the plasma wakefield accelerator (PWFA) concept where a plasma wave wake is excited by a particle beam and a trailing beam surfs on the wake. For a linear collider, the energy transfer efficiency from the drive beam to the wake and from the wake to the trailing beam must be large, while the emittance and energy spread of the trailing bunch must be preserved. One way to simultaneously achieve this when accelerating electrons is to use longitudinally shaped bunches and nonlinear wakes. In the linear regime, there is an analytical formalism to obtain the optimal shapes. In the nonlinear regime, however, the optimal shape of the driver to maximize the energy transfer efficiency cannot be precisely obtained because currently no theory describes the wake structure and excitation process for all degrees of nonlinearity. In addition, the ion channel radius is not well defined at the front of the wake where the plasma electrons are not fully blown out by the drive beam. We present results using a novel optimization method to effectively determine a current profile for the drive and trailing beam in PWFA that provides low energy spread, low emittance, and high acceleration efficiency. We parameterize the longitudinal beam current profile as a piecewise-linear function and define optimization objectives. For the trailing beam, the algorithm converges quickly to a nearly inverse trapezoidal trailing beam current profile similar to that predicted by the ultrarelativistic limit of the nonlinear wakefield theory. For the drive beam, the beam profile found by the optimization in the nonlinear regime that maximizes the transformer ratio also resembles that predicted by linear theory. The current profiles found from the optimization method provide higher transformer ratios compared with the linear ramp predicted by the relativistic limit of the nonlinear theory.