结构化光电阴极的周期单元与有限束团动力学耦合
Coupling periodic-cell and finite-bunch dynamics for structured photocathodes
- Deutsches Elektronen-Synchrotron DESY（德国电子同步辐射中心）
机构由 AI 辅助整理，请以论文原文为准。
AI总结：
提出一种耦合周期单元与有限束团动力学的方法，通过结合有限平面阴极计算与周期差异计算，高效模拟结构化光电阴极的束流特性，在保持精度的同时大幅降低计算成本。
AI中文摘要：
对光电阴极进行亚微米特征图案化，可通过集中光场增强非线性光发射，但表面几何结构也可能增加发射电子的横向动量展宽。在完整的射频电子枪模拟中，解析注入器尺度照明区域内的纳米级表面场在计算上要求极高。为耦合这些尺度，我们开发了一种方法，将自洽的有限平面阴极计算与匹配的结构化与平面周期计算之间的粒子分辨差异相结合。有限计算确定宏观束团演化与空间电荷场。周期差异确定由表面引起的局部变化。对于一个有限但相对较小的测试问题，与完全分辨的有限阵列WarpX计算相比，均方根能散差异为1.3%，投影归一化发射度差异小于0.1%。随后，我们将该方法应用于具有代表性的自由电子激光光注入器参数，发射源为100 pC，分布在超过五十万个周期上，并跟踪组合束团通过L波段射频电子枪和螺线管。在射频加速和螺线管聚焦后，投影水平与垂直发射度之间的初始差异变得小得多。在阴极下游1.52米处，投影发射度几乎相等，且两个平面均不超过匹配平面阴极参考值的2%。中心切片发射度在水平方向超过参考值约3%，垂直方向约5%。
英文摘要：
Patterning a photocathode with submicrometre features can enhance nonlinear photoemission by concentrating the optical field, but the surface geometry can also increase the transverse momentum spread of the emitted electrons. Resolving nanoscale surface fields across an injector-scale illuminated area within a full rf gun simulation is computationally demanding. To couple these scales, we developed an approach which combines a self-consistent finite flat-cathode calculation with the particle-resolved difference between matched structured and flat periodic calculations. The finite calculation determines the macroscopic bunch evolution and space-charge field. The periodic difference determines the local change caused by the surface. For a finite but relatively small test problem, comparison with a fully resolved finite-array WarpX calculation gives differences of 1.3% in rms energy spread and less than 0.1% in projected normalized emittance. We then apply the method to representative FEL photoinjector parameters with a 100 pC emitted source distributed over more than half a million periods, and track the composed bunch through an L-band rf gun and solenoid. The initial difference between the projected horizontal and vertical emittances becomes much smaller after rf acceleration and solenoid focusing. At 1.52 m downstream of the cathode, the projected emittances are nearly equal and exceed those of the matched flat-cathode reference by less than 2% in both planes. The central-slice emittances exceed the reference values by approximately 3% horizontally and 5% vertically.
