Reducing the intrinsic emittance of photocathodes is one of the most promising routes to improving the brightness of electron sources. However, when emittance growth occurs during beam transport (for example, due to space-charge), it is possible that this emittance growth overwhelms the contribution of the photocathode and, thus, in this case source emittance improvements are not beneficial. Using multi-objective genetic optimization, we investigate the role intrinsic emittance plays in determining the final emittance of several space-charge dominated photoinjectors, including those for high repetition rate free electron lasers and ultrafast electron diffraction. We introduce a new metric to predict the scale of photocathode emittance improvements that remain beneficial and explain how additional tuning is required to take full advantage of new photocathode technologies. Additionally, we determine the scale of emittance growth due to point-to-point Coulomb interactions with a fast tree-based space-charge solver. Our results show that in the realistic high brightness photoinjector applications under study, the reduction of thermal emittance to values as low as 50 pm/ (1 meV MTE) remains a viable option for the improvement of beam brightness.

中文翻译：

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现代光注入器亮度低的固有发射率

降低光电阴极的固有发射率是提高电子源亮度的最有希望的途径之一。但是，当在光束传输过程中发生发射率增长时（例如由于空间电荷），该发射率增长可能会淹没光电阴极的作用，因此，在这种情况下，提高源发射率是无益的。使用多目标遗传优化，我们研究了固有发射率在确定几种以空间电荷为主的光注入器（包括用于高重复频率自由电子激光器和超快电子衍射的光注入器）的最终发射率中的作用。我们引入了一种新的度量标准来预测仍然有益的光电阴极发射率改善的规模，并说明如何充分利用新的光电阴极技术需要进行额外的调整。此外，我们确定了基于树的快速空间电荷求解器的点对点库仑相互作用所导致的发射率增长的规模。我们的结果表明，在正在研究的实际高亮度光电注入器应用中，将热辐射降低至低至50 pm /（1 meV MTE）的值仍然是提高光束亮度的可行选择。