Beam-driven Plasma Wakefield Acceleration

该项目将涉及开展与束流驱动等离子体尾场加速有关的理论、计算和实验研究，特别是使用等离子体光电阴极方法进行电子注入。该项目将特别关注一种潜在的基于等离子体的激光光束同步诊断。所有研究都将与设在德国汉堡市DESY的FlashForward实验密切合作。刘易斯·博尔顿的博士工作将致力于开发一种基于等离子体余辉的新的、通用的和健壮的电子束诊断方法。这项研究将由汉堡的DESY和FlashForward小组共同资助，FlashForward小组还同意接待刘易斯进行实验性活动。FlashForward是一个电子束驱动的等离子体尾波场加速实验，目前已经酝酿了几年，将在2019年看到第一个光束时间。该实验利用闪光软X射线自由电子激光光源的电子束来驱动等离子体波。这种方法是激光驱动的等离子体尾迹加速的“兄弟”，这是苏格兰等离子体加速器应用中心SCAPA的基石，它允许利用实质性的协同效应。2013-2017年间，在B.Hidding领导的斯坦福大学SLAC小平面设施的E210：特洛伊木马合作中，首次观察到了等离子体余辉效应和相关的等离子火炬注入效应。在这里，电子束与预先产生的等离子体相互作用，根据操作模式的不同，要么可以利用几乎非侵入性的相互作用，要么可以利用包括注入和产生电子的严重微扰相互作用，与驱动光束相比，电子的注入和产生具有显著改善的特性。在非侵入式工作模式下，电子束只与微小的激光产生的等离子体细丝非常短暂地耦合。然后，等离子体细丝电子被局部加热，并留下高度特异的“指纹”，这可以用最简单的方法检测到，只需使用简单的CCD操作，并对其中一条等离子体余辉谱线使用带通过滤器，例如，过去Hidding的研究团队在斯坦福大学使用的氦作为媒介的~589 nm。斯坦福大学的测量结果目前正在自然物理研究所进行审查。虽然斯坦福大学的这种设置和测量只是将总光子产额积分为一个简单的数字，但模拟表明，通过在空间、光谱和时间上解析余辉信号具有巨大的好处。Lewis应通过实施这些功能来开发这种诊断，例如使用光学光谱仪、条纹相机和高空间分辨率。通过这种方式，可以解决电子束电荷、大小、持续时间和电流以及局部等离子体密度的逐个变化。通过从本地化交互中检索尽可能多的信息，可以获取高度特定的交互指纹。因此，刘易斯还将开发允许明确检索交互数据的算法，这可能需要机器学习方法。如果相互作用发生在下面的等离子体介质中，例如氢，基于激光的局域氦电离可以导致全光学版本的等离子体下坡注入，这被称为等离子体炬注入。这种效应的第一个特征已经在斯坦福大学的E210中看到，但现在该方案将在DESY以补充SLAC的参数范围进行详细的探索和利用。在他之前在DESY的研究中，Hidding曾投资约100kEURO用于未来在FlashForward提供飞秒同步能力，现在将在未来几年利用这一能力。

This project will involve carrying out theoretical, computational and experimental studies related to Beam Driven Plasma Wakefield acceleration, in particular using the plasma photocathode method for electron injection. The project in particular will focus on a potential plasma based laser-beam synchronisation diagnostic. All studies will involve close collaboration with the FLASHForward experiment based at DESY Hamburg.Lewis Boulton's PhD work will aim at developing a novel, versatile and robust electron beam diagnostics method based on plasma afterglow. This research will be co-funded by DESY in Hamburg by the FLASHForward group, who also agreed to host Lewis for experimental campaigns. FLASHForward is an experiment for electron beam-driven plasma wakefield acceleration which is now a couple of years in the making, and will see first beamtime in 2019. The experiment uses the electron beam of the FLASH soft x-ray free-electron laser light source in order to drive a plasma wave. This approach is the 'sibling' of laser-driven plasma wakefield acceleration, a cornerstone at SCAPA, the Scottish Centre for the Application of Plasma-based Accelerators, which allows substantial synergies to be harnessed. The plasma afterglow effect and the connected plasma torch injection effect has been observed for the first time within the E210: Trojan horse collaboration, led by B. Hidding at Stanford's SLAC FACET facility from 2013-2017. Here, the electron beam interacts with pre-generated plasma and depending of the mode of operation, either nearly non-intrusive interaction can be harnessed, or heavily perturbative interaction which includes the injection and generation of electrons with dramatically improved characteristics compared to the driver beam. In the non-intrusive mode of operation, the electron beam couples only very briefly with a tiny laser-generated plasma filament. Plasma filament electrons are then locally heated and leave a highly specific 'fingerprint', which can be detected in the most simple approach with a simple CCD operating with a bandpass filter for one of the plasma afterglow lines, for example ~589 nm for helium as a medium, as used in the past in Stanford by Hidding's research team. The Stanford measurement results are currently under review at Nature Physics. While this setup and measurement at Stanford simply integrated the total photon yield to retrieve a simple number, simulations indicate that there is huge benefit by resolving the afterglow signature spatially, spectrally and temporally resolved. Lewis shall develop this diagnostic by implementing these capabilities, for example by using optical spectrometers, streak cameras and high spatial resolution. This way, shot-to-shot variations of electron beam charge, size, duration and current but also as regards local plasma density can be resolved. By retrieving as much information from the localized interaction as possible, highly specific interaction fingerprints can be taken. Lewis will therefore also develop algorithms which will allow to retrieve interaction data unambiguously, which may require machine learning approaches. If the interaction takes place in an underlying plasma medium, such as hydrogen, the localized laser-based helium ionization can lead to an all-optical version of plasma downramp injection, which is termed 'plasma torch' injection. First signatures of this effect have been seen in E210 in Stanford, but now the scheme shall be explored and exploited in detail at DESY in a complementary parameter range to SLAC. In his previous research at DESY, Hidding had invested ~100kEURO into future provision of femtosecond synchronization capability at FLASHForward, which shall now be exploited in the coming years.