Beam-driven Plasma Wakefield Acceleration

该项目将涉及开展与束驱动等离子体韦克菲尔德加速有关的理论、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。该项目将特别关注基于激光束同步诊断的潜在等离子体。所有的研究都将涉及与位于DESY汉堡的FLASHForward实验的密切合作。刘易斯博尔顿的博士工作将旨在开发一种基于等离子体余辉的新颖、通用和强大的电子束诊断方法。这项研究将由汉堡的DESY和FLASHForward小组共同资助，该小组还同意接待刘易斯进行实验活动。FLASHForward是一个电子束驱动等离子体韦克菲尔德加速的实验，现在已经进行了几年，并将在2019年看到第一个beamtime。该实验使用FLASH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等离子体韦克菲尔德加速的“兄弟”，是SCAPA的基石，SCAPA是苏格兰等离子体加速器应用中心，它允许利用大量的协同效应。在B领导的E210：特洛伊木马合作项目中，首次观察到了等离子体余辉效应和连接的等离子体炬注入效应。2013年至2017年藏在斯坦福大学的SLAC FACET设施。这里，电子束与预生成的等离子体相互作用，并且取决于操作模式，可以利用几乎非侵入性的相互作用，或者包括与驱动器束相比具有显著改善的特性的电子的注入和生成的严重扰动的相互作用。在非侵入式操作模式下，电子束仅与微小的激光产生的等离子体灯丝非常短暂地耦合。然后等离子体灯丝电子被局部加热并留下高度特异性的“指纹”，这可以用最简单的方法检测到，用一个简单的CCD操作，带通滤波器用于等离子体余辉线之一，例如，对于作为介质的氦，约589 nm，如Hidding的研究小组过去在斯坦福大学使用的那样。斯坦福大学的测量结果目前正在自然物理学进行审查。虽然斯坦福大学的这种设置和测量只是简单地积分了总光子产额以检索一个简单的数字，但模拟表明，通过在空间、光谱和时间上分辨余辉特征，可以获得巨大的好处。刘易斯应通过实施这些功能（例如，使用光谱仪、条纹相机和高空间分辨率）来开发此诊断。这样，可以解决电子束电荷、尺寸、持续时间和电流以及关于局部等离子体密度的发射到发射的变化。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，则基于局部激光的氦电离可以导致等离子体下坡注入的全光学版本，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DESY将在SLAC的补充参数范围内详细探索和利用该方案。在他之前在DESY的研究中，Hidding已经投资了大约100千欧元用于未来在FLASHForward提供飞秒同步能力，现在将在未来几年内开发。

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.