Laser-based selective preionization of plasma wakefield accelerator stages

粒子束驱动等离子体韦克菲尔德加速（PWFA）是近年来国内外加速器研究的热点。在大型加速器中心（如SLAC）旁边，激光等离子体加速器（如斯特拉斯克莱德大学的SCAPA中心或LMU慕尼黑的CALA中心）也可以通过使用来自激光等离子体加速器（LWFA）的电子束作为PWFA级的驱动器[1，2，3]。作为PWFA介质的宽预电离等离子体通道的产生是产生高能高质量电子束的关键任务[4，5]。另一个关键特征是在多组分气体-等离子体中仅吸收一种组分，使得电离的组分可用于实现等离子体光电阴极。这些是基于这样的特征，即电子驱动等离子体韦克菲尔德加速不需要过多的峰值电驱动场来激发强等离子体波，这是由于其单极电驱动束场分布。由于电子束的振荡电场结构，能够激发这种波所需的电子束的峰值电场比高功率激光脉冲的峰值电场低许多数量级。该特征允许通过利用显著不同的隧穿电离阈值的物种（诸如氢和氦）来将尾流激发与电子聚束注入解耦。因此，可以利用峰值电场局部超过高电离阈值介质的峰值电场的激光脉冲，以在任意时空位置以受控方式释放和注入电子。这是等离子体阴极的主要吸引力，可以实现，允许控制和高度可调的注入电子人口与极低的所谓的电子束发射率，因此，高亮度，许多数量级优于国家的最先进的。这样的能力可能是变革的相干和非相干光子科学源，高场和高能物理。进而，这意味着需要低电离阈值组分和高电离阈值组分的选择性电离。这包括在宽而长的区域中均匀地预电离低电离阈值分量，等离子体上升和下降的成形，以及用于等离子体光阴极注入的较高电离阈值分量的局部非常受限的电离。这是这个博士学位的核心研发主题，并意味着两个主要目标：-选择性激光预电离低电离阈值介质，如氢。这里的目标是一个长达一米的等离子体通道，宽度可达一毫米，没有热点，这将阻碍相关的较高电离阈值介质-基于激光的高电离阈值介质（如氦）的局部隧道电离-入射电子和激光脉冲，等离子体介质和产生的电子脉冲的计量该项目是在欧洲与LMU合作实现的，LMU是主要合作伙伴。[1]藏起来了，B。..卡尔施，S。例如，混合激光-等离子体韦克菲尔德加速器中的单能能量倍增，物理评论快报。104，195002（2010）[2]激光加速电子束诱导的等离子体波和动力学的直接观测，M. F.吉尔约翰B。藏起来S. Karsch，Physical Review X 9，011046（2019）[3] T. Kurz，T.海涅曼等人，激光加速电子束驱动的紧凑型等离子体加速器的演示，arXiv：1909.06676[4] G.G. Manahan.. Hidding，B.，单级基于等离子体的相关能量扩展补偿6D亮度电子束，Nat. Communications 8，15705（2017）[5] A.邓Hidding，B.，等离子体光电阴极的电子束产生，Nat. Physics（2019）

Particle beam-driven plasma wakefield acceleration (PWFA) is an area of strongly increasing interest in the world-wide accelerator community. Next to large accelerator centres such as SLAC, laser-plasma accelerators such as at the SCAPA centre at University of Strathclyde or the CALA centre at LMU Munich can also engage, by using electron beams from laser-plasma-accelerators (LWFA) as drivers for the PWFA stage [1,2,3]. Generation of wide preionized plasma channels as medium for PWFA is a key task for production of electron beams with high energies and high-quality [4,5]. A further key feature is to ionize only one component in a multi-component gas-plasma, such that an ionized component is available for realization of plasma photocathodes. These are based on the feature that electron-driven plasma wakefield acceleration does not require excessive peak electric driver fields to excite strong plasma waves, due to its unipolar electric drive beam field distribution. The peak electric field of electron beams required capable to excite such waves is many orders of magnitude lower than those of high power laser pulses due to their oscillating electric field structure. This feature allows to decouple wake excitation from electron bunch injection by exploiting species of significantly different tunnelling ionization thresholds such as hydrogen and helium. A laser pulse with peak electric fields locally exceeding that of the high ionization threshold medium can therefore be exploited to release and inject electrons in a controlled way at arbitrary spatiotemporal positions. The chief attraction of this is that plasma cathodes can be realized which allow controlled and highly tunable injection of electron populations with extremely low so-called electron beam emittance and therefore ultrahigh brightness, many orders of magnitude better than state-of-the-art. Such capabilities may be transformative for coherent and incoherent photon science sources, high field and high energy physics. In turn, this means that selective ionization of the low ionization threshold component and the high ionization threshold component is required. This includes preionization of the low ionization threshold component uniformly in a wide and long region, shaping of plasma upramps and downramps, as well as locally very confined ionization of the higher ionization threshold component for plasma photocathode injection. This is the core R&D theme of this PhD and implies two main objectives: - Selective laser-based preionization of low-ionization threshold media such as hydrogen. The aim here is an up to metre-long plasma channel with width up to a millimetre, without hot spots which would ionize relevant higher ionization threshold media - Laser-based localized tunneling ionization of high-ionization threshold media such as helium- Metrology of incoming electron and laser pulses, plasma medium and produced electron pulsesThe project is realized in a European collaboration with LMU as main partner. [1] Hidding, B. .. Karsch, S. et al., Monoenergetic Energy Doubling in a Hybrid Laser-Plasma Wakefield Accelerator, Phys. Rev. Lett. 104, 195002 (2010)[2] Direct observation of plasma waves and dynamics induced by laser-accelerated electron beams, M. F. Gilljohann .. B. Hidding .. S. Karsch, Physical Review X 9, 011046 (2019)[3] T. Kurz, T. Heinemann et al., Demonstration of a compact plasma accelerator powered by laser-accelerated electron beams, arXiv:1909.06676[4] G.G. Manahan .. Hidding, B., Single-stage plasma-based correlated energy spread compensation for ultrahigh 6D brightness electron beams, Nat. Communications 8, 15705 (2017)[5] A. Deng .. Hidding, B., Electron bunch generation from a plasma photocathode, Nat. Physics (2019)