Laser-based selective preionization of plasma wakefield accelerator stages

粒子束驱动的等离子体尾迹加速(PWFA)是近年来加速器领域研究的热点。除了SLAC这样的大型加速器中心，斯特拉斯克莱德大学的SCAPA中心或LMU慕尼黑的CALA中心等激光等离子体加速器也可以使用来自激光等离子体加速器(LWFA)的电子束作为PWFA阶段的驱动程序[1，2，3]。产生宽的预电离等离子体通道作为PWFA的介质，是产生高能高质量电子束的关键任务[4，5]。另一个关键特征是只电离多组分气体等离子体中的一个组分，使得电离的组分可用于实现等离子体光电阴极。这是基于电子驱动的等离子体尾迹场加速不需要过多的峰值电驱动场来激发强等离子体波的特点，因为它是单极的电驱动束场分布。由于其振荡的电场结构，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个数量级。这一特征允许通过利用显著不同的隧道电离阈值的物种，如氢和氦，将尾迹激发与电子束注入解耦。因此，可以利用峰值电场局部超过高电离阈值介质的激光脉冲，以受控的方式在任意时空位置释放和注入电子。这样做的主要吸引力在于可以实现等离子体阴极，它允许以极低的所谓电子束发射度控制和高度可调的电子布居注入，因此具有超高亮度，比最先进的水平高出许多个数量级。这种能力可能会对相干和非相干光子科学源、高场和高能物理产生变革。这又意味着需要对低电离阈值分量和高电离阈值分量进行选择性电离。这包括低电离阈值分量在宽和长区域内均匀的预电离，等离子体上斜坡和下斜坡的整形，以及用于等离子体光电阴极注入的高电离阈值分量的局部非常受限的电离。这是本博士的核心研发主题，并暗示了两个主要目标：-基于选择性激光的预电离低电离阈值介质，如氢。这里的目标是一个长达一米，宽度可达一毫米的等离子体通道，没有会电离相关较高电离阈值介质的热点-基于激光的高电离阈值介质的局域隧穿电离，如传入电子和激光脉冲、等离子体介质和产生的电子脉冲的氦计量。[1]Hidding，B.Karsch，S.等人，在混合激光-等离子体尾场加速器中的单能倍增，物理。莱特牧师。10,195002(2010年)[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.吉尔约汉.B.隐藏..孙建国，张晓明，等.激光加速电子束驱动的小型等离子体加速器的研究进展.北京：北京：北京，2005(2)：343-843.超高6维亮度电子束的单级等离子体关联能散补偿，NAT。通讯8,15705(2017年)[5]A.邓..Hidding，B.，等离子体光电阴极的电子束产生，NAT。《物理》(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)