Femtosecond-micrometer spatiotemporal alignment of particle and laser beams

阿拉斯泰尔·纳特将开发一种新的通用方法，用于粒子和激光的时空对准和同步。我们的战略合作伙伴之一斯坦福SLAC国家加速器实验室的高级加速器实验测试设施(FEET)的E210合作，通过多年的实验活动发现了潜在的原理。该项目将由我们在德国德累斯顿-罗森多夫海姆霍兹研究所(HZDR)的合作伙伴共同出资完成。HZDR不仅同意直接共同资助这个博士项目，而且还将为蓬勃发展的合作提供大量资源，在他们的PW级激光系统中使用光束时间，每年约10周，并可能额外投资于重要的SCAPA光束时间(SCAPA 350 TW激光器的日费为GB 3300)。这名学生将驻扎在斯特拉斯克莱德，但也将被安排在HZDR延长时间，以符合斯特拉斯克莱德CDT关于基于等离子体的粒子和光源http://ppals.phys.strath.ac.uk/的国际方面。强激光与电子束的相互作用发挥着越来越大的作用，例如在各种泵浦-探测实验和超快、超微小的成像中。这需要精确的技术来测量和控制相互作用的时间和空间重叠。我们已经发现了一种基本效应，它允许在一个强大的设备中实现飞秒和微米级精度的强激光和电子束的等离子体-光子同步和对准。激光产生的冷等离子体灯丝不同程度地拾取了瞬变电子束的电场，种子等离子体电子通过这种耦合被加热，并产生复杂的振荡，延伸到环境气体中。在这里，它们的非相对论能量正好可以通过碰撞电离产生大量额外的等离子体。这就像一个‘放大镜’，将特定的飞秒-微米相互作用信号转换为可在微秒毫米尺度上观察到的可见等离子体复合/去激发光。我们已经利用这一基本效应演示了SLAC Stanford 20GeV电子束与聚焦的钛宝石激光器的时空同步和对准，其精度为飞秒-微米。这种方法使先进的诊断学成为可能，这将大大改善泵浦-探测实验的广泛范围，并允许实现迄今不可行的先进的时空分辨实验。这些现象也揭示了碰撞电离以前隐藏的重要性，例如在先进的等离子体加速器实验中。支持粒子在细胞中的模拟揭示了在广泛的时间和长度尺度上令人着迷的动力学：电子束的初始飞秒尺度的冲击通过ps尺度上的等离子体密度波和相应的GV/m尺度的场沿着细小的种子细丝传播，然后碰撞电离和复合过程在ns-us尺度上发生。在斯坦福大学，我们一直在使用这种效应的基本版本，使用在传统的直线加速器中产生的电子束。阿拉斯泰尔的博士工作将进一步发展这种时空对准的方法，这样当斯坦福大学的后续等离子体尾迹加速器设施Facet-II将于2019年上线进行用户辅助调试时，这种方法将以先进的形式提供。然而，这种效应具有更广泛的适用性，可以用于在SCAPA或HZDR等激光等离子体加速器设施中实现先进的等离子体尾场加速。为此，电子束不是由传统的加速器产生的，而是由激光等离子体加速器产生的。

Alastair Nutter will be developing a novel versatile method for spatiotemporal alignment and synchronization of particle and laser beams. The underlying principle was discovered as a key result of a multi-year experimental campaign obtained by the E210 collaboration at the Facility for Advanced Accelerator Experimental Tests (FACET) at the SLAC National Accelerator Laboratory at Stanford, one of our strategic partners. The project will be done co-funded by our partner at Helmholtz-Institute Dresden-Rossendorf (HZDR) in Germany. HZDR did not only agree to co-fund this PhD project directly, but will also be providing substantial resources into the burgeoning collaboration in terms of using beamtime at their PW-class laser system to the tune of ~10 weeks per year, and may be additionally investing into significant SCAPA beamtime (daily fees for the 350 TW laser at SCAPA are £ 3300). The student will be based at Strathclyde, but will also be placed at HZDR for extended times, in line with the international aspect of the Strathclyde CDT on Plasma-based Particle and Light Sources http://ppals.phys.strath.ac.uk/ . The coordinated interaction of intense laser and electron beams plays an ever-increasing role, for example for various pump-probe experiments and imaging of the ultrafast and ultrasmall. This requires precise techniques to measure and control temporal and spatial overlap of the interaction. We have discovered a fundamental effect which permits plasma-photonic synchronization and alignment of intense laser and electron beams with femtosecond and micrometer precision in a single robust apparatus. A laser-generated cold plasma filament picks up the electric field of a transient electron beam to varying degree, seed plasma electrons are heated by this coupling and perform complex oscillations which extend into ambient gas. Here, their non-relativistic energies are just right to generate substantial amount of additional plasma via impact ionization. This acts as a 'magnifying glass', transforming the specific femtosecond-micrometer interaction signature into visible plasma recombination / de-excitation light, which is observable on microsecond-millimeter scales.We have already exploited this basic effect to demonstrate combined spatiotemporal synchronization and alignment of the SLAC Stanford 20 GeV electron beam with a focused Ti:Sapphire laser with femtosecond-micrometer accuracy. This approach enables advanced diagnostics which will improve a wide range of pump-probe experiments substantially, and allows to realise advanced spatiotemporally resolved experiments which are hitherto not feasible. The phenomena also shed light on the previously hidden importance of impact ionization for instance in advanced plasma accelerator experiments. Supporting particle-in-cell simulations reveal fascinating dynamics over a wide range of time and length scales: The initial fs-scale kick by the electron beam is spread along the thin seed filament by plasma density waves and corresponding GV/m scale fields on the ps-scale, and impact ionization and recombination processes then take place on the ns-us time scale.In Stanford, we have been using a basic version of this effect, using an electron beam generated in a conventional linear accelerator. Alastair's PhD work will develop this method of spatiotemporal alignment further, such that it will be available in advanced shape when FACET-II, the follow-up plasma wakefield accelerator facility in Stanford will come online in 2019 for user-assisted commissioning. However, the effect has even wider applicability and can be used to enable advanced plasma wakefield acceleration at laser-plasma accelerator facilities such as SCAPA or HZDR. For this, the electron beam is generated not by a conventional accelerator, but by a laser-plasma-accelerator.