Femtosecond-micrometer spatiotemporal alignment of particle and laser beams

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.