Laser-based selective preionization of plasma wakefield accelerator stages

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 pulses
The 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)