Exploratory study of PWFA-driven FEL at CLARA

This project "Exploratory Study of PWFA-FEL at CLARA" will develop electron-beam driven plasma wakefield accelerator (PWFA) based free-electron-lasers (FEL). FEL's are advanced coherent light sources which allow imaging of ultrasmall structures and ultrafast processes. These 'engines of discovery', today driven exclusively by linac-generated electron beams, are a major part of the STFC strategy https://stfc.ukri.org/files/fel-report-2016/ . Plasma-based accelerators, either driven by laser pulses in laser wakefield accelerators (LWFA) or driven by electron pulses in plasma wakefield accelerators (PWFA), are alternative acceleration methods known to produce short and intense, high energy electron beams. They are therefore increasingly considered as drivers for FEL's in future compact systems. However, a well-known substantial challenge of plasma accelerators is the production of electron beams with sufficiently low emittance and energy spread in order to achieve lasing in an undulator setup. Recent conceptual breakthroughs by the Strathclyde/UCLA-led "E210: Trojan Horse PWFA" collaboration and first experimental evidence obtained at SLAC FACET suggests that PWFA can produce electron beams with emittance and energy spread as well as brightness many orders of magnitude better than state-of-the-art. This would not only allow plasma accelerator electron output to meet the strict requirements to achieve lasing in an FEL, but also to exceed the state-of-the art of the best linac-based machines such as LCLS or European XFEL by many orders of magnitude as regards emittance as well as 5D and 6D brightness. Such beams could have transformative impact, as they may allow the realization of compact and cost-effective FEL's with a performance exceeding those of existing hard x-ray FEL's such as LCLS or the EU XFEL, including novel capabilities such as coherent attosecond-scale, high brilliance, single-spike hard X-rays pulses. We aim to develop this technology as a next-generation photon source for the time after current and upcoming linac-driven FEL's.

The development, and eventually realization of plasma-(X)-FEL's with performance similar to cutting edge 1 billion-pound facilities has transformative impact on both capacities and capabilities across a wide range of natural, material and life sciences.
We see three main beneficiaries:
1) Light source facility providers
With the first generation of FEL light sources now on-line for users, there is significant research being conducted, towards improving their output qualities and reducing their footprint to enable their wider use. Those facilities able to provide such improved output will inevitably attract the high research impact users who are able to utilise the output for world-first experiments. The opportunity exists for the UK to extend beyond the first generation FEL facilities and invest in the research and design of a facility with the reduced costs that a plasma accelerator driven FEL may offer. The combination of new methods, together with the advanced FEL modelling capability detailed in this proposal, can direct and inform the underpinning experiments and later designs that would enable the UK to build such a second generation FEL user facility.
2) Users of light sources
The current users of short wavelength FEL facilities are multifarious. The research proposed here would help enable X-ray FEL source development to address issues such as creation and study of properties of warm dense matter), understanding the Physics of Life such as 3D imaging of in-vivo biological samples at the atomic and molecular scales and several material science such as understanding magnetisation at the electronic level; energy and underlying dynamical processes in inertial confinement fusion, healthcare technologies such as understanding functional processes at cell membranes leads to basis for the development of new medicines, manufacturing processes of the future such as improved silicon wafer lithography and catalysis. The FEL is an instrument that enables the exploration of matter and its functioning, at a ubiquitous fundamental level that has previously been inaccessible. This ability has a significant potential impact in the diverse research infrastructure of university-scale facilities. Plasma accelerator driven FELs offer this potential within the UK. The research proposed here will greatly enhance this prospect. Due to the ultrahigh gain and short bunch length of pulses form the plasma photocathode, even sub-femtosecond coherent x-ray single spike pulses which may allow to investigate e.g. single electron notion on natural timescales may become possible.
A plasma accelerator driven FEL in an industrial context will be a significantly different beast from that in a research environment. Their design and size requirements will require exhaustive simulation studies that examine the statistical nature of the output due e.g. to plant fluctuations. Some initial conventional RF-linac driven FEL design studies for a commercially confidential collaboration with a large industrial company with a multi-billion pound turnover has been conducted to investigate such issues. It would be prudent to enable similar studies with the smaller scale plasma driven FEL.
3) Other beneficiaries
Given the broad range of science that FELs cover, the potential benefits to wider society can be significant. Examples of potential societal and economic impact range from energy generation by internal confinement fusion to the development of new pharmaceuticals. In 10-50 years, new manufacturing methods, products, pharmaceuticals and the way in which some aspects of society functions, will undoubtedly be able to be traced back to discoveries made as a result of research conducted using short-wavelength Free Electron Lasers. This impact has the potential to be accelerated by the use of smaller plasma accelerator driven FEL.