Generating coherent light from laser-accelerator electrons

Novel research points to developing table-top x-ray sources for future applications ​

​A new way of producing coherent light in the ultra-violet spectral region, which points the way to developing brilliant table-top x-ray sources, has been produced in research led by researchers at the Cockcroft Institute and with ASTeC staff from STFC Daresbury Laboratory.

In a new open-access paper available today in Nature Scientific Reports, predictions are made that intense, extremely short-wavelength ultraviolet light can be obtained by direct emission from ultra-short electron bunches driven by laser wakefield acceleration (LWFA).

Undulator radiation obtained using GPT for a LWFA modelled using the particle-in-cell code FBPIC. (a) Electric field generated for 110 MeV mean energy and (b) 130 MeV mean energy. (c) Spatial distribution generated for 110 MeV mean energy and (d) 130 MeV mean energy.

Coherent light sources are powerful tools that enable research in many areas of medicine, biology, material sciences, chemistry and physics. An LWFA-based coherent source can be highly compact, fitting into a single laboratory room and enabling ‘table-top’ research using femtosecond pulses of light.

A proof-of-principle experiment is planned for autumn at the SCAPA beamline at Strathclyde University to characterise the performance of such sources, pointing the way to generation of X-rays in a similar way.

Professor Dino Jaroszynski, of Strathclyde’s Department of Physics, led the research. He said, “This work significantly advances the state-of-the-art of synchrotron sources by proposing a new method of producing short-wavelength coherent radiation, using a short undulator and attosecond duration electron bunches.

This is more compact and less demanding on the electron beam quality than free-electron lasers and could provide a paradigm shift in light sources, which would stimulate a new direction of research. It proposes to use bunch compression – as in chirped pulse amplification lasers – within the undulator to significantly enhance the radiation brightness. The new method presented would be of wide interest to a diverse community developing and using light sources.

Electrons passing through the undulator radiate incoherent light if the electron are far apart from each other. By generating them carefully in a short bunch around 1 femtosecond in length, the electrons emit light as if they were a single ‘big’ electron. The emitted intensity is then proportional to the square of the number of electrons rather than being linear as it is for incoherent light. This doesn’t sound important, but even a tiny bunch with 1 picocoulomb of charge contains a billion electrons; that tiny bunch if short enough therefore has its emission boosted by a billion times. Generating and controlling bunches to have short durations therefore allows compact accelerators to make intense light pulses for researchers.

The research collaboration involved the Universities of Strathclyde and Manchester, Pulsar Physics in the Netherlands, and ASTeC at STFC Daresbury Laboratory. The study benefited from EPSRC funding to support the wider Lab in a Bubble project, and from STFC.

The new paper can be read on the Nature Scientific Reports website: