Towards compact Gamma-ray Free Electron Lasers

ver the past few decades, X-ray free-electron lasers (XFELs) have become the pre-eminent tool for studying the structure and dynamics of materials at the molecular scale. Their bright, coherent, ultra-short X-ray pulses are ideal for imaging and manipulating dynamical molecular processes, which has been vital, for example, in the development of new pharmaceuticals and novel materials. A source of coherent gamma-ray pulses would enable analogous processes on the nuclear scale, with potential applications in imaging, security, energy storage, and fundamental science. Unfortunately, the current theoretical and computational tools essential for modelling XFELs cannot describe emission of gamma rays, where recoil effects play a significant role.

One practical limitation of XFELs is their physical scale: with undulators on the order of 100 m, and kilometre-long linear accelerators, there is scope for only a small number of such facilities worldwide, which limits their availability to users. The use of alternative undulator technologies with periods on the scale of microns rather than centimetres, would permit the use of lower energy electrons (100 MeV rather than 10s GeV), and hence greatly reduce the footprint of these facilities. As well as increasing availability of XFELs, this would also open the prospect of producing coherent gamma rays, since the lower energy electrons could take advantage of “quantum purification”, which prevents recoil effects degrading the beam quality and suppressing the FEL process.

The aim of this project is to develop new theoretical and computational tools for modelling gamma-ray FELs, combining classical and quantum descriptions. These will be used to analyse two candidates for short-period undulators based on intense laser beams and periodically deformed crystals, to assess the feasibility of producing coherent gamma-ray pulses.

Proposed Scheme of Work

In the early stages, the student will develop simplified quantum models of FELs, generalising existing models to account for non-periodic effects of laser focussing, and the anharmonic interatomic fields of “bent” crystals. In parallel, they will use the classical Particle-in-Cell code Puffin to conduct detailed simulations of the same physical systems, albeit while neglecting the effects of quantised photon recoil.

Building on this foundation, the student will then develop new methods and routines to model quantum photon recoil effects, and incorporate these into existing PIC codes. These will be benchmarked against cases where recoil effects are negligible, and idealised cases described by simplified recoil models.

In the final stage, the student will apply both the simplified recoil models and the recoil-enabled PIC code to explore the generation of coherent gamma rays, and identify promising parameter ranges. Three scenarios will be considered: traditional magnetic undulators with centimetre-scale periods;

laser-based undulators with micron-scale periods; and crystal-based undulators, with periods ranging from millimetres down to microns.

From the beginning, the student will attend graduate-level lectures at the Cockcroft Institute and the Scottish Universities Physics Alliance, to ensure a thorough grounding in the necessary theory of particle accelerators, Free-Electron Lasers, and quantum optics. Throughout the project, the student will work closely with the supervisory team, as well as colleagues at STFC ASTeC, Lancaster University, and the University of Plymouth.