Laser-driven creation of the next-generation ultra-bright gamma-ray sources: a case study

The quest for a compact source of high-energy and high-brilliance gamma-ray beams is motivated by their paramount importance for a wide range of practical applications, which include cancer treatment, photo-fission, high-energy radiography, and active interrogation of materials. For all these applications, a high energy per photon is required: as possible examples, cancer treatment demands for a photon energy in the multi-MeV range, whereas the Giant Dipole Resonance occurs for most materials in an energy window of 15 to 30 MeV and can lead to disintegration of the nucleus. By detecting the by-products, a clear recognition of the material under study can be obtained, a technique usually known as active interrogation of materials, which is actively pursued for its dramatic potential impact for military as well as for home-land security applications. Traditionally, high-quality gamma-ray and hard X-ray beams have been generated exploiting either bremsstrahlung radiation, or undulation along a series of alternated magnets of a high-energy electron beam. The size and cost of these machines is however calling for more compact schemes and laser-driven sources have been recently proposed as a viable alternative. Within this framework, encouraging results have been obtained using betatron radiation, whereby ultra-bright and coherent x-ray sources have been reported; however their relatively low-energy per photon (sub-MeV), rules out their use for the aforementioned applications. Higher energies have been reported using bremsstrahlung sources but the comparatively big source size (hundreds of microns) and divergence (tens of mrad) does not allow for high-brightness to be achieved. A mechanism that might ensure both high-energy and high-brightness is theoretically predicted to be Thomson scattering of an ultra-relativistic electron beam in an intense electromagnetic field. The experimental challenges associated with this scheme include micron-scale spatial overlap and femtosecond-scale synchronisation of the electron beam with the laser, spectral detection of the generated gamma-ray beam, and shot-to-shot reproducibility of the main parameters of the laser and electron beam. Even though proof-of-principles experimental campaigns have already demonstrated the feasibility of using this technique for the generation of high-quality gamma-ray beams, these technical issues are thus far preventing a systematic application and commercialisation of this scheme. The Centre for Plasma Physics at the Queen’s University of Belfast has a long-lasting world-wide expertise on high-intensity laser-matter interactions and the experimental group lead by Dr. G. Sarri has recently experimentally obtained the highest brightness (>1019 photons/s1/mm2/mrad2 x0.1% BW) of multi-MeV laser-driven photon beams. The successful student will join this vibrant group and carry out theoretical studies aimed at identifying the best laser and electron beam parameters for a stable and reproducible source of high-brightness and multi-MeV gamma ray beams. The results will be tested in experimental campaign and, if successful, contribute towards commercialisation of this technique for medical as well as home-land security applications. Project supervisor details: Dr. G. Sarri Room G050, Centre for Plasma Physics, The Queen’s University of Belfast Telephone: 02890973575 E-mail: g.sarri@qub.ac.uk Website: http://pure.qub.ac.uk/portal/en/persons/gianluca-sarri(a0bcd35b-798b-41fe-a6e5-3f1a54cc05e2).html