Laser Induced X-ray 'RADAR' Particle Physics Model

The technique of high-power laser-induced plasma acceleration can be used to generate a variety of diverse effects including the emission of X-rays, electrons, neutrons, protons and radio-frequency radiation. A compact variable source of this nature could support a wide range of potential applications including single-sided through-barrier imaging, cargo and vehicle screening, infrastructure inspection, oncology and structural failure analysis. This paper presents a verified particle physics simulation which replicates recent results from experiments conducted at the Central Laser Facility at Rutherford Appleton Laboratory (RAL), Didcot, UK. The RAL experiment demonstrated the generation of backscattered X-rays from test objects via the bremsstrahlung of an incident electron beam, the electron beam itself being produced by Laser Wakefield Acceleration. A key initial objective of the computer simulation was to inform the experimental planning phase on the predicted magnitude of the backscattered X-rays likely from the test objects. This objective was achieved and the computer simulation was used to show the viability of the proposed concept (Laser-induced X-ray 'RADAR'). At the more advanced stages of the experimental planning phase, the simulation was used to gain critical knowledge of where it would be technically feasible to locate key diagnostic equipment within the experiment. The experiment successfully demonstrated the concept of X-ray 'RADAR' imaging, achieved by using the accurate timing information of the backscattered X-rays relative to the ultra-short laser pulse used to generate the electron beam. By using fast response X-ray detectors it was possible to derive range information for the test objects being scanned. An X-ray radar 'image' (equivalent to a RADAR B-scan slice) was produced by combining individual X-ray temporal profiles collected at different points along a horizontal distance line scan. The same image formation process was used to generate images from the modelled data. The simulated images show good agreement with the experimental images both in terms of the temporal and spatial response of the backscattered X-rays. The computer model has also been used to simulate scanning over an area to generate a 3D image of the test objects scanned. Range gating was applied to the simulated 3D data to show how significant signal-to-noise ratio enhancements could be achieved to resulting 2D images when compared to conventional backscatter X-ray images. Further predictions have been made using the computer simulation including the energy distribution of the backscatter X-rays, as well as multi-path and scatter effects not measured in the experiment. Multi-path effects were shown to be the primary contributor to undesirable image artefacts observed in the simulated images. The computer simulation allowed the sources of these artefacts to be identified and highlighted the importance of mitigating these effects in the experiment. These predicted effects could be explored and verified through future experiments. Additionally the model has provided insight into potential performance limitations of the X-ray RADAR concept and informed on possible solutions. Further model developments will include simulating a more realistic electron beam energy distribution and incorporating representative detector characteristics.