Student Seminar - December 2019

Matthew Selwood

The application of coded apertures for high-energy high-resolution imaging requirements

Laser-plasma x-ray sources have garnered interest from various communities due to their ability to generate high photon-energies from a small source size. The passive imaging of high energy x-rays and neutrons is also a useful diagnostic in laser-driven fusion capsules as well as laboratory astrophysics experiments which aim to study small samples of transient electron-positron plasmas. 

This talk demonstrates a coded aperture with scatter and partial attenuation included, which we have dubbed a `CASPA', and compare them to the more common method of pinhole imaging. As well as discussing the well-known increased throughput of coded apertures, we also show that the decoding algorithm relaxes the need for a thick substrate. We simulate a 511 keV x-ray source through ray-tracing and Geant4 simulations to show how incomplete attenuation of the source by the mask may be less detrimental to imaging using a CASPA than when using a standard pinhole system. Finally, we suggest further ideas for relaxing common high aspect ratio imaging pitfalls and potential new approaches for the application of CASPA imaging systems. 

Harry Dudding

Improving the isotope scaling of quasilinear transport models

Quasilinear transport models are essential for simulating tokamak plasmas globally on confinement time scales. These models are comprised of reduced linear dynamical equations, and a ‘quasilinear rule’ – a black-box function that attempts to reconstruct the full nonlinear result from the linear physics. Quasilinear rules are by definition semi-empirical, and as such have been historically tuned to deuterium plasmas. This tuning, as well as their approximate nature, causes the quasilinear models to perform poorly when simulating tokamak plasmas of other isotopes, in many cases deviating strongly from the results calculated using nonlinear gyrokinetic models. With the coming of ITER, and the subsequent move to DT experiments, the issue of transport scaling with isotope in these quasilinear models needs to be resolved, to enable accurate predictive modelling for future reactors. By comparing the quasilinear model results with nonlinear gyrokinetic simulations, the root causes of the deviations can be found, and relevant improvements identified. The improved models will then be validated against the upcoming JET isotope experiments. 

Michel Osca Engelbrecht

TBC