Phased array imaging of two dimensional Doppler microwave backscattering from spherical tokamak edge plasmas

Doppler backscattering (DBS) in 1D is an established and powerful fusion plasma diagnostic technique. In this thesis we explore the capability of the novel Synthetic Aperture Microwave Imaging diagnostic (SAMI) in performing proof-of-principle 2D DBS experiments on the Mega Ampere Spherical Tokamak (MAST) and the National Spherical Torus eXperiment Upgrade (NSTX-U). Phenomena observed previously using 1D DBS systems such as intrinsic plasma spin up, momentum injection from neutral beams and sharp changes in power and turbulence velocity coinciding with the L-H transition are re-observed. In addition, SAMI’s unique 2D DBS capability has enabled the first ever 2D maps of Doppler backscattered radiation to be constructed. These 2D maps reveal that, due to turbulence elongated along field lines, Doppler backscattered power is concentrated in directions perpendicular to the magnetic field. This distribution of backscattered power allows magnetic pitch angle to be measured. Results from the utilisation of this technique are presented using MAST and NSTX-U data. This procedure constitutes a new independent channel for diagnosing magnetic pitch angle and is the first case of pitch angle being measured using a microwave diagnostic. A method utilising microwave diagnostics is of particular interest as this presents the possibility of high temporal and spatial magnetic pitch measurements enabling, through application of Amp`ere’s law, measurement of edge current density: an important parameter in governing pedestal stability. The new capabilities and limitations resulting from implementation of a 2D DBS phased array system are discussed. How such a 2D device might be further optimised is examined and areas of further study are proposed.