Design of the Synthetic Aperture Microwave Imager-2 for measurement of the edge current density on MAST-U

The Synthetic Aperture Microwave Imager-2 (SAMI-2) is a 2D Doppler backscattering (DBS) diagnostic designed for multiple high quality simultaneous measurements of the edge pitch angle on the Mega-Ampere Spherical Tokamak Upgrade (MAST-U). The specification, design and testing of the SAMI-2 microwave front end, predominantly antennas and mixer circuitry, are described in this thesis. Dual-polarisation sinuous antennas are designed and shown to fulfil their performance criteria over the 20-40 GHz range. A four-channel microwave in-phase and quadrature (IQ) downconverter is designed to mix the radio frequency (RF) signals from each antenna in the SAMI-2 phased array to a lower frequency, for digitisation. The down-converter is tested and found to achieve its performance specifications over the entire frequency range of the antennas.
The 2D DBS technique was demonstrated by Synthetic Aperture Microwave Imager (SAMI) on the Mega-Ampere Spherical Tokamak (MAST) to measure the edge magnetic pitch angle. A radial profile of the edge pitch angle enables calculation of the edge current density, a difficult quantity to measure, which is valuable for the validation of models and understanding of pedestal dynamics and edge plasma instabilities, e.g. ELMs. In active probing mode, the SAMI-2 diagnostic is designed to make the first measurements of the edge current density by a DBS diagnostic. In passive mode, SAMI-2 will measure Bernstein wave mode conversion, to inform spherical tokamak microwave heating systems.