Commissioning of the Synthetic Aperture Microwave Imager-2 (SAMI-2) diagnostic system on MAST-U for edge magnetic pitch angle measurements*

The SAMI-2 diagnostic is a synthetic aperture microwave imaging diagnostic using phased array technology to probe the edge plasma of the MAST-U tokamak based at Culham in the UK. SAMI-2 is currently in a commissioning phase, with the project aims being to create a reliable working diagnostic package, producing magnetic pitch angle measurements at the edge of the MAST-U plasma.

SAMI-2 uses a monostatic antenna array and measures the doppler shift of back-scattered radiation from the edge of the plasma. Turbulence elongated along the magnetic field lines causes the back-scattering, and as the plasma is rotating, there will be separate doppler red and blue-shifted regions. The bisection of these two regions is orthogonal to the magnetic field lines and will produce pitch angle measurements. It may be possible to use these pitch angle measurements to infer the edge current density, making SAMI-2 one of the first reliable diagnostics for edge current density measurements. This would help for defining the parameter regimes that lead to ELM events, and may provide groundwork for future control systems.

SAMI-2 uses a phased array that does not include optical focusing components. Phased array systems are used in the military for radar applications and would provide a unique solution to maintenance issues of optical components undergoing neutron irradiation. Work will be presented on the calibration methods of the diagnostic including the phased antenna array and potential initial results from the MAST-U experiment.