New Techniques for Coherence Imaging Fusion Plasmas

Imaging diagnostic techniques are desirable for fusion plasma experiments for their wide coverage and high spatial resolution, which allows for a more complete comparison with the predictions made by plasma physics models than traditional techniques. Benchmarking models against measurements made on current experiments improves our understanding of the physics and reduces the uncertainties involved with designing future experiments and reactors. This thesis presents new techniques for coherence imaging (CI), an interferometric narrowband spectral imaging technique used to measure the brightness, shift and width of spectral lines emitted by the plasma in the visible range. From these measurements, 2-D maps of emitting species flow velocity, and temperature can be inferred via Doppler shifts and broadening respectively. For passive hydrogen Balmer series emission in the tokamak divertor, Stark broadening is strong enough to provide a 2-D map of electron density $n_e$. First, we introduce novel CI instrument designs based on pixelated phase-mask (PPM) interferometry, which improve spatial resolution and robustness over typical linear carrier designs. Secondly, we introduce a new method for absolute calibration of CI flow velocity measurements using emission lines from standard gas-discharge lamps instead of a tuneable laser. This method significantly reduces hardware costs while maintaining high measurement accuracy — ±1 km/s compared to typical ion flows in the tokamak plasma edge of < 30 km/s. Lastly, we present improved methods for CI measurement of ne, using modern lineshape models to improve accuracy and using a multi-delay PPM-CI instrument design to minimise errors caused by Doppler broadening, extending the valid measurement range to lower $n_e$. This is demonstrated with experimental measurements of H$_\gamma$ and H$_\delta$ emission on the Magnum-PSI linear plasma experiment with a direct comparison to Thomson scattering measurements