Coherence imaging spectroscopy with a polarization-sensitive sensor to visualize the plasma flows in fusion devices

Plasma flows play a fundamental role in many aspects of the power and particle exhaust of fusion devices, such as impurity transport, helium exhaust, detachment processes, and divertor in-out asymmetries. Coherence imaging spectroscopy (CIS) is a camera-based interferometer technique that can obtain 2D images of line-of-sight particle flows from the Doppler shift in their line emission. In this paper, we describe three improvements to the traditional spatial heterodyne CIS setup, aiming to enhance both the velocity precision and spatial resolution. The analyzing linear polarizer was replaced with a polarization-sensitive sensor, providing a higher spatial resolution and eliminating Fourier demodulation ambiguities. A field-widened delay reduced the gradient in the delay across the image to improve signal demodulation. Finally, Yittrium Orthovanadate (YVO4) instead of alpha Barium Borate (α-BBO) birefringent crystals were used, reducing the temperature sensitivity of the system. Excellent agreement was found between multi-chord high-resolution spectroscopy and CIS from observations of the linear plasma in Upgraded Pilot-PSI, with CIS showing a higher velocity precision (+-0.2 km s-1) and spatial resolution (0.1 mm, 1232x1028 resolution). Demonstration cases of CIS include the near-surface plasma flows in the linear plasma generator Magnum-PSI, and the divertor plasma flows in the TCV tokamak. The H and Ar+ flows in Magnum-PSI illustrate collisional coupling between the main plasma ion species and the impurities within the pre-sheath. TCV observations indicate strong gradients in the velocity profile across the last-closed flux surface.