2D measurements of plasma electron density using coherence imaging with a pixelated phase mask

In this paper, the pixelated phase mask (PPM) method of interferometry is applied to coherence imaging (CI)-a passive, narrowband spectral imaging technique for diagnosing the edge and divertor regions of fusion plasma experiments. Compared to previous CI designs that use a linear phase mask, the PPM method allows for a higher possible spatial resolution. The PPM method is also observed to give a higher instrument contrast (analogous to a more narrow spectrometer instrument function). A single-delay PPM instrument is introduced as well as a multi-delay system that uses a combination of both pixelated and linear phase masks to encode the coherence of the observed radiation at four different interferometer delays simultaneously. The new methods are demonstrated with measurements of electron density n_e, via Stark broadening of the H_γ emission line at 434.0 nm, made on the Magnum-PSI linear plasma experiment. A comparison of the Abel-inverted multi-delay CI measurements with Thomson scattering shows agreement across the 3 × 10¹⁹ < n_e < 1 × 10²¹ m^-3 range. For the single-delay CI results, agreement is found for n_e > 1 × 10²⁰ m^-3 only. Accurate and independent interpretation of single-delay CI data at lower n_e was not possible due to Doppler broadening and continuum emission.