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

Abstract

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 ne, via Stark broadening of the Hγ 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 < ne < 1 × 1021 m−3 range. For the single-delay CI results, agreement is found for ne > 1 × 1020 m−3 only. Accurate and independent interpretation of single-delay CI data at lower ne was not possible due to Doppler broadening and continuum emission.

The role of plasma–atom and molecule interactions on power & particle balance during detachment on the MAST Upgrade Super-X divertor

“…The region in which the IRVB data can be inverted to obtain 2D distributions of the radiation is reduced with respect to the absolute coverage, as there has to be a sufficient number of lines of sight (corresponding to each pixel) intersecting the poloidal plane at each location (figure 2) [36]. A Multi-Wavelength Imaging (MWI) [32,37] diagnostic, similar to MANTIS at TCV [38], is used to monitor the lower divertor, which has also one channel dedicated to Coherence Imaging Spectroscopy [39,40].…”

2D coherence imaging measurements of C2+ ion temperatures in the divertor of Wendelstein 7-X

“…This is presently being investigated within our group for the next experimental campaign. New results ( [15]), published very recently, propose the use of a pixelated phase mask method of interferometry, which could be helpful in this regard. If the effect of the Zeeman contrast could be sufficiently reduced across the given field of view, a single estimate of both B and θ might become acceptable for evaluating the Zeeman contrast of the entire CIS image.…”

“…N is the total group delay induced by the birefringent plates. In case the entire observed spectrum solely consists of spectral lines of equal shape, one can derive plasma parameters like T i from Doppler broadening [13] or n e from Stark broadening [14,15]. This has been successfully achieved with CIS for atomic lines, which were completely dominated by either Doppler or Stark broadening only.…”

Characterisation of detachment in the MAST-U Super-X divertor using multi-wavelength imaging of 2D atomic and molecular emission processes

“…The final MWI channel is reserved for a coherence imaging cell that should allow the MWI to measure plasma flow or electron density directly. Coherence imaging will not be addressed here, but further details are reported in a dedicated publication [29].…”

“…One promising diagnostic to constrain electron density is the coherence imaging cell of the MWI itself [29], which will be explored in upcoming campaigns. 2D electron temperature and density constraints can also be derived from helium imaging.…”