Design and implementation of a prototype infrared video bolometer (IRVB) in MAST Upgrade.

A prototype infrared video bolometer (IRVB) was successfully deployed in the Mega Ampere Spherical Tokamak Upgrade (MAST Upgrade or MAST-U), the first deployment of such a diagnostic in a spherical tokamak. The IRVB was designed to study the radiation around the lower x-point, another first in tokamaks, and has the potential to estimate emissivity profiles with spatial resolution beyond what is achievable with resistive bolometry. The system was fully characterized prior to installation on MAST-U, and the results are summarized here. After installation, it was verified that the actual measurement geometry in the tokamak qualitatively matches the design; this is a particularly difficult process for bolometers and was done using specific features of the plasma itself. The installed IRVB measurements are consistent both with observations from other diagnostics, including magnetic reconstruction, visible light cameras, and resistive bolometry, as well as with the IRVB-designed view. Early results show that with conventional divertor geometry and only intrinsic impurities (for example, C and He), the progression of radiative detachment follows a similar path to that observed for large aspect ratio tokamaks: The peak of the radiation moves along the separatrix from the targets to the x-point and high-field side midplane with a toroidally symmetric structure that can eventually lead to strong effects on the core plasma inside the separatrix.

Wavelength calibration of birefringent interferometers for 2-D measurement of plasma flow.

Imaging birefringent interferometers are used to measure plasma flow in 2-D via the Doppler shift of a spectral emission line. Applications include plasma physics study in fusion energy experiments and in the Earth's upper atmosphere. We present a new, to the best of our knowledge, method for wavelength calibration that does not require measurement at the rest wavelength of the targeted spectral line, nor measurement using a tuneable laser source. This is useful when such light sources are not available. Fringes measured at known wavelengths from the emission lines of gas-discharge lamps are used to constrain an instrument model which can generate the required calibration data. In the process, optical path difference, dispersion and misalignments are characterized. The "2π ambiguity" of interferogram phase data is handled using circular statistics, allowing the wavelength span of the calibration lines to far exceed the unambiguous measurement range of the instrument. The technique is demonstrated to an accuracy of ±1 pm (±0.7 km/s flow-equivalent) over a 40 nm visible wavelength range.