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

Abstract

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.

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

“…Hence, we expect the total hydrogenic radiative losses to peak near the ionisation region and move upstream with the ionisation source. As the ionisation source and hydrogenic radiative losses move out of the divertor chamber, the imaging X-point bolometry system-the IRVB [36]-detects an increase in the total radiative losses above the divertor entrance.…”

“…High-n n ⩾ 9 Balmer line measurements from a deeply detached L-mode discharge (# 45370) are used in section 5.1. The magnetic geometries are shown in figure 2, together with the spatial coverage of the DMS and imaging bolometer (IRVB [36], which monitors the X-point region) diagnostics. 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].…”

“…The magnetic geometries are shown in figure 2, together with the spatial coverage of the DMS and imaging bolometer (IRVB [36], which monitors the X-point region) diagnostics. 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].…”

“…Although the IRVB [36] in the first MAST-U campaign could not reconstruct the 2D radiative emissivity profile in the divertor chamber, it can provide an estimate of the integrated radiation in the divertor chamber below the baffle entrance [43]. The IRVB facilitates a direct comparison against the divertor chamber integrated hydrogenic (DMS analysis) radiative losses (figure 7).…”

Investigations of atomic and molecular processes of NBI-heated discharges in the MAST Upgrade Super-X divertor with implications for reactors

“…(a) Radiative power losses integrated downstream the baffle entrance for # 46860. Black-total radiation (imaging bolometer-IRVB[40] within the shaded region of figure3(c)), from repeat discharge # 47958. Total atomic hydrogenic radiation ('Hydro'-magenta) consisting of radiation from electron impact excitation ('EIE'-red), electron-ion recombination ('EIR'-blue) and plasma-molecular interactions ('PMI'-green), inferred from spectroscopic analysis.…”