Experimental tests of an infrared video bolometer on Alcator C-Mod

Developing a robust sensor for infrared imaging bolometers

A new type of large area sensor for infrared imaging bolometers has been developed. It replaces the thin and fragile free-standing metal foils, which typically have been used, with a multi-layer coated sapphire (or diamond) substrate. Sapphire is transparent to mid-infrared wavelengths, is robust against transients, and can be thick enough to even be the vacuum window. The primary radiation absorber is still a thin deposited metal layer, but now it is partially insulated from the supporting sapphire substrate by a black (carbon-based) layer, which also acts as a blackbody remitter. Test results indicate 6× more noise equivalent power density (estimated NEPD = 23 W/m2 at 5 ms camera exposure time, foil temperature decay time 60 ms) for a 2 μm gold-coated sapphire disk compared to estimated NEP = 4 W/m2 at 1.8 ms exposure time, with foil decay time 420 ms, for a nominal 2.5 μm thick platinum-free-standing foil.

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.

Signal to noise ratio of upgraded imaging bolometer for KSTAR

An InfraRed imaging Video Bolometer (IRVB) was installed on KSTAR in 2012 having a ∼2 μm × 7 cm × 9 cm Pt foil blackened with graphite and a 5 mm × 5 mm aperture located 7.65 cm from the foil with 16 × 12 channels and a time resolution of 10 ms. The IR camera was an Indigo Phoenix (InSb, 320 × 256 pixels, 435 fps, <25 mK). In 2017, the IRVB was upgraded by replacing the IR camera with a FLIR SC7600 (InSb, 640 × 512 pixels, 105 fps, <25 mK). The aperture area was reduced by approximately half to 3.5 mm × 3.5 mm, and the number of channels was quadrupled to 32 × 24. A synthetic image derived using the projection matrix for the upgraded IRVB from a Scrape Off Layer Plasma Simulator (SOLPS) model with 146 kW of total radiated power had a maximum signal of 7.6 W/m² and a signal to noise ratio (SNR) of 11. Experimental data for a plasma with parameters similar to the SOLPS model (total radiated power of 158 kW) had a maximum signal of 12.6 W/m² and noise equivalent power density (SNR) of 0.9 W/m² (14).

Improvement of infrared imaging video bolometer for application to deuterium experiment on the large helical device

An infrared imaging video bolometer was improved for application to a neutron environment in fusion plasma devices, i.e., the Large Helical Device (LHD). In order to calibrate the thermal characteristics of the activated foil absorber inside the plasma vacuum vessel, the remote-controlled in situ calibration system was improved with high-surface-flatness mirrors. Furthermore, the carbon coating method was improved by introducing a vacuum evaporation technique instead of the conventional spray technique to realize the coating on both sides of the absorber with reproducibility and uniformity. The optimal thickness of the coating was also determined. Owing to these coating improvements, the reproducibility of the effective emissivity on both sides especially was improved. Finally, the variation with the neutron irradiation of the thermal characteristics of the foil absorber was investigated. It was found that the effect was not significant for the total neutron emission of 3.6 × 10¹⁸ on LHD.