Increased accuracy and signal-to-noise ratio through recent improvements in Infra-Red Video Bolometer fabrication and calibration

TL;DR

This paper presents recent improvements in Infra-Red Video Bolometer fabrication and calibration, leading to higher accuracy, better spatial resolution, and more uniform sensing elements for plasma diagnostics.

Contribution

It introduces a new multi-material foil design using Titanium and Platinum, enhancing signal strength and spatial resolution, along with a simplified calibration method for thermal property measurement.

Findings

Higher temperature signals (2-3x increase)

Improved spatial resolution (about 4x)

Enhanced foil uniformity and calibration accuracy

Abstract

The Infra-Red Video Bolometer (IRVB) is a diagnostic equipped with an infra-red camera that measures the total radiated power in thousands of LOSs within a large field of view (FOV). Recently validated in MAST-U, it offers a high spatial resolution map of the radiated power in the divertor region, where large gradients are expected. The IRVB's sensing element comprises a thin layer of high Z absorbing material, typically Platinum, usually coated with Carbon to reduce reflections. It is here explored the possibility of using a relatively inert material like Titanium, that can be produced in layers up to 1mum compared to 2.5mum for Pt, and then coat it with Pt of the desired thickness (0.3mum per side here) and Carbon. This leads to a higher temperature signal (2 to 3 times), and better spatial resolution (about 4 times), resulting in higher accuracy in the measured power. This assembly is also expected to improve foil uniformity, as the Pt layer is obtained via deposition rather than mechanical processes. Given its multi-material composition, measuring the thermal properties of the foil assembly is vital. Various methods using a calibrated laser as a heat source have been developed, analysing the temperature profile shape or fitting the calculated laser power for different intensities and frequencies. It is here presented a simpler approach, that relies on analysing the separate components of the foil heat equation for a single laser exposure in a given area. This can then be iterated over the entire foil to capture local deviations.