Recent Progress on Radial Fiber Optic Bolometer Array at DIII-D

A radial fiber-optic bolometer (FOB) array is operational at DIII-D for measuring plasma radiated power near the X-point and divertor regions. The FOB demonstrates performance comparable to resistive bolometers while avoiding electromagnetic (EM) interference by employing a Fabry-Pérot resonator to detect small temperature variations associated with incident power. The array, comprising 20 sensors in two groups of 10, is installed with a radial view into the DIII-D tokamak. Operating at 1 kHz, FOB signals correlate well with D-alpha bursts observed during ELMs via filter scope diagnostics acquiring at 100 kHz. Time traces show agreement with resistive bolometers along similar sightlines. Absolute power comparisons, performed via forward modeling using tomographic reconstructions from resistive bolometers, yielded inconclusive results due to inconsistencies and variability in the reconstructions. Post-bake testing revealed altered thermal time constants in some channels, attributed to sensor displacement (~tens of µm) relative to the thermal sink. Several approaches are currently being investigated to mitigate the issue to continue development with the aim of deploying the FOB on future devices.