The First Tokamak Plasma Testing of Single-Channel Fiber Optic Bolometers at DIII-D *

A novel bolometer utilizing fiber-optic based interferometry to measure radiated power was successfully tested on a tokamak for the first time at DIII-D. The fiber optic bolometer (FOB) avoids electromagnetic interference (EMI) using a Fabrey-Pérot resonator system to encode small temperature changes related to the incoming power. Off-line impulse-response calibration using a laser source is used to characterize the frequency response of the FOB and provides a means of solving for the radiated power by deconvolution. The FOB response during operations at DIII-D was compared with off-line results and DIII-D resistive bolometers. The FOB showed no increase in noise during operations (0.29mK) compared to the benchtop result (0.30mK) showing that FOBs do not suffer from EMI in a fusion environment. The absolute values of plasma brightness from the FOB matched the values calculated from the resistive bolometers in time through plasma discharges. The brightness was compared to the average brightness of two resistive bolometers with a similar path length and the back-calculated brightness from the tomographic reconstruction of resistive bolometers.