Demonstration of Neutral Pressure Sensitivity in an Interferometric Fiber Optic Temperature Sensor

An interferometric fiber optic temperature sensor (FOTS) was demonstrated to have neutral pressure sensitivity in various vacuum regimes. The sensor was based on a fiber optic Fabry–Perot interferometer that showed negligible susceptibility to electromagnetic interference (EMI) in a fusion environment. The compact design ( $600~\mu \text{m}$ diameter and $154~\mu \text{m}$ height) of the FOTS will be beneficial for flexible implementation in a tokamak environment. Significant changes in the thermal responsivity and response time of the sensor from different vacuum pressures were the basis for vacuum pressure measurements. Three different methods were demonstrated from 760 to 1.8E−5 Torr. The constant-heating method utilized the constant sensor heating from the signal carrying light. The main sensitivity for the method was in the 1E−3–1E−1 range. The measured temperature with the constant heating drastically increased from ~10 to 1.5E−3 Torr due to the increased responsivity. The pressure measurement at this range showed an accuracy of 5.1%. At high vacuum (below 1.5E−3 Torr), the pressure measurement had an accuracy of 20.7% following a different trend. The method required ~30-s wait time for temperature saturation. A square wave and pulse heating of 1-Hz cycle from an external laser were used to avoid the wait time. The measured amplitude of the square wave differed at different vacuum pressures due to the varying responsivity. The square-wave signal method was operable for low vacuum (from 760 to ~10 Torr) and medium vacuum (from ~10 to 1.5E−2 Torr) regimes with different trends and accuracies of < 20%. The pulse signal method utilized the transition in the measured temperature to additionally avoid sensitivity to heating power. Significant changes in the decay constant from a pulse temperature decay, which correspond to the changes in the response time, were distinguished above the signal variation at the medium vacuum regime (from ~10 to 1.5E−2 Torr) with an accuracy of 5.4%. The operating range of the FOTS could complement the ASDEX type or Penning gauges deployed in the divertor region where the neutral pressure could increase drastically. In addition, the FOTS does not have the disadvantage of capacitance manometer-type gauges that need to be further away from the divertor region.