title: Development of plasma bolometers using fiber-optic temperature sensors.
abstract: Measurements of radiated power in magnetically confined plasmas are important for exhaust studies in present experiments and expected to be a critical diagnostic for future fusion reactors. Resistive bolometer sensors have long been utilized in tokamaks and helical devices but suffer from electromagnetic interference (EMI). Results are shown from initial testing of a new bolometer concept based on fiber-optic temperature sensor technology. A small, 80 μm diameter, 200 μm long silicon pillar attached to the end of a single mode fiber-optic cable acts as a Fabry-Pérot cavity when broadband light, λo∼ 1550 nm, is transmitted along the fiber. Changes in temperature alter the optical path length of the cavity primarily through the thermo-optic effect, resulting in a shift of fringes reflected from the pillar detected using an I-MON 512 OEM spectrometer. While initially designed for use in liquids, this sensor has ideal properties for use as a plasma bolometer: a time constant, in air, of ∼150 ms, strong absorption in the spectral range of plasma emission, immunity to local EMI, and the ability to measure changes in temperature remotely. Its compact design offers unique opportunities for integration into the vacuum environment in places unsuitable for a resistive bolometer. Using a variable focus 5 mW, 405 nm, modulating laser, the signal to noise ratio versus power density of various bolometer technologies are directly compared, estimating the noise equivalent power density (NEPD). Present tests show the fiber-optic bolometer to have NEPD of 5-10 W/m2when compared to those of the resistive bolometer which can achieve <0.5 W/m2in the laboratory, but this can degrade to 1-2 W/m2or worse when installed on a tokamak. Concepts are discussed to improve the signal to noise ratio of this new fiber-optic bolometer by reducing the pillar height and adding thin metallic coatings, along with improving the spectral resolution of the interrogator.
authors.affiliation: Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors.forename: M L
authors.initials: ML
authors.lastname: Reinke
authors.affiliation: University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA.
authors.forename: M
authors.initials: M
authors.lastname: Han
authors.affiliation: University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA.
authors.forename: G
authors.initials: G
authors.lastname: Liu
authors.affiliation: Dutch Institute for Fundamental Energy Research, De Zaale 20, 5612 AJ Eindhoven, The Netherlands.
authors.forename: G G
authors.initials: GG
authors.lastname: van Eden
authors.affiliation: Technobis, Pyrietstraat 2, 1812 SC Alkmaar, The Netherlands.
authors.forename: R
authors.initials: R
authors.lastname: Evenblij
authors.affiliation: Technobis, Pyrietstraat 2, 1812 SC Alkmaar, The Netherlands.
authors.forename: M
authors.initials: M
authors.lastname: Haverdings
authors.affiliation: Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, New Jersey 08543, USA.
authors.forename: B C
authors.initials: BC
authors.lastname: Stratton