Design and implementation of a prototype infrared video bolometer (IRVB) in MAST Upgrade.

A prototype infrared video bolometer (IRVB) was successfully deployed in the Mega Ampere Spherical Tokamak Upgrade (MAST Upgrade or MAST-U), the first deployment of such a diagnostic in a spherical tokamak. The IRVB was designed to study the radiation around the lower x-point, another first in tokamaks, and has the potential to estimate emissivity profiles with spatial resolution beyond what is achievable with resistive bolometry. The system was fully characterized prior to installation on MAST-U, and the results are summarized here. After installation, it was verified that the actual measurement geometry in the tokamak qualitatively matches the design; this is a particularly difficult process for bolometers and was done using specific features of the plasma itself. The installed IRVB measurements are consistent both with observations from other diagnostics, including magnetic reconstruction, visible light cameras, and resistive bolometry, as well as with the IRVB-designed view. Early results show that with conventional divertor geometry and only intrinsic impurities (for example, C and He), the progression of radiative detachment follows a similar path to that observed for large aspect ratio tokamaks: The peak of the radiation moves along the separatrix from the targets to the x-point and high-field side midplane with a toroidally symmetric structure that can eventually lead to strong effects on the core plasma inside the separatrix.

Fiber-optic silicon Fabry-Perot interferometric bolometer with improved detection limit for magnetic confinement fusion.

Fiber-optic bolometers (FOBs) intended for plasma radiation measurement in magnetically confined fusion have been previously developed using a silicon pillar that functions as both a Fabry-Perot interferometer (FPI) for temperature measurement and an absorber for the radiation. We report an FOB design that can significantly improve the detection sensitivity over earlier designs by engineering the absorber of the FOB. Our design uses the fact that, compared with a silicon pillar, a gold film with the same x-ray absorption thickness will show a much higher temperature rise from a given power density of the radiation. Therefore, the responsivity of an FOB can be improved by attaching a large gold disk to the silicon FPI as the absorber. We have developed a fabrication method for FOBs of such design and obtained an FOB with a 4-µm-thick, 0.6-mm-diameter gold disk attached to a 200-µm-diameter, 100-µm-thick silicon FPI. We have characterized the noise, responsivity, response time, and noise-equivalent power density (NEPD) and compared these with the earlier design where the absorber is mainly the silicon FPI itself. The experimental result suggests that the FOB with the gold disk achieves a responsivity of ∼2.8 mK/(W/m2) and a noise-equivalent-power-density of 0.13 W/m2, which are, respectively, more than nine times larger and six times smaller compared to the FOB using a previous design. Improved NEPD and good absorption over a broad frequency range will make the FOB more attractive for applications in magnetic-confinement fusion devices.

A Silicon-tipped Fiber-optic Sensing Platform with High Resolution and Fast Response.

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This FOSP relies on a silicon Fabry-Perot interferometer (FPI) attached to the fiber end, referred to as Si-FOSP in this work. The Si-FOSP generates an interferogram determined by the optical path length (OPL) of the silicon cavity. Measurand alters the OPL and thus shifts the interferogram. Due to the unique optical and thermal properties of the silicon material, this Si-FOSP exhibits an advantageous performance in terms of sensitivity and speed. Furthermore, the mature silicon fabrication industry endows the Si-FOSP with excellent reproducibility and low cost toward practical applications. Depending on the specific applications, either a low-finesse or high-finesse version will be utilized, and two different data demodulation methods will be adopted accordingly. Detailed protocols for fabricating both versions of the Si-FOSP will be provided. Three representative applications and their according results will be shown. The first one is a prototype underwater thermometer for profiling the ocean thermoclines, the second one is a flow meter to measure flow speed in the ocean, and the last one is a bolometer used for monitoring exhaust radiation from magnetically confined high-temperature plasma.

A fiber-optic bolometer based on a high-finesse silicon Fabry-Pérot interferometer.

We report a fiber-optic bolometer based on a high-finesse silicon Fabry-Perot interferometer (FPI). The silicon FPI absorbs and converts the incident radiation into temperature variations, which are interrogated by the shift of the reflection spectrum of the FPI. The FPI is a silicon pillar with one side coated with a high-reflectivity dielectric mirror and the other side coated with a gold mirror. A multimode fiber collimator is applied between the FPI and lead-in single-mode fiber to reduce the round-trip diffraction loss, giving rise to a high-finesse of 35 of the FPI. The sensor is demodulated using a low-cost distributed feedback diode laser. A dummy bolometer was used to effectively reduce the common noises from the laser wavelength drift and ambient temperature variations. Experimental results show that, compared with a previously reported fiber-optic bolometer, the one reported here has a 5-fold decrease in noise and a 7-fold increase in responsivity with a noise equivalent power density (NEPD) of 0.27 W/m2, which is comparable with the NEPDs of the state-of-the-art resistive bolometers.