Experimental tests of an infrared video bolometer on Alcator C-Mod.

A prototype of an infrared imaging bolometer (IRVB) was successfully tested on the Alcator C-Mod tokamak at the end of its 2016 campaign. The IRVB method interprets the power radiated from the plasma by measuring the temperature rise of a thin, ∼2 µm, Pt absorber that is placed in the torus vacuum and exposed, using a pinhole camera, to the full-spectrum of plasma's photon emission. The IRVB installed on C-Mod viewed the poloidal cross section of the core plasma and observed Ohmic and ion cyclotron range of frequency (ICRF)-heated plasmas. Analysis of total radiated power and on-axis emissivity from IRVB is summarized, and quantitative comparisons made to data from both resistive bolometers and AXUV diodes. IRVB results are clearly within a factor of two, but additional effort is needed for it to be used to fully support power exhaust research. The IRVB is shown to be immune to electromagnetic interference from ICRF which strongly impacts C-Mod's resistive bolometers. Results of the bench-top calibration are summarized, including a novel temperature calibration method useful for IRVBs.

Oscillatory vapour shielding of liquid metal walls in nuclear fusion devices.

Providing an efficacious plasma facing surface between the extreme plasma heat exhaust and the structural materials of nuclear fusion devices is a major challenge on the road to electricity production by fusion power plants. The performance of solid plasma facing surfaces may become critically reduced over time due to progressing damage accumulation. Liquid metals, however, are now gaining interest in solving the challenge of extreme heat flux hitting the reactor walls. A key advantage of liquid metals is the use of vapour shielding to reduce the plasma exhaust. Here we demonstrate that this phenomenon is oscillatory by nature. The dynamics of a Sn vapour cloud are investigated by exposing liquid Sn targets to H and He plasmas at heat fluxes greater than 5 MW m-2. The observations indicate the presence of a dynamic equilibrium between the plasma and liquid target ruled by recombinatory processes in the plasma, leading to an approximately stable surface temperature.Vapour shielding is one of the interesting mechanisms for reducing the heat load to plasma facing components in fusion reactors. Here the authors report on the observation of a dynamic equilibrium between the plasma and the divertor liquid Sn surface leading to an overall stable surface temperature.

Design and characterization of a prototype divertor viewing infrared video bolometer for NSTX-U.

The InfraRed Video Bolometer (IRVB) is a powerful tool to measure radiated power in magnetically confined plasmas due to its ability to obtain 2D images of plasma emission using a technique that is compatible with the fusion nuclear environment. A prototype IRVB has been developed and installed on NSTX-U to view the lower divertor. The IRVB is a pinhole camera which images radiation from the plasma onto a 2.5 µm thick, 9 × 7 cm2 Pt foil and monitors the resulting spatio-temporal temperature evolution using an IR camera. The power flux incident on the foil is calculated by solving the 2D+time heat diffusion equation, using the foil's calibrated thermal properties. An optimized, high frame rate IRVB, is quantitatively compared to results from a resistive bolometer on the bench using a modulated 405 nm laser beam with variable power density and square wave modulation from 0.2 Hz to 250 Hz. The design of the NSTX-U system and benchtop characterization are presented where signal-to-noise ratios are assessed using three different IR cameras: FLIR A655sc, FLIR A6751sc, and SBF-161. The sensitivity of the IRVB equipped with the SBF-161 camera is found to be high enough to measure radiation features in the NSTX-U lower divertor as estimated using SOLPS modeling. The optimized IRVB has a frame rate up to 50 Hz, high enough to distinguish radiation during edge-localized-modes (ELMs) from that between ELMs.

Preliminary design of a tangentially viewing imaging bolometer for NSTX-U.

The infrared imaging video bolometer (IRVB) measures plasma radiated power images using a thin metal foil. Two different designs with a tangential view of NSTX-U are made assuming a 640 × 480 (1280 × 1024) pixel, 30 (105) fps, 50 (20) mK, IR camera imaging the 9 cm × 9 cm × 2 µm Pt foil. The foil is divided into 40 × 40 (64 × 64) IRVB channels. This gives a spatial resolution of 3.4 (2.2) cm on the machine mid-plane. The noise equivalent power density of the IRVB is given as 113 (46) µW/cm2 for a time resolution of 33 (20) ms. Synthetic images derived from Scrape Off Layer Plasma Simulation data using the IRVB geometry show peak signal levels ranging from ∼0.8 to ∼80 (∼0.36 to ∼26) mW/cm2.

Development of plasma bolometers using fiber-optic temperature sensors.

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/m2 when compared to those of the resistive bolometer which can achieve <0.5 W/m2 in the laboratory, but this can degrade to 1-2 W/m2 or 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.

Self-Regulated Plasma Heat Flux Mitigation Due to Liquid Sn Vapor Shielding.

A steady-state high-flux H or He plasma beam was balanced against the pressure of a Sn vapor cloud for the first time, resulting in a self-regulated heat flux intensity near the liquid surface. A temperature response of the liquid surface characterized by a decoupling from the received heating power and significant cooling of the plasma in the neutral Sn cloud were observed. The plasma heat flux impinging on the target was found to be mitigated, as heat was partially dissipated by volumetric processes in the vapor cloud rather than wholly by surface effects. These results motivate further exploration of liquid metal solutions to the critical challenge of heat and particle flux handling in fusion power plants.