Bidirectional reflectance measurement of tungsten samples to assess reflection model in WEST tokamak

This paper presents the measurement of the bidirectional reflectance distribution function of tungsten (W) samples and the resulting reflection models in the nuclear fusion device WEST (tokamak). For this, an experimental gonio-spectrophotometer was developed to fully characterize the material's optical and thermal-radiative properties of metallic samples with different roughnesses. Ray-tracing photonic simulation was then carried out to predict the photon behavior in a fully metallic environment as a function of reflectance measurement. Low emissivity (0.1 at 4 μm) and highly specular reflectance (fitting with a Gaussian distribution around the specular direction with a small width lower than 10°) are found for W samples. These measurements have been used as input for the photonic simulation, and the resulting synthetic image reproduced the reflection features well on the upper divertor, detected in WEST infrared experimental images.

Development of visible spectroscopy diagnostics for W sources assessment in WEST

The present work concerns the development of a W sources assessment system in the framework of the tungsten-W environment in steady state tokamak project that aims at equipping the existing Tore Supra device with a tungsten divertor in order to test actively cooled tungsten Plasma Facing Components (PFCs) in view of preparing ITER operation. The goal is to assess W sources and D recycling with spectral, spatial, and temporal resolution adapted to the PFCs observed. The originality of the system is that all optical elements are installed in the vacuum vessel and compatible with steady state operation. Our system is optimized to measure radiance as low as 10¹⁶ Ph/(m² s sr). A total of 240 optical fibers will be deployed to the detection systems such as the "Filterscope," developed by Oak Ridge National Laboratory (USA) and consisting of photomultiplier tubes and filters, or imaging spectrometers dedicated to Multiview analysis.

2D surface temperature measurement of plasma facing components with modulated active pyrometry

In nuclear fusion devices, such as Tore Supra, the plasma facing components (PFC) are in carbon. Such components are exposed to very high heat flux and the surface temperature measurement is mandatory for the safety of the device and also for efficient plasma scenario development. Besides this measurement is essential to evaluate these heat fluxes for a better knowledge of the physics of plasma-wall interaction, it is also required to monitor the fatigue of PFCs. Infrared system (IR) is used to manage to measure surface temperature in real time. For carbon PFCs, the emissivity is high and known (ɛ ∼ 0.8), therefore the contribution of the reflected flux from environment and collected by the IR cameras can be neglected. However, the future tokamaks such as WEST and ITER will be equipped with PFCs in metal (W and Be/W, respectively) with low and variable emissivities (ɛ ∼ 0.1-0.4). Consequently, the reflected flux will contribute significantly in the collected flux by IR camera. The modulated active pyrometry, using a bicolor camera, proposed in this paper allows a 2D surface temperature measurement independently of the reflected fluxes and the emissivity. Experimental results with Tungsten sample are reported and compared with simultaneous measurement performed with classical pyrometry (monochromatic and bichromatic) with and without reflective flux demonstrating the efficiency of this method for surface temperature measurement independently of the reflected flux and the emissivity.

Modeling of the ITER-like wide-angle infrared thermography view of JET

Infrared (IR) thermography systems are mandatory to ensure safe plasma operation in fusion devices. However, IR measurements are made much more complicated in metallic environment because of the spurious contributions of the reflected fluxes. This paper presents a full predictive photonic simulation able to assess accurately the surface temperature measurement with classical IR thermography from a given plasma scenario and by taking into account the optical properties of PFCs materials. This simulation has been carried out the ITER-like wide angle infrared camera view of JET in comparing with experimental data. The consequences and the effects of the low emissivity and the bidirectional reflectivity distribution function used in the model for the metallic PFCs on the contribution of the reflected flux in the analysis are discussed.

Wall reflection issues for optical diagnostics in fusion devices

The problem of light reflection has been raised as a high priority issue for optical diagnostics in next step fusion devices where metallic wall environment will generate significant perturbations in the diagnostics measurements. Tore Supra is a large size tokamak equipped with water-cooled stainless-steel panels used to sustain the plasma long shot radiations. These panels are highly reflective and affect significantly optical systems. In particular, we show that the infrared imaging diagnostic, which surveys the plasma facing component surface temperature for safety purposes, can give incorrect information due to reflected light coming from the bottom limiter. In the visible range, motional Stark effect and Zeff measurements experience important drifts during the plasma heating phases due to parasitic light coming from the limiter, but also from the plasma itself when the viewing lines are facing the reflecting walls. In the next step fusion devices such as ITER, the possibility to use optical measurements needs to be accessed by a modeling of the diagnostic light in its machine environment and the development of new techniques of online correction.

Development of a realistic photonic modeling for the evaluation of infrared reflections in the metallic environment of ITER

In nuclear fusion experiments, the plasma facing components are exposed to high heat fluxes and infrared (IR) imaging diagnostics are routinely used for surveying their surface temperature for preventing damages. However the future use of metallic components in the ITER tokamak adds complications in temperature estimation. Indeed, low and variable emissivity of the observed surface and the multiple reflections of the light coming from hot regions will have to be understood and then taken into account. In this paper, a realistic photonic modeling based on Monte Carlo ray-tracing codes is used to predict the global response of the complete IR survey system. This also includes the complex vessel geometry and the thermal and optical surface properties using the bidirectional reflectivity distribution function that models the photon-material interactions. The first results of this simulation applied to a reference torus are presented and are used as a benchmark to investigate the validity of the global model. Finally the most critical key model parameters in the reflected signals are identified and their contribution is discussed.