Infrared imaging systems for wall protection in the W7-X stellarator (invited)

Measurements of emissivity and temperature for polarized long-wavelength infrared light from tungsten under simulating tokamak conditions

Tungsten is regarded as the baseline first wall material in tokamaks. This work provides a polarized method for measuring the emissivity and temperature of the tungsten using an infrared camera and a polarizer under simulating tokamak conditions. In the experiment, a polarizer with an adjustable polarization direction is set up in front of an infrared camera. A rotatable fixture is used to fix the sample and change the angle between the surface and the normal. The sample is rotated from 0° to 80°, and the polarized emissivity first increases and then decreases with increasing rotation angle. The uncertainty in emissivity resulting from this polarized method and non-polarized method is analyzed. To compare the effects of the polarized method and the non-polarized method, the rotation angle is adjusted to 0°, and a fitting model is used to describe the relationship between emissivity and temperature. Errors between the calculated temperature and measured temperature are used as a scale, and the polarized method improves the accuracy of temperature measurement. This polarized method provides a technical way to measure the emissivity and temperature in a tokamak and can be applied in other similar applications.

Implementation of Thermal Event Image Processing Algorithms on NVIDIA Tegra Jetson TX2 Embedded System-on-a-Chip

Advances in Infrared (IR) cameras, as well as hardware computational capabilities, contributed towards qualifying vision systems as reliable plasma diagnostics for nuclear fusion experiments. Robust autonomous machine protection and plasma control during operation require real-time processing that might be facilitated by Graphics Processing Units (GPUs). One of the current aims of image plasma diagnostics involves thermal events detection and analysis with thermal imaging. The paper investigates the suitability of the NVIDIA Jetson TX2 Tegra-based embedded platform for real-time thermal events detection. Development of real-time processing algorithms on an embedded System-on-a-Chip (SoC) requires additional effort due to the constrained resources, yet low-power consumption enables embedded GPUs to be applied in MicroTCA.4 computing architecture that is prevalent in nuclear fusion projects. For this purpose, the authors have proposed, developed and optimised GPU-accelerated algorithms with the use of available software tools for NVIDIA Tegra systems. Furthermore, the implemented algorithms are evaluated and benchmarked on Wendelstein 7-X (W7-X) stellarator experimental data against the corresponding alternative Central Processing Unit (CPU) implementations. Considerable improvement is observed for the accelerated algorithms that enable real-time detection on the embedded SoC platform, yet some encountered limitations when developing parallel image processing routines are described and signified.

The Langmuir probe system in the Wendelstein 7-X test divertor

The design and evaluation of the Langmuir probe system used in the first divertor operation phase of Wendelstein 7-X is described. The probes are integrated into the target plates and have individually facetted surfaces to keep the angle of incidence of the magnetic field within an appropriate range for different magnetic configurations. Multiple models for the derivation of plasma parameters from current-voltage characteristics are introduced. These are analyzed with regard to their assumptions and limitations, generalized, and adapted to our use case. A detailed comparison is made to determine the most suitable model. It is found that the choice of model has a large impact, for example, resulting in a change in the inferred temperatures of up to a factor two. This evaluation is implemented in a Bayesian modeling framework and automated to allow for joint analysis with other diagnostics and a replacement of ad hoc assumptions. We rigorously treat parameter uncertainties, revealing strong correlations between them. General and flexible model formulations permit an expansion to additional effects.

Coherence imaging spectroscopy at Wendelstein 7-X for impurity flow measurements

In the last decade, Coherence Imaging Spectroscopy (CIS) has shown distinctive results in measuring ion flow velocities in the edge of magnetically confined plasma devices. Its 2D spatially resolved measurement capabilities and its high optical throughput are ideal for investigating the impurity behavior in the complex 3D magnetic island topology edge of Wendelstein 7-X (W7-X). However, a highly precise and stable calibration method is required for a reliable diagnostic operation. A new level of precision and stability has been achieved for the two CIS systems installed at W7-X with the use of a new calibration source, a continuous tunable laser commercially available only since 2015. A specific prototype model was successfully adapted to the challenging requirements of W7-X, granting high accuracy (±0.01 pm) and flexibility (spectral range: 450-650 nm) in the wavelength calibration required for measuring low-Z impurity ion flow velocities. These features opened up new investigation possibilities on temperature stability and wavelength response of the CIS components, allowing to fully characterize and validate the W7-X systems. The CIS diagnostic was operational throughout the last W7-X experimental campaign. Measured velocities on the order of ∼20-30 km/s were observed, corroborated by comparisons with measurements with Mach probes.

First Observation of a Stable Highly Dissipative Divertor Plasma Regime on the Wendelstein 7-X Stellarator

For the first time, the optimized stellarator Wendelstein 7-X has operated with an island divertor. An operation regime in hydrogen was found in which the total plasma radiation approached the absorbed heating power without noticeable loss of stored energy. The divertor thermography recorded simultaneously a strong reduction of the heat load on all divertor targets, indicating almost complete power detachment. This operation regime was stably sustained over several energy confinement times until the preprogrammed end of the discharge. The plasma radiation is mainly due to oxygen and is located at the plasma edge. This plasma scenario is reproducible and robust at various heating powers, plasma densities, and gas fueling locations. These experimental results show that the island divertor concept actually works and displays good power dissipation potential, producing a promising exhaust concept for the stellarator reactor line.

Towards a new image processing system at Wendelstein 7-X: From spatial calibration to characterization of thermal events

Wendelstein 7-X (W7-X) is the most advanced fusion experiment in the stellarator line and is aimed at proving that the stellarator concept is suitable for a fusion reactor. One of the most important issues for fusion reactors is the monitoring of plasma facing components when exposed to very high heat loads, through the use of visible and infrared (IR) cameras. In this paper, a new image processing system for the analysis of the strike lines on the inboard limiters from the first W7-X experimental campaign is presented. This system builds a model of the IR cameras through the use of spatial calibration techniques, helping to characterize the strike lines by using the information given by real spatial coordinates of each pixel. The characterization of the strike lines is made in terms of position, size, and shape, after projecting the camera image in a 2D grid which tries to preserve the curvilinear surface distances between points. The description of the strike-line shape is made by means of the Fourier Descriptors.