Approaches for quantitative study of divertor heat loads on W7-X

Abstract

Aimed to show the fusion-reactor relevance of optimized stellarators, Wendelstein 7-X (W7-X) [1], as the world's largest stellarator (plasma volume 30 m 3 , major radius 5.5 m, minor radius 0.5 m), completed successfully its first experimental campaign with island divertor in December 2017. To demonstrate the capabilities of steady-state operation with reactor-relevant plasma parameters, island divertor concept is commissioned for a tolerable plasma power and particle exhaust. Pre-defined magnetic islands at the plasma boundary are cut open by the divertor plates, so that these open field lines will lead the unconfined plasmas to the graphite tiles, which are designed to allow a high power load of up to 10 MW/m 2 .Infrared thermography system has been developed in many fusion devices as a key diagnostic to monitor the temperature and to study the heat loads on the plasma facing components, thus to prevent them from excessive damage. W7-X are designed with superconducting coils for plasma pulses with 30 mins duration at a heating power of 10 MW. For the safety of the experimental exploration aimed at high performance long discharges, a full coverage of the field of view by infrared diagnostics for the first wall and divertor plates is essential. In this campaign, ten thermography systems were installed for the first time, with each infrared camera monitoring one half module of the machine, realized by the welldesigned wide-angle optics [2]. Figure 1 shows the field of view of the nine infrared cameras behind the immersion tubes, the temperature distributions indicate a rather symmetry target heat loads. The systems have been operated through the whole campaign with great liability, producing temperature evolution on the plasma facing components during pulses.The infrared cameras were controlled to record the thermal emissions with a frame rate of 100 Hz in a wavelength range of 8 -10 μm. The sensor array contains 768 x 1024 pixels, with a varied spatial resolution at different parts of the divertor surface from ~ 3 mm (low iota part) to ~ 10 mm (high iota tail). The exposure time has been set manually (1 to 10 μs) for each discharge, depending on the heating power, to generate accurate digital levels. The Planck's law calibration under the designed optical routes, against a cavity radiator with known emissivity and temperature, has been carried out in the laboratory before the campaign. The relation between the received digital levels and the temperature has been compiled into look-up tables. The camera control and temperature conversion have been incorporated into a software platform for a real time observation of surface temperature during the experiments [3].Except for being one of the key diagnostics for the safety of the machine [4], the divertor thermal footprints provide important inputs for the study of the intrinsic three dimensional magnetic topology and the heat transport inside the boundary islands. A quantitative analysis and comparison of the divertor temperature in different machine modules are requ...

Application of improved analysis of convective heat loads on plasma facing components to Wendelstein 7-X

“…Exploration problem: Magnetic configuration space exploration or FLD parameter scans in the search for magnetic configurations or parameters with particular properties [23,24] Inverse problem: or discharge reconstruction [15,25] Forward problem: or discharge planning [26] As motivated before, evolving plasma parameters induce changes in the magnetic configuration and thus dynamic PFC heat loads, which results in the need of multiple simulations for forward problems and even more so for inverse problems. Thus all of the categories above require multiple simulations.…”

“…FLD applications can mostly be assigned to one ore more of the following categories: Exploration problem: Magnetic configuration space exploration or FLD parameter scans in the search for magnetic configurations or parameters with particular properties [23,24]. Inverse problem: or discharge reconstruction [15,25]. Forward problem: or discharge planning [26].…”

“…To be very clear: In the scope of the present article no artificial neural networks (NNs) or other machine learning (ML) techniques are applied although they are envisioned. So far heat load visualizations have either been realized by a camera-matrix projection, where the complex curved geometry is distorted [14,15], or by one-dimensional heat load quantification along a predefined trajectory [15].…”

Neural network regression approaches to reconstruct properties of magnetic configuration from Wendelstein 7-X modeled heat load patterns

“…These experimental data and according simulations are heat load patterns on the graphite limiters which were the main PFC during the first operational phase. Since 2017 ten island divertors (see figure 1) are installed, spatially separating the core plasma from the plasma edge and thus making W7-X suitable for high performance discharges [29][30][31]. For geometrical reasons 4 , the limiter heat load patterns are much less diverse as compared to the heat load distribution on the divertor.…”