Assessment of potential heat flux overload of target and first wall components in Wendelstein 7-X finite-beta magnetic configurations and choice of locations for temperature monitoring

https://doi.org/10.1016/j.fusengdes.2020.111902Get rights and content

Highlights

  • The potential arising of overloads onto in vessel components of W7-X has been investigated.

  • Properly selected finite β magnetic configurations have been studied.

  • A recently developed innovative approach has been adopted to calculate, map and represent the heat flux onto the tiles of the W7-X in vessel components.

  • Assuming 8.0 MW of convected power and halving the limit on heat flux, a considerable number of overloads is predicted onto baffle tiles.

  • Results have allowed selecting proper locations for thermocouples aimed at monitoring the tiles temperature in the selected magnetic configurations.

Abstract

Within the framework of R&D activities on the Wendelstein 7-X (W7-X) stellarator machine, the assessment of heat loads onto the plasma facing components (PFCs) is an important aspect. So far, W7-X was operated in short pulses without water cooling of the PFCs. Presently, the device is being prepared for future operation phases with water cooling. The target plates, which receive the highest heat loads, are monitored by a thermography system. The rest of the first wall (heat shield) is not designed to receive convective particle loads, and it is in part poorly monitored by the infrared cameras. Recent studies have shown that for many plasma configurations there are locations on the heat shield which do receive significant convective heat flux. This is in particular true for the heat shields adjacent to the target plates (called baffles) and for configurations with finite plasma pressure, where the magnetic configuration is modified by plasma currents. On the other side, the heat flux limit to the baffle tiles had to be reduced from 0.50 to 0.25 MW/m2.

In this work an evaluation of the heat flux to targets and baffles in plasma configurations with finite plasma pressure is presented. To this end, Field Line Diffusion (FLD) calculations have been performed to obtain the heat load pattern distributions for the considered magnetic configurations. The results have been assessed statistically to achieve a measure of certainty in the prediction of an overload in a certain location. The possibility of overloads onto the baffles due to plasma radiation has also been investigated. The results of the entire analysis show that local temperature monitoring by thermocouples in a rather limited number of locations will be sufficient to avoid heat flux overloads of the baffles and heat shields in all magnetic configurations considered.

Introduction

Within the framework of the European roadmap to the realization of fusion energy [1], the construction and operation of the Wendelstein 7-X (W7-X) stellarator machine represents one of the big milestones. The W7-X is a large stellarator with superconducting coils, operated at the Greifswald site of the Max-Planck-Institut für Plasmaphysik [2]. The first goal of the W7-X operation has been to show the feasibility of building a superconducting modular stellarator with the required precision which has been successfully shown [3,4,5]. A second primary objective is to demonstrate the accessibility of plasma parameters close to those of a future Fusion Power Plant (FPP) [6,7], which is ongoing work [8,9]. The last goal will be to prove the possibility of high-power steady-state operation.

W7-X can be operated in different magnetic configurations, controlled by the currents in the different types of field coil [10]. Those configurations envisaged for high-power operation are characterised by a chain of natural magnetic islands at the plasma boundary. In any toroidal cross section, the magnetic flux surfaces in the island region form an O point in the centre of each island and an X point between two adjacent islands [11]. In the 3D view the corresponding O and X points of each cross section are connected by field lines closing upon themselves after a low number of toroidal revolutions. The islands are intersected by the target plates (island divertor concept) [9,12,13]. Following the typical five-fold toroidal symmetry and the up-down flip symmetry (stellarator symmetry), the targets are arranged in ten identical divertor units (Fig. 1).

So far, W7-X was operated with uncooled test divertor units (TDU) [14]. For future operation phases, a water-cooled high heat flux (HHF) divertor will be installed [15]. It is then planned to operate W7-X in steady-state discharges of up to 30 min with 10 MW of heating power. Adjacent to the target plates, where lower heat loads are expected, so called baffles are installed (Fig. 1). The remaining surface of the plasma vessel is covered partly by wall protection tiles of the same design as the baffles [16], partly by steel panels [17]. In the future stellarator FPPs, almost all the plasma vessel internal wall will be covered by breeding blanket modules [[18], [19], [20], [21]] aimed at removing the thermal power generated by the fusion plasma, shielding the magnets from neutron and gamma radiation and ensuring the tritium breeding of the plant.

For high-power long-pulse operation of W7-X, it is essential to protect the Plasma Facing Components (PFCs) listed above from heat loads exceeding the design specifications. This is particularly important for the baffles and targets, which are the most loaded PFCs because of the convective heat power deposited by charged particles. During the first divertor operation phase of W7-X, baffle loads above the design values were derived from infrared (IR) camera images [22], presumably because anomalous transport perpendicular to the magnetic field is higher than assumed during the design of the PFCs. At the same time, refined thermomechanical analysis of the baffles indicated that the maximum thermal load to these components should be reduced [23]. Whereas the target plates and part of the baffles is well monitored by IR cameras (Fig. 2), there are locations in which baffle or wall protection tiles cannot be observed.

It was therefore decided to install thermocouples in the heat sinks of selected baffle and wall protection tiles in order to avoid thermal overload to these components.

We are using field line diffusion (FLD) [24] to simulate the convective power load to PFCs. Whereas this has been done before mostly for the W7-X vacuum reference configurations [25,26] and only for a few cases with finite plasma pressure, and with a focus on the target loads [27,28], we shall here investigate configurations with finite plasma pressure. These are the so-called finite-β configurations, where β = p/(B2/(2μ0) is the ratio between plasma pressure and magnetic pressure. The magnetohydrodynamic equilibria with finite plasma pressure in a toroidally confined plasma are radially shifted toward the torus outboard side relative to the vacuum flux surfaces, such that we expect potential overload to occur on baffle tiles on the torus outboard side.

In this paper, we shall describe the methodology adopted to select a limited number of baffle positions for temperature monitoring. The basic philosophy is described in section 2, together with the approach to model the heat load onto PFCs for some magnetic configurations and to assess its statistical significance. Section 3 reports the main outcomes of the overload calculations whereas in section 4 we present the locations selected for temperature monitoring. Lastly, conclusions are given in section 5 and a complete overview of the overload calculations carried out is reported in the Appendix.

Section snippets

Calculation of thermal loads to wall components

In order to calculate the thermal loads onto W7-X target and wall components for a certain magnetic configuration, the following procedure was used. Whereas for a vacuum configuration (i. e., the changes in magnetic field due to plasma currents are negligible) the magnetic field inside the W7-X plasma vessel was calculated from the currents in the field coils (represented as filaments), for a finite-β plasma the magnetic field of a magnetohydrodynamic equilibrium was calculated by the VMEC [30]

Results of overload analysis

In order to investigate the overloads arising onto baffle and divertor tiles in finite-β configurations, FLD calculations for the magnetic configurations listed in Table 2 are performed. In particular, 7 different vacuum field configurations are chosen (called standard, low shear (orig./mod.), outward shifted, low iota, high mirror and high iota) and, for each of them, three different values of β are chosen for VMEC calculations. Configurations with finite plasma current are not covered in this

Choice of thermocouple locations

As discussed in section 2.4, the wall protection tiles with overloads in some of the magnetic configurations considered are clustered in a limited number of locations, and the same very few tiles within each cluster are predicted to receive the highest load even in different magnetic configurations. Those are the obvious choice for the thermocouples placement. In addition, care is taken on the one hand side to provide temperature monitoring in those critical locations that are not visible in

Conclusion

In the framework of W7-X R&D activities, an assessment of the overloads arising onto divertor, baffle and heat shield in finite-β magnetic configurations is reported in this paper. To this purpose a calculation procedure, aimed at finding out those tiles where the predicted convective heat flux exceeds the limit, is applied. The study is carried out assuming, for the baffle and heat shield, limits of 0.50 MW/m2 and 0.25 MW/m2. A maximum convected heat power of 8.0 MW is considered.

Results show

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

G. Bongiovì: Formal analysis, Investigation, Writing - original draft, Visualization. D. Böckenhoff: Methodology, Software, Resources, Writing - review & editing. A. Carls: Resources. M. Endler: Project administration, Conceptualization, Resources, Supervision, Writing - review & editing, Visualization. J. Fellinger: Project administration, Supervision, Writing - review & editing. J. Geiger: Resources.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

Authors want to thank M. Krause for providing the CAD views of the baffles and shields.

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

References (35)

  • R.C. Wolf

    Major results from the first plasma campaign of the Wendelstein 7-X stellarator

    Nucl. Fusion

    (2017)
  • T. Sunn Pedersen

    Confirmation of the topology of the Wendelstein 7-X magnetic field to better than 1:100000, Nature Communications 7

    (2016)
  • T. Klinger

    Performance and properties of the first plasmas of Wendelstein 7-X

    Plasma Physics and Controlled Fusion

    (2017)
  • A. Dinklage

    Magnetic configuration effects on the Wendelstein 7-X stellarator

    Nature Physics

    (2018)
  • C. Beidler

    Physics and Engineering Design for Wendelstein VII-X

    Fusion Science and Technology

    (1990)
  • T. Sunn Pedersen

    Key results from the first plasma operation phase and outlook for future performance in Wendelstein 7-X

    Physics of Plasmas

    (2017)
  • T. Sunn Pedersen

    First results from divertor operation in Wendelstein 7-X

    Plasma Phys. Control. Fusion

    (2019)
  • Cited by (2)

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