The two leading concepts for confining high-temperature fusion plasmas are the tokamak and the stellarator. Tokamaks are rotationally symmetric and use a large plasma current to achieve confinement, whereas stellarators are non-axisymmetric and employ three-dimensionally shaped magnetic field coils to twist the field and confine the plasma. As a result, the magnetic field of a stellarator needs to be carefully designed to minimize the collisional transport arising from poorly confined particle orbits, which would otherwise cause excessive power losses at high plasma temperatures. In addition, this type of transport leads to the appearance of a net toroidal plasma current, the so-called bootstrap current. Here, we analyse results from the first experimental campaign of the Wendelstein 7-X stellarator, showing that its magnetic-field design allows good control of bootstrap currents and collisional transport. The energy confinement time is among the best ever achieved in stellarators, both in absolute figures (τE > 100 ms) and relative to the stellarator confinement scaling. The bootstrap current responds as predicted to changes in the magnetic mirror ratio. These initial experiments confirm several theoretically predicted properties of Wendelstein 7-X plasmas, and already indicate consistency with optimization measures. Results from the first experimental campaign of the Wendelstein 7-X stellarator demonstrate that its magnetic-field design grants good control of parasitic plasma currents, leading to long energy confinement times.

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Magnetic configuration effects on the Wendelstein 7-X stellarator

The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, currently under construction in Greifswald, Germany. Steady-state operation is an intrinsic feature of stellarators, and one key element of the Wendelstein 7-X mission is to demonstrate steady-state operation under plasma conditions relevant for a fusion power plant. Steady-state operation of a fusion device, on the one hand, requires the implementation of special technologies, giving rise to technical challenges during the design, fabrication and assembly of such a device. On the other hand, also the physics development of steady-state operation at high plasma performance poses a challenge and careful preparation. The electron cyclotron resonance heating system, diagnostics, experiment control and data acquisition are prepared for plasma operation lasting 30 min. This requires many new technological approaches for plasma heating and diagnostics as well as new concepts for experiment control and data acquisition.

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Technical challenges in the construction of the steady-state stellarator Wendelstein 7-X

Wendelstein 7-X is the first comprehensively optimized stellarator aiming at good confinement with plasma parameters relevant to a future stellarator power plant. Plasma operation started in 2015 using a limiter configuration. After installing an uncooled magnetic island divertor, extending the energy limit from 4 to 80 MJ, operation continued in 2017. For this phase, the electron cyclotron resonance heating (ECRH) capability was extended to 7 MW, and hydrogen pellet injection was implemented. The enhancements resulted in the highest triple product (6.5 × 1019 keV m−3 s) achieved in a stellarator until now. Plasma conditions [Te(0) ≈ Ti(0) ≈ 3.8 keV, τE > 200 ms] already were in the stellarator reactor-relevant ion-root plasma transport regime. Stable operation above the 2nd harmonic ECRH X-mode cutoff was demonstrated, which is instrumental for achieving high plasma densities in Wendelstein 7-X. Further important developments include the confirmation of low intrinsic error fields, the observation of current-drive induced instabilities, and first fast ion heating and confinement experiments. The efficacy of the magnetic island divertor was instrumental in achieving high performance in Wendelstein 7-X. Symmetrization of the heat loads between the ten divertor modules could be achieved by external resonant magnetic fields. Full divertor power detachment facilitated the extension of high power plasmas significantly beyond the energy limit of 80 MJ.

Performance of Wendelstein 7-X stellarator plasmas during the first divertor operation phase

The first physics operation phase on the stellarator experiment Wendelstein 7-X was successfully completed in March 2016 after about 10 weeks of operation Experiments in this phase were conducted with five graphite limiters as the primary plasma-facing components Overall, the results were beyond the expectations published shortly before the start of operation [Sunn Pedersen et al, Nucl Fusion 55, 126001 (2015)] both with respect to parameters reached and with respect to physics themes addressed We report here on some of the most important plasma experiments that were conducted The importance of electric fields on global confinement will be discussed, and the obtained results will be compared and contrasted with results from other devices, quantified in terms of the fusion triple product Expected values for the triple product in future operation phases will also be described and put into a broader fusion perspective

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

Wendelstein 7-X (W7-X) represents the continuation of fusion experiments of the stellarator type at the Max-Planck Institute for Plasma Physics (IPP). The aim of W7-X is to demonstrate the suitability for a fusion reactor of this alternative type of magnetically confined plasma experiment. W7-X is being built at Greifswald in the northeast of Germany. The size of device (725 tons, height of 5 m, diameter 16 m) and the superconductive magnet system distinguish W7-X from earlier stellarators at IPP. The paper provides a summary of the status of the main components, the mastering of the technical challenges during component acceptance testing and during machine assembly. Latest results of the assembly work are especially highlighted. The scope of the construction of W7-X was modified and additional acceleration measures were implemented to mitigate risks and delays. Some aspects of these changes are explained in this paper.

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Status of Wendelstein 7-X construction

With the aim of starting plasma operation in 2014 the Wendelstein 7-X (W7-X) project has developed a scenario with an intermediate operational phase in which a reduced number of in-vessel components are installed prior to going to the fully equipped, full performance phase. An important part of this scenario is the test divertor (TDU), an adiabatically cooled, reduced pulse length device that will allow machine operation over a comparable range of plasma configurations as the final long pulse high-heat flux (HHF) divertor, which will be installed after two years operation of the TDU. The design and development of the TDU has started by clearly defining the requirements of the TDU with regard to performance, material selection, geometry, interfaces to other components, diagnostic integration, installation requirements and tolerances and quality management. In fact, the whole TDU development will follow the procedures laid down for design and development of components within the W7-X project. This paper provides a description of the design and development of the TDU to date.

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Progress in the design and development of a test divertor (TDU) for the start of W7-X operation

Engineering design for the magnetic diagnostics of Wendelstein 7-X

The Wendelstein 7-X (W7-X) modular stellarator is in the assembly phase at the Max-Planck-Institut fur Plasmaphysik (IPP) in Greifswald, Germany. The goal of the project is to demonstrate that this type of machine is a viable option for a fusion power-plant. The “pentagonal” magnet system of the machine encompasses 50 non-planar and 20 planar superconducting coils with sophisticated support structure. Structural reliability of components as well as resulting deformations and displacements during various modes of operation have to be considered not only for the magnet system but also throughout the whole cryostat whose main components are the plasma vessel, outer vessel, ports, and thermal insulation. A reliable prediction of the W7-X structural behaviour is only possible by employing complex finite element (FE) simulations with a hierarchical set of FE models. A special strategy has been developed and implemented for the task. The design is basically completed, main parameters are defined, and most of the W7-X components are manufactured. Therefore, the focus in the analysis is being shifted to the creation of parametric FE models which allow performing fast analyses of possible non-conformities, changes in the assembly procedure, and future exploration of operational limits. This paper gives an overview of the implemented analysis strategy, the applied safety margins, and focuses on the most remarkable results.

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Structural analysis of W7-X: Overview

Thermo-mechanical analysis of Wendelstein 7-X plasma facing components

For long-pulse plasma operation of the W7-X stellarator, one of the most serious challenges for the design of in-vessel diagnostic systems is the thermo-mechanical problem. Thermal load from convective losses and from plasma radiation can reach up to 500 kW/m 2 at some locations close to plasma. The typical thermal load from plasma radiation alone ranges from 10 kW/m 2 to 100 kW/m 2 as derived from 3D Monte-Carlo simulations. Most plasma facing diagnostic components require active cooling for the long-pulse plasma operation. Especially the design of optical components must be optimized to minimize thermal deformations. Finite element analyses (FEA) are performed for better understanding of thermo-mechanical problems on plasma facing components of diagnostics, and to guide the design of the diagnostic system. As examples the paper presents the FEA results of the Corner-Cube Retro-Reflectors for the CO 2 -laser interferometer and the movable pop-up probe arrays in the divertor.

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M. Y. Ye

Papers

Progress in the design and development of a test divertor (TDU) for the start of W7-X operation

Engineering design for the magnetic diagnostics of Wendelstein 7-X

Structural analysis of W7-X: Overview

Thermo-mechanical analysis of Wendelstein 7-X plasma facing components

Thermo-mechanical analysis of plasma facing components of diagnostics in the Wendelstein 7-X stellarator