The optimized, superconducting stellarator Wendelstein 7-X went into operation and delivered first measurement data after 15 years of construction and one year commissioning. Errors in the magnet assembly were confirmend to be small. Plasma operation was started with 5 MW electron cyclotron resonance heating (ECRH) power and five inboard limiters. Core plasma values of keV, keV at line-integrated densities were achieved, exceeding the original expectations by about a factor of two. Indications for a core-electron-root were found. The energy confinement times are in line with the international stellarator scaling, despite unfavourable wall conditions, i.e. large areas of metal surfaces and particle sources from the limiter close to the plasma volume. Well controlled shorter hydrogen discharges at higher power (4 MW ECRH power for 1 s) and longer discharges at lower power (0.7 MW ECRH power for 6 s) could be routinely established after proper wall conditioning. The fairly large set of diagnostic systems running in the end of the 10 weeks operation campaign provided first insights into expected and unexpected physics of optimized stellarators.

https://iopscience.iop.org/article/10.1088/0741-3335/59/1/014018/pdf

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

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

Fusion energy research has in the past 40 years focused primarily on the tokamak concept, but recent advances in plasma theory and computational power have led to renewed interest in stellarators. The largest and most sophisticated stellarator in the world, Wendelstein 7-X (W7-X), has just started operation, with the aim to show that the earlier weaknesses of this concept have been addressed successfully, and that the intrinsic advantages of the concept persist, also at plasma parameters approaching those of a future fusion power plant. Here we show the first physics results, obtained before plasma operation: that the carefully tailored topology of nested magnetic surfaces needed for good confinement is realized, and that the measured deviations are smaller than one part in 100,000. This is a significant step forward in stellarator research, since it shows that the complicated and delicate magnetic topology can be created and verified with the required accuracy.

https://pure.tue.nl/ws/files/53837977/ncomms13493.pdf

Confirmation of the topology of the Wendelstein 7-X magnetic field to better than 1:100,000

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

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

For the Wendelstein 7-X stellarator, an open divertor was chosen as a first step in divertor development for the expected extended magnetic and plasma parameter range. Particularly, the three-dimensional (3-D) geometry of the boundary and the provided stationary operation are challenges for the design. So far, simplified models have been used to specify the geometry of the divertor and the performance of the high-heat-load surfaces. By applying the 3-D codes that are now available, the results concerning local heat load and particle exhaust can have more detailed evaluation and can be confirmed generally. Together with the development of improved high-heat-load components, a significant reduction of the target area in comparison with the previous design is possible. The new specifications will be characterized.

Physical Aspects and Design of the Wendelstein 7-X Divertor

Wendelstein 7-X, currently under commissioning at the Max-Planck-Institut fur Plasmaphysik in Greifswald, Germany, is a modular advanced stellarator, combining the modular coil concept with optimized properties of the plasma. Most of the envisaged magnetic configurations of the machine are rather sensitive to symmetry breaking perturbations which are the consequence of unavoidable manufacturing and assembly tolerances. This overview describes the successive tracking of the Wendelstein 7-X magnet system geometry starting from the manufacturing of the winding packs up to the modelling of the influence of operation loads. The deviations found were used to calculate the resulting error fields and to compare them with the compensation capacity of the trim coils.

https://iopscience.iop.org/article/10.1088/0029-5515/55/6/063025/pdf

Tracking of the magnet system geometry during Wendelstein 7-X construction to achieve the designed magnetic field

The magnetic configurations of the Wendelstein 7-X (W7-X) stellarator are sensitive to perturbations of the magnetic field resonant with ι`/2π = 1. Such perturbations can be caused by deviations of the current filament positions of the real coil system from the design due to the accuracy achievable during the manufacture of the coils and assembly of the magnet system. The sensitivity of the magnetic field to the different types of error has been investigated by introducing randomly distributed errors to the coil shapes and positions within the given tolerances. A statistical analysis of these error distributions was performed. This procedure will be used to assess the magnetic configuration of W7-X before the completion of each assembly step.

Analysis of the Magnetic Field Perturbations During the Assembly of Wendelstein 7-X

The in-vessel components of Wendelstein 7-X (W7-X) with a total surface of 265 m 2 comprise the divertor and the wall protection. The high heat flux (HHF) and lower heat flux (LHF) target, the baffle, the end plates closing the divertor chamber, a cryo vacuum pump (CVP) and a control coil form one divertor unit. Steel panels and the graphite heat shield protect the wall, including the ports. The HHF target elements, the steel panels and the control coils are manufactured by industry. The remaining components will be manufactured by the Max-Planck-Institute fur Plasmaphysik (IPP) at its Garching workshops. For all components the final acceptance tests will be performed by IPP. This paper summarizes the main aspects for manufacturing, the preceding development and qualification tests as well as the final acceptance tests for the in-vessel components.

J. Kißlinger

Papers

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

Physical Aspects and Design of the Wendelstein 7-X Divertor

Tracking of the magnet system geometry during Wendelstein 7-X construction to achieve the designed magnetic field

Analysis of the Magnetic Field Perturbations During the Assembly of Wendelstein 7-X

Manufacturing of the Wendelstein 7-X divertor and wall protection