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

Zhang, D; König, R.; Feng, Y.; Burhenn, R.; Brezinsek, S.; Jakubowski, M.; Buttenschön, B.; Niemann, H.; Pavone, A.; Krychowiak, M.; Kwak, S.; Svensson, J.; Gao, Yu; Pedersen, T. S.; Alonso, A.; Baldzuhn, J.; Beidler, C. D.; Biedermann, C.; Bozhenkov, S.; Brunner, K. J.; Damm, H.; Hirsch, M.; Giannone, L.; Drewelow, P.; Effenberg, F.; Fuchert, G.; Hammond, K. C.; Höfel, U.; Killer, C.; Knauer, J.; Laqua, H. P.; Laube, R.; Pablant, N.; Pasch, E.; Penzel, F.; Rahbarnia, K.; Reimold, F.; Thomsen, H.; Winters, V.; Wagner, F.; Klinger, T.; Team, W X

doi:10.1103/physrevlett.123.025002

Cited by 36 publications

(43 citation statements)

References 35 publications

(26 reference statements)

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“…Plasma radiation is measured by two bolometer cameras-each with arrays of detectors that have a 5 μm blackened gold-foil absorberinstalled in one of the mid-field-period toroidal-symmetry planes of W7-X, and with spatial resolution of between 3 and 4 cm (ref. 55 ). Line-integrated signals from 65 channels are used to obtain radiation intensity distributions by tomographic reconstruction with 'relative gradient smoothing' as regularization functional (to be published).…”

Section: Data Availabilitymentioning

confidence: 99%

Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

Beidler

Smith

Alonso³

et al. 2021

Nature

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Research on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X)2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator’s non-turbulent ‘neoclassical’ energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lacking a comparably high level of neoclassical optimization.

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Section: Data Availabilitymentioning

confidence: 99%

Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

Beidler

Smith

Alonso³

et al. 2021

Nature

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“…The overall radiation measured by bolometer decreased by about six times as shown in Fig. 5 [13]. The total radiation before boronization was quite larger and more discrete than the radiation after boronization.…”

Section: Impurity Identificationmentioning

confidence: 86%

Impurity sources and fluxes in W7-X: from the plasma-facing components to the edge layer

Wang¹,

Brezinsek²,

Sereda³

et al. 2020

Phys. Scr.

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Wendelstein 7-X (W7-X) is a nearly full-carbon machine with graphite divertors, baffles and shields in Operation Phase 1.2b (OP 1.2b). Divertor spectrometer measurements showed that an amount of helium and oxygen impurities existed in the predominately hydrogen plasma, which resulted in a high carbon impurity level by enhanced physical and chemical sputtering by these impurities in comparison with the pure impinging proton yields. In order to improve the wall conditions, especially to reduce the oxygen content, boronizations were applied in OP1.2b. After the boronization, an oxygen decrease by more than an order of magnitude was observed. Helium disappeared in comparison with OP1.2a due to reduced application of helium wall conditioning after introduction of boronizations. The overall radiation normalized to line integrated density was reduced by a factor of six. In addition, local CH4 injection was applied in the divertor in order to quantify the chemical sputtering by hydrogen on divertor plates. The experimentally determined effective D/XB of the A-X band of CH resulting from CH4 was [ 𝐷 𝑋𝐵 ] 𝐴 2 ∆→𝑋 2 Π 𝐶𝐻 4 →𝐶𝐻 = 16 at Te ≈ 20 eV and ne ≈ 5e18 m -3 . It was applied to determine the hydrocarbon fluxes and further to deduce the particle flux ratio Г CH4 /Г H on divertor plates.

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“…In a steady-state regime, W7-X will operate with continuous pellet fueling. It was rather straightforward to transition into the detachment with a sequence of pellets raising the plasma density [76]. Also an experiment was performed to see if it is possible to achieve stable detached scenarios with the plasma density stabilized with low-frequency pellet injection.…”

Section: Detachment In Pellet Fueled Dischargementioning

confidence: 99%

Overview of the results from divertor experiments with attached and detached plasmas at Wendelstein 7-X and their implications for steady-state operation

et al. 2021

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Wendelstein 7-X (W7-X), the largest advanced stellarator, is built to demonstrate high power, high performance quasi-continuous operation. Therefore, in the recent campaign, experiments were performed to prepare for long pulse operation, addressing three critical issues: the development of stable detachment, control of the heat and particle exhaust, and the impact of leading edges on plasma performance. The heat and particle exhaust in W7-X is realized with the help of an island divertor, which utilizes large magnetic islands at the plasma boundary. This concept shows very efficient heat flux spreading and favourable scaling with input power. Experiments performed to overload leading edges showed that the island divertor yields good impurity screening. A highlight of the recent campaign was a robust detachment scenario, which allowed reducing power loads even by a factor of ten. At the same time, neutral pressures at the pumping gap entrance yielded the particle removal rate close to the values required for stable density control in steady-state operation.

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

Cited by 36 publications

References 35 publications

Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

Impurity sources and fluxes in W7-X: from the plasma-facing components to the edge layer

Overview of the results from divertor experiments with attached and detached plasmas at Wendelstein 7-X and their implications for steady-state operation

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