Analysis of the neutral fluxes in the divertor region of Wendelstein 7-X under attached and detached conditions using EMC3-EIRENE

This paper analyzes the neutral fluxes in the divertor region of the W7-X standard configuration for different input powers, both under attached and detached conditions. The performed analysis is conducted through EMC3-EIRENE simulations. They show the importance of the horizontal divertor to generate neutrals, and resolve the neutral plugging in the divertor region. Simulations of detached cases show a decrease in the number of generated neutrals compared to the attached simulations, in addition to a higher fraction of the ion flux arriving on the baffles during detachment. As the ionization takes place further inside the plasma during detachment, a larger percentage of the generated neutral particles leave the divertor as neutrals. The leakage in the poloidal and toroidal direction increases, just as the fraction of collected particles at the pumping gap. The fraction of pumped particles increases with a factor two, but stays below one percent. This demonstrates that detachment with the current target geometry, although it improves the power exhaust, is not yet leading to an increased particle exhaust

Ion temperature clamping in Wendelstein 7-X electron cyclotron heated plasmas

The neoclassical transport optimization of the Wendelstein 7-X stellarator has not resulted in the predicted high energy confinement of gas fueled electron-cyclotron-resonance-heated (ECRH) plasmas as modelled in (Turkin et al 2011 Phys. Plasmas 18 022505) due to high levels of turbulent heat transport observed in the experiments. The electron-turbulent-heat transport appears non-stiff and is of the electron temperature gradient (ETG)/ion temperature gradient (ITG) type (Weir et al 2021 Nucl. Fusion 61 056001). As a result, the electron temperature Te can be varied freely from 1 keV–10 keV within the range of PECRH = 1–7 MW, with electron density ne values from 0.1–1.5 × 1020 m−3. By contrast, in combination with the broad electron-to-ion energy-exchange heating profile in ECRH plasmas, ion-turbulent-heat transport leads to clamping of the central ion temperature at Ti ∼ 1.5 keV ± 0.2 keV. In a dedicated ECRH power scan at a constant density of 〈ne〉 = 7 × 1019 m−3, an apparent \u27negative ion temperature profile stiffness\u27 was found in the central plasma for (r/a < 0.5), in which the normalized gradient ∇Ti/Ti decreases with increasing ion heat flux. The experiment was conducted in helium, which has a higher radiative density limit compared to hydrogen, allowing a broader power scan. This \u27negative stiffness\u27 is due to a strong exacerbation of turbulent transport with an increasing ratio of Te/Ti in this electron-heated plasma. This finding is consistent with electrostatic microinstabilities, such as ITG-driven turbulence. Theoretical calculations made by both linear and nonlinear gyro-kinetic simulations performed by the GENE code in the W7-X three-dimensional geometry show a strong enhancement of turbulence with an increasing ratio of Te/Ti. The exacerbation of turbulence with increasing Te/Ti is also found in tokamaks and inherently enhances ion heat transport in electron-heated plasmas. This finding strongly affects the prospects of future high-performance gas-fueled ECRH scenarios in W7-X and imposes a requirement for turbulence-suppression techniques

Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

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

Bayesian modelling of a thermal helium beam for measurement of electron density and temperature in the W7-X divertor plasma

In Greifswald/Germany W7-X, a new stellarator-type fusion plasma experiment, is currently being built. For the investigation of the divertor plasma two thermal helium beams are foreseen. This diagnostic is routinely used on several fusion plasma experiments and is capable of measuring radial profiles of electron density and temperature with good spatial and temporal resolution in the range of typical edge plasma parameters ne = 1018–1019 m−3 and Te = 20–200 eV. The penetration depth of the beam is limited by electron collisional ionization of the helium atoms and amounts to 3–8 cm in this parameter range. In this paper we investigate the beam propagation for detached plasma conditions in the W7-X divertor region (based on a background plasma simulated with a 3D plasma and neutral transport code EMC3/EIRENE), in which the electron density in the divertor may well exceed 1020 m−3, as observed in the predecessor experiment W7-AS. In this regime the beam penetration drops to 1–2 cm. Through a Bayesian approach, we include uncertainties of all rate coefficients for electronic excitation and ionization used in the collisional–radiative model of atomic helium based on a steady-state approximation valid for a relaxed thermal or supersonic beam. Bayesian inversion of simulated signals for W7-X conditions provides a reliable quantitative estimation of the propagation of uncertainties of the atomic data to the ne and Te errors as well as input for potential improvements of the diagnostic setup. For example, the temperature error at Te = 5 eV and ne = 1020 m−3 can be reduced from approximately 50% to 9% by absolute calibration of the observation system and fitting of three absolute line intensities instead of two line intensity ratios to the model

Endoscopes for observation of plasma-wall interactions in the divertor of Wendelstein 7-X

The stellarator Wendelstein 7-X is being prepared for long pulse operation. This includes diagnostics for investigation of plasma wall interaction processes. A versatile optical observation system has been developed for local characterization of the divertor plasma and the divertor target surface. The optical systems consist of two endoscopes each with perpendicular fields of view and the opportunity of tomographic reconstruction. Mirror based optics has been chosen in order to assure good optical properties independent of the wavelength. A narrow field of view allows for high spatial resolution while rotation of the first mirror covers the full poloidal divertor sections. An integrated shutter mechanism and a vacuum window far back minimize coating of optical components. For assessment of change of light transmission, a relative calibration function is implemented. The output light is split into wavelength ranges. Both, cameras equipped with narrow band filters as well as spectrometers are connected. The first endoscope was mounted at W7-X after successfully passing mechanical, optical and functional tests

通过 3-D 建模分析 Wendelstein 7-X 启动情景的螺旋刮层中的热通量特性

International audienceA crucial topic for the stellarator W7-X is the power dissipation by impurities for future island divertor scenarios. The investigation of the related heat flux distribution and profiles including the radial power fall-off length λq in the 3D stellarator SOL is less straight forward as in toroidally symmetric tokamaks. Studies with the 3D plasma edge transport code EMC3-Eirene predicted a modulation of plasma parameters with LC and correlated heterogenous heat and particle loads onto the limiters during start-up operation. The relative simple start up geometry at W7-X allows for a detailed analysis of the heat fluxes in separate helical transport channels featuring different ∥ to ⊥ transport ratios. It is shown that the SOL has two characteristic fall off domains - a near SOL and a far SOL domain which both have different power decay lengths. An increase of λq with LC in the order of ~1-1.5cm in the near SOL and ~1.8-2.8cm in the far SOL for a power scan in the range of P=0.5-2MW at nLCFS=2×1018m−3 has been found. First comparisons with IR camera data will be discussed.Un sujet crucial pour le stellarator W7-X est la dissipation de puissance par les impuretés pour les futurs scénarios de divertor insulaire. L'étude de la distribution et des profils de flux de chaleur associés, y compris la longueur de chute de puissance radiale λq dans le SOL du stellarator 3D, est moins simple que dans les tokamaks à symétrie toroïdale. Des études avec le code de transport de bord de plasma 3D EMC3-Eirene ont prédit une modulation des paramètres du plasma avec LC et des charges de chaleur et de particules hétérogènes corrélées sur les limiteurs pendant le démarrage. La géométrie de démarrage relativement simple à W7-X permet une analyse détaillée des flux de chaleur dans des canaux de transport hélicoïdaux séparés présentant différents rapports de transport ∥ à ⊥. Il est montré que le SOL a deux domaines de décroissance caractéristiques - un domaine SOL proche et un domaine SOL lointain qui ont tous deux des longueurs de décroissance de puissance différentes. Une augmentation de λq avec LC de l'ordre de ~1-1,5 cm dans le SOL proche et de ~1,8-2,8 cm dans le SOL lointain pour un balayage de puissance dans la plage de P=0,5-2MW à nLCFS=2×1018m−3 a été trouvé. Les premières comparaisons avec les données des caméras IR seront discutées

通过 3-D 建模分析 Wendelstein 7-X 启动情景的螺旋刮层中的热通量特性

International audienceA crucial topic for the stellarator W7-X is the power dissipation by impurities for future island divertor scenarios. The investigation of the related heat flux distribution and profiles including the radial power fall-off length λq in the 3D stellarator SOL is less straight forward as in toroidally symmetric tokamaks. Studies with the 3D plasma edge transport code EMC3-Eirene predicted a modulation of plasma parameters with LC and correlated heterogenous heat and particle loads onto the limiters during start-up operation. The relative simple start up geometry at W7-X allows for a detailed analysis of the heat fluxes in separate helical transport channels featuring different ∥ to ⊥ transport ratios. It is shown that the SOL has two characteristic fall off domains - a near SOL and a far SOL domain which both have different power decay lengths. An increase of λq with LC in the order of ~1-1.5cm in the near SOL and ~1.8-2.8cm in the far SOL for a power scan in the range of P=0.5-2MW at nLCFS=2×1018m−3 has been found. First comparisons with IR camera data will be discussed.Un sujet crucial pour le stellarator W7-X est la dissipation de puissance par les impuretés pour les futurs scénarios de divertor insulaire. L'étude de la distribution et des profils de flux de chaleur associés, y compris la longueur de chute de puissance radiale λq dans le SOL du stellarator 3D, est moins simple que dans les tokamaks à symétrie toroïdale. Des études avec le code de transport de bord de plasma 3D EMC3-Eirene ont prédit une modulation des paramètres du plasma avec LC et des charges de chaleur et de particules hétérogènes corrélées sur les limiteurs pendant le démarrage. La géométrie de démarrage relativement simple à W7-X permet une analyse détaillée des flux de chaleur dans des canaux de transport hélicoïdaux séparés présentant différents rapports de transport ∥ à ⊥. Il est montré que le SOL a deux domaines de décroissance caractéristiques - un domaine SOL proche et un domaine SOL lointain qui ont tous deux des longueurs de décroissance de puissance différentes. Une augmentation de λq avec LC de l'ordre de ~1-1,5 cm dans le SOL proche et de ~1,8-2,8 cm dans le SOL lointain pour un balayage de puissance dans la plage de P=0,5-2MW à nLCFS=2×1018m−3 a été trouvé. Les premières comparaisons avec les données des caméras IR seront discutées