Plasma radiation behavior approaching high-radiation scenarios in W7-X

Zhang, D.; Burhenn, R.; Feng, Y.; König, R.; Buttenschön, B.; Beidler, C. D.; Hacker, P.; Reimold, F.; Thomsen, H.; Laube, R.; Klinger, T.; Giannone, L.; Penzel, F.; Pavone, A.; Krychowiak, M.; Beurskens, M.; Bozhenkov, S.; Brunner, Jakob; Effenberg, F.; Fuchert, G.; Gao, Yu; Geiger, J.; Hirsch, M.; Höfel, U.; Jakubowski, M.; Knauer, J.; Kwak, S.; Laqua, H. P.; Niemann, H.; Otte, M.; Pedersen, T. S.; Pasch, E.; Pablant, N.; Svensson, J.; Blackwell, B. D.; Drews, P.; Endler, M.; Rudischhauser, L.; Wang, E.; Weir, G.; Winters, V.

doi:10.1088/1741-4326/ac2b75

Cited by 9 publications

(8 citation statements)

References 52 publications

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“…The dashed vertical lines mark the selected time points for performing bolometer tomography. reduction of impurity radiation from the core at medium f rad is also observed in discharges conducted after wall boronization, which have a lower Z eff value, usually below 1.5 [55]. We believe that the evident core impurity radiation in figures 10(a) and (d) is related to the weak screening effects of the magnetic islands on both impurities and neutral hydrogen at the low edge densities of 0.6-0.8 × 10 19 m −3 (see figure 8) [28].…”

Section: Tomographic Reconstructionsmentioning

confidence: 78%

Bolometer tomography on Wendelstein 7-X for study of radiation asymmetry

et al. 2021

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The algorithm for bolometer tomography at Wendelstein 7-X (W7-X) has been recently improved using a novel regularization functional, based on relative gradient smoothing of the sought radiation profile. It has been validated using radiation patterns provided by 3D modeling under real plasma conditions as phantoms and then applied to bolometer measurements performed during the first divertor operation phase of W7-X. The following results are presented: (1) edge-localized 2D radiation patterns with clearly resolved magnetic island radiation structures, (2) an up-down asymmetry in the impurity radiation that is not captured by the 3D edge plasma transport modeling, (3) reversal of the asymmetry with reversed magnetic field direction. Further analysis reveals a poloidal variation of the emissivity in the outer confined plasma region with a field-direction dependent asymmetry, also supported by the soft x-ray measurements. This asymmetry is considered to be related to asymmetric impurity distributions, driven by the pronounced ion-impurity friction force at the plasma edge where the collisionality of the W7-X plasma is sufficiently high to develop impurity asymmetry as predicted by neoclassical theory of parallel impurity transport.

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Section: Tomographic Reconstructionsmentioning

confidence: 78%

Bolometer tomography on Wendelstein 7-X for study of radiation asymmetry

et al. 2021

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“…The energy transfer between the electrons and ions Q e-i is a sink for the electrons and a source for the ions, and the power density profile p e-i is calculated from the kinetic profiles using equation (2.1). The radiated heat flux Q rad is the volume-integrated power density profile p rad obtained from a poloidal integration of the tomographic reconstruction of the bolometer multiple lines of sight [47,48]. Finally, a sink for the ions is the charge exchange loss Q cx between the hot ions and cold neutrals entering the plasma.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

“…The ECRH heat deposition profile is calculated from the launch geometry using VMEC for equilibrium and the TRAVIS code for ray tracing [39]. The radiation loss profile is estimated from bolometer inversion profiles and is located towards the edge of the plasma [47,48], as can be seen in appendix A. The stored energy and normalized pressure of this plasma are W th = 0.5 MJ with a normalized pressure of β ∼ 0.5%, and τ E,exp /τ ISS04 = 0.65.…”

Section: Neoclassical Transport Compared To Experimental Findingsmentioning

confidence: 99%

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

et al. 2021

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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 T e can be varied freely from 1 keV-10 keV within the range of P ECRH = 1-7 MW, with electron density n e values from 0.1-1.5 × 10 20 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 T i ∼ 1.5 keV ± 0.2 keV. In a dedicated ECRH power scan at a constant density of n e = 7 × 10 19 m −3 , an apparent 'negative ion temperature profile stiffness' was found in the central plasma for (r/a < 0.5), in which the normalized gradient ∇T i /T i 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 'negative stiffness' is due to a strong exacerbation of turbulent transport with an increasing ratio of T e /T i 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 T e /T i . The exacerbation of turbulence with increasing T e /T i is also found in tokamaks and inherently enhances ion heat transport in electron-heated

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“…as one of its possible actuators in future experiments. Additional radiation scaling studies for W7-X based directly on bolometer data can be found in [34].…”

Section: Dependency Of the C 2+ Radiation Intensity On The Electron D...mentioning

confidence: 99%

2D measurements of parallel counter-streaming flows in the W7-X scrape-off layer for attached and detached plasmas

et al. 2021

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Investigations of particle parallel flow velocities have been carried out for the scrape-off layer (SOL) of the Wendelstein 7-X (W7-X) stellarator, in order to gain insights on the SOL transport properties during attached and detached plasma scenarios. The experimental evidence is based on the coherence imaging spectroscopy (CIS) diagnostic, able to measure 2D impurity emission intensity and flow velocity. The impurity monitored by CIS is C2+, characterized by a line-emission intensity observed to be linearly proportional to the total plasma radiated power in both attached and detached plasmas. The related C2+ velocity shows a strong dependence on the line-averaged electron density while remaining insensitive to the input power. During attached plasmas, the velocity increases with increasing line-averaged density. The tendency reverses in the transition to and during detachment, in which the velocity decreases by at least a factor of 2. The sharp drop in velocity, together with a rise in line-emission intensity, is reliably correlated to the detachment transition and can therefore be used as one of its signatures. The impurity flow velocity appears to be well coupled with the main ions’ one, thus implying the dominant role of impurity-main ion friction in the parallelimpurity transport dynamics. In view of this SOL impurity transport regime, the CIS measurement results are here interpreted with the help of EMC3-Eirene simulations, and their major trends are already explainable with a simple 1D fluid model.

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Plasma radiation behavior approaching high-radiation scenarios in W7-X

Cited by 9 publications

References 52 publications

Bolometer tomography on Wendelstein 7-X for study of radiation asymmetry

Bolometer tomography on Wendelstein 7-X for study of radiation asymmetry

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

2D measurements of parallel counter-streaming flows in the W7-X scrape-off layer for attached and detached plasmas

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