Understanding detachment of the W7-X island divertor

Abstract

The fundamental behavior of the W7-X island divertor under detached conditions, which has been theoretically predicted with the EMC3-Eirene code, is re-examined here under the experimental conditions achieved so far and compared with the first experimental results. Both simulations and experiments cover a range of divertor configurations and plasma parameters, and show the following common trends: (1) with rising impurity radiation, the target heat load decreases ‘uniformly’ over the entire target surface in the sense that both the peak and average heat loads can drop by an order of magnitude. Impurity radiation (mainly from intrinsic carbon) occurs primarily at the plasma edge and the resulting negative impact on the stored energy is less than 10%. (2) When the total radiation exceeds a critical level, the target particle flux (the recycling flux Γrecy) begins to fall and can drop by a factor of 3–5 at high radiation levels without an obvious indication of significant volume recombination. (3) While Γrecy decreases, the divertor neutral pressure continues to build up and reaches a maximum, at which point Γrecy has declined significantly. (4) During detachment, the electron temperature at the last closed flux surface falls in a way that is not quantitatively understandable from parallel classical heat conduction processes. This paper presents a physical explanation of the numerical/experimental results described above. Furthermore, using the EMC3-Eirene code as a diagnostic tool, we are able, apparently for the first time, to provide a full quantitative analysis of each transport channel in the island divertor, aiming to clarify how the island divertor plasma self-regulates to maintain particle, energy, and momentum balance under detached conditions.

First attempt to quantify W7-X island divertor plasma by local experiment-model comparison

“…To facilitate direct comparison, the TS data along the laser beam is mapped to the radial grid coordinates defined by cylindrically approximating the volume enclosed by each radial grid surface. This approach is meaningful because the radial grid surfaces follow the basic shape of the magnetic flux surfaces formed by the 3D coils [1]. The comparison results for the attached case (t1) are displayed in figure 11.…”

“…The work in this paper fills this gap by comparing EMC3-Eirene simulations with different local diagnostics. A detailed illustration of the discharge history can be found in [1]. Here, we give a brief description of the basic behavior of the most relevant plasma parameters.…”

“…A combination with the configuration variation yields three simulation series, as listed in table 1. Further details of the simulation setup and a more comprehensive Computation setup of three simulation series (derived from [1]).…”

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As a complement to our recent work [1], which focused on understanding the basic detachment physics and general experimental and numerical trends observed in the W7-X island divertor, this paper compares EMC3-Eirene simulation results with different local diagnostics, including IR cameras, Langmuir probes, H α -cameras, and Thomson scattering. The main purposes are to (1) justify the simulation setup in the previous work, (2) identify the application limitations of the current EMC3-Eirene model, (3) verify the consistency of different diagnostics, and (4) isolate the main geometric and physical effects that need to be prioritized in further developing the EMC3-Eirene code and improving diagnostic capabilities.

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Turbulent transport in the scrape-off layer of Wendelstein 7-X

“…Even within uncertainties of up to 50% due to spectral filter choices and probe pin configurations, these values are smaller than typical tokamak experimental results or modelling assumptions [30,32,35,37]. In EMC3-EIRENE modelling of W7-X, D is typically assumed between 0.5 m 2 s −1 -1.5 m 2 s −1 to obtain plausible modelling results [36,38,39]. Therefore, the level of turbulent transport observed here advocates to use 0.5 m 2 s −1 or less in EMC3-EIRENE simulations.…”