Observation of Marfes in the Wendelstein 7-X stellarator with inboard limiters

Wenzel, U.; Biedermann, C.; Kocsis, G.; Szepesi, T.; Blackwell, B. D.; Klose, S.; Knauer, J.; Krychowiak, M.; Stephey, L.; Schmitz, O.; Harris, J. H.; König, R.; Pedersen, T. S.

doi:10.1088/1741-4326/aacb86

Cited by 5 publications

(3 citation statements)

References 27 publications

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“…The beginning of the end of the plasma features a pronounced neutral pressure spike, and an initially poloidally localized, rotating radiation belt before the plasma shrinks and extinguishes itself. This radiation belt is similar to the MARFE observed in tokamaks and it is described in detail in [31].…”

Section: Analysis Of N 2 Seeded Dischargessupporting

confidence: 77%

Radiative edge cooling experiments in Wendelstein 7-X start-up limiter campaign

et al. 2019

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Impurity seeding experiments during the start-up limiter campaign of Wendelstein 7-X provide first evidence for a localization effect of the three-dimensional magnetic edge structure on the seeded impurities and their radiation distribution and cooling effect. Moreover, species dependencies have been seen. Nitrogen was observed to cool the entire edge plasma, with a stronger electron temperature reduction measured at the downstream position at the limiter. The radiation was limited at the periphery of the confined region. Both the temperature reduction and the radiation enhancement were directly correlated to the injection of the coolant gas. Mitigation of the limiter heat loads was also measured. Neon was observed to affect also the confined plasma with a long-lasting radiation and a cooling of the entire plasma.

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Section: Analysis Of N 2 Seeded Dischargessupporting

confidence: 77%

Radiative edge cooling experiments in Wendelstein 7-X start-up limiter campaign

et al. 2019

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“…The latter typically occurs at low plasma temperatures, where the radiative cooling coefficient is increasing with decreasing temperature (for light impurities like carbon and oxygen the relevant temperature range for this is a few hundred eV). If this happens only locally, it can lead to formation of a Marfe [10], which has also been observed in W7-X [11]. In both cases, the stored energy decreases close to the collapse since a large fraction of the heating power is lost as radiation and is, therefore, not available to the plasma.…”

Section: Introductionmentioning

confidence: 73%

Increasing the density in Wendelstein 7-X: benefits and limitations

et al. 2020

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In stellarators, increasing the density is beneficial for the energy confinement. While there is no single reason for this observation, it is still very robust across different devices and this is reflected in the empirical energy confinement time scaling for stellarators, ISS04. In order to study whether this is also true for Wendelstein 7-X, the density scaling of the energy confinement time is analyzed and compared to ISS04 for the first divertor experiments. When the density is increased beyond a critical density, however, radiative collapses are frequently observed. Existing analytical models for the critical density are revisited to assess whether they can predict the accessible density range. Furthermore, since close to the collapse the radiation losses increase substantially, the impact on the global energy confinement is investigated. It is found that in plasmas with high radiation the density scaling of the energy confinement time becomes weaker, the reason for this observation is not yet clear. In the second half of the first divertor campaign, boronization was applied to W7-X for the first time. This broadened the operational window, allowing for operation at higher density and, hence, higher stored energy.

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“…A credible path to high performance in stellarators is to increase the plasma density. Often the achievable density, and the control of it, is limited by edge radiation instabilities that occur at low temperatures, usually referred to as MARFEs [23] and also seen in W7-X limiter operation [24]. Reducing the interaction between the edge plasma and edge neutrals often allows higher densities to be reached, and therefore localized and penetrating fueling methods such as pellets or supersonic gas injection with nozzles placed close to the last closed flux-surface are advantageous.…”

Section: Fueling and Detachmentmentioning

confidence: 99%

First divertor physics studies in Wendelstein 7-X

Pedersen¹,

König²,

Jakubowski³

et al. 2019

Nucl. Fusion

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The Wendelstein 7-X (W7-X) optimized stellarator fusion experiment, which went into operation in 2015, has been operating since 2017 with an un-cooled modular graphite divertor. This allowed first divertor physics studies to be performed at pulse energies up to 80 MJ, as opposed to 4 MJ in the first operation phase, where five inboard limiters were installed instead of a divertor. This, and a number of other upgrades to the device capabilities, allowed extension into regimes of higher plasma density, heating power, and performance overall, e.g. setting a new stellarator world record triple product. The paper focuses on the first physics studies of how the island divertor works. The plasma heat loads arrive to a very high degree on the divertor plates, with only minor heat loads seen on other components, in particular baffle structures built in to aid neutral compression. The strike line shapes and locations change significantly from one magnetic configuration to another, in very much the same way that codes had predicted they would. Strike-line widths are as large as 10 cm, and the wetted areas also large, up to about 1.5 m 2 , which bodes well for future operation phases. Peak local heat loads onto the divertor were in general benign and project below the 10 MW/m 2 limit of the future water-cooled divertor when operated with 10 MW of heating power, with the exception of low-density attached operation in the high-iota Submitted to Nuclear Fusion configuration. The most notable result was the complete (in all 10 divertor units) heat-flux detachment obtained at highdensity operation in hydrogen.

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Observation of Marfes in the Wendelstein 7-X stellarator with inboard limiters

Cited by 5 publications

References 27 publications

Radiative edge cooling experiments in Wendelstein 7-X start-up limiter campaign

Radiative edge cooling experiments in Wendelstein 7-X start-up limiter campaign

Increasing the density in Wendelstein 7-X: benefits and limitations

First divertor physics studies in Wendelstein 7-X

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