The evolution of the bound particle reservoir in Wendelstein 7-X and its influence on plasma control

Schlisio, G.; Wenzel, U.; Naujoks, D.; Pedersen, T. S.; Grote, H.; Winters, V.; Niemann, H.; Mulsow, Matthias; Krychowiak, M.; Drewelow, P.; Gao, Yu; Jakubowski, M.; Sitjes, A. Puig; Laqua, H. P.; Knauer, J.; Brunner, K. J.

doi:10.1088/1741-4326/abd63f

Cited by 8 publications

(9 citation statements)

References 43 publications

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“…Particles can be deposited in the walls and released when the walls heat up by plasma wall interaction. A detailed study of long-pulse discharges in W7-X showed that the density control was lost at the moment when the wall source exceeded the amount of pumped particles [4]. Therefore, a sufficiently large particle exhaust is necessary to ensure density control and to close the fuel cycle in a future reactor.…”

Section: Introductionmentioning

confidence: 99%

Gas exhaust in the Wendelstein 7-X stellarator during the first divertor operation

Wenzel

Schlisio²,

Drewelow³

et al. 2022

Nucl. Fusion

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The optimized superconducting stellarator Wendelstein 7-X is equipped with an island divertor for energy control and efficient pumping. We investigated the performance of the island divertor in terms of gas exhaust. For this purpose we have installed 18 pressure gauges in the vacuum vessel. This allowed us to determine the exhaust efficiency, the leakage, the collection efficiency and the compression ratio of the island divertor. These quantities depend strongly on the magnetic configuration. The best performance was obtained in the high-iota configuration. The exhaust efficiency was 2.9 %, significantly higher than in the standard configuration (0.44 %), and the maximum neutral compression was about 80. The high-iota configuration appears particularly promising for long-pulse operation of Wendelstein 7-X.

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Section: Introductionmentioning

confidence: 99%

Gas exhaust in the Wendelstein 7-X stellarator during the first divertor operation

Wenzel

Schlisio²,

Drewelow³

et al. 2022

Nucl. Fusion

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“…If particles are not fueled externally, or originate from wall desorption, they must be a recycling particle. For the discharges investigated here, the gas balance analysis method described in [37], showed that the wall pumping flow was negligible compared to the recycling flow. The neutral particle flow from the wall Γ wall can therefore be assumed equal to the recycling flow Γ recy in this scenario.…”

Section: γ Wall a Recycling Dominated Source Termmentioning

confidence: 89%

“…To find out whether the wall is releasing or pumping particles, one has to consider the gas balance [37]. Principally, the determined particle flux based on photon flux measurements does not distinguish between a recycling, or a desorbing neutral.…”

Section: γ Wall a Recycling Dominated Source Termmentioning

confidence: 99%

“…As a global measurement, it cannot be used to resolve local effects that arise through the 3D system at W7-X, but it stands as an effective system confinement time that defines recycling and density control [45]. Based on gas balance results, the wall absorbs or desorbs particles on the order of 1% of the total recycling flow and is therefore seen as negligible, as shown in figure 11 [37]. Under this assumption, external fueling is the only way to achieve a constant plasma density in an actively pumped system.…”

Section: Effective Confinement Time τ * P and Rmentioning

confidence: 99%

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Analysis of hydrogen fueling, recycling, and confinement at Wendelstein 7-X via a single-reservoir particle balance

et al. 2022

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A single-reservoir particle balance for the main plasma species hydrogen has been established for Wendelstein 7-X (W7-X). This has enabled the quantitative characterization of the particle sources in the standard island divertor configuration for the first time. Findings from attached scenarios with two different island sizes with a boronized wall and turbo molecular pumping are presented. Fueling efficiencies, particle flows and source locations were measured and used to infer the total particle confinement time $\tau_{\rm{p}}$. Perturbative gas injection experiments served to measure the effective particle confinement time $\tau_{\rm{p}}^*$. Combining both confinement times provides access to the global recycling coefficient $\bar{R}$. Hydrogen particle inventories have been addressed and the knowledge of particle sources and sinks reveals the core fueling distribution and provides insight into the capability of the magnetic islands to control exhaust features. Measurements of hydrogen fueling efficiencies were sensitive to the precise fueling location and measured between 12~\% and 31~\% with the recycling fueling at the strike line modeled at only 6~\%, due to much higher densities. 15~\% of the total \SI{5.2E+22}{a/s} recycling flow ionizes far away from the recycling surfaces in the main chamber. It was shown that 60~\% of recycled particles ionize above the horizontal and 18~\% above the vertical divertor target, while the remainder of the recycling flow ionizes above the baffle (7~\%). Combining these source terms with their individual fueling efficiencies resolves the core fueling distribution. Due to the higher fueling efficiency in the main chamber, up to 51~\% of the total \SI{5.1E+21}{1/s} core fueling particles are entering the confined plasma from the main chamber. $\tau_{\rm{p}}$ values in the range of 260 ms were extracted for these discharges. Together with $\tau_{\rm{p}}$, the global recycling coefficient $\bar{R}$ was resolved for every $\tau_{\rm{p}}^*$ measurement and a typical value close to unity was obtained. An increase of the island size, resulted in no change of $\tau_{\rm{p}}$, but doubled $\tau_{\rm{p}}^*$, indicating the feasibility of the control coils as an actuator to control exhaust features without affecting core confinement properties.

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“…Divertor pumping, where neutral particles are compressed and exhausted efficiently with a baffle structure [1], was developed for this task. Density control is challenging for long-pulse discharge because wall recycling changes with time [2][3][4][5]. For example, in the Large Helical Device (LHD) [6], one of the largest superconducting helical/stellarator fusion devices, dynamic wall retention was observed in 48 min of long-pulse discharge [7].…”

Section: Introductionmentioning

confidence: 99%

Particle control in long-pulse discharge using divertor pumping in LHD

et al. 2022

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Density control is crucial for maintaining stable confined plasma. Divertor pumping, where neutral particles are compressed and exhausted in the divertor region, was developed for this task for the Large Helical Device. In this study, neutral particle pressure, which is related to recycling, was systematically scanned in the magnetic configuration by changing the magnetic axis position. High neutral particle pressure and compression were obtained in the divertor for a high plasma electron density and the inner magnetic axis configuration. Density control using divertor pumping with gas puffing was applied to electron cyclotron heated plasma in the inner magnetic axis configuration, which provides high neutral particle compression and exhaust in the divertor. Stable plasma density and electron temperature were maintained with divertor pumping. A heat analysis shows that divertor pumping did not affect edge electron heat conductivity, but it led to low electron heat conductivity in the core caused by electron-internal-transport-barrier-like formation.

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The evolution of the bound particle reservoir in Wendelstein 7-X and its influence on plasma control

Cited by 8 publications

References 43 publications

Gas exhaust in the Wendelstein 7-X stellarator during the first divertor operation

Gas exhaust in the Wendelstein 7-X stellarator during the first divertor operation

Analysis of hydrogen fueling, recycling, and confinement at Wendelstein 7-X via a single-reservoir particle balance

Particle control in long-pulse discharge using divertor pumping in LHD

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