Island divertor experiments on the Wendelstein 7-AS stellarator

McCormick, K.; Grigull, P.; Burhenn, R.; Brakel, R.; Ehmler, H.; Feng, Y.; Fischer, R.; Gadelmeier, F.; Giannone, L.; Hildebrandt, D.; Hirsch, M.; Holzhauer, E.; Jaenicke, R.; Kißlinger, J.; Klinger, T.; Klose, S.; Knauer, J.; König, R.; Kühner, G.; Laqua, H. P.; Naujoks, D.; Niedermeyer, H.; Pasch, E.; Ramasubramanian, N.; Rust, N.; Sardei, F.; Wagner, F.; Weller, A.; Wenzel, U.; Werner, A.

doi:10.1016/s0022-3115(02)01506-4

Cited by 36 publications

(21 citation statements)

References 21 publications

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“…This indicates, that not only the change of the core density profiles is responsible for the global impurity confinement but also the edge transport. This is additionally confirmed by the totally different confinement properties of H* [19] mode plasmas, namely long confinement times [4] and the occurrence of impurity accumulation, although density and temperature profile shapes were similar to those in HDH mode. Simulations show, that slight changes of the width, the size and the radial position of the artificially enhanced diffusive layer with respect to the inward convection layer, can reproduce all confinement qualities from H* to HDH [4] (Fig.4,right column).…”

Section: Elmfree H-mode (H*) and High Density H-mode (Hdh)mentioning

confidence: 57%

“…In high densitiy NBI plasmas, finally, the plasma energy is noticeably affected by strong radiation losses. However, it could be demonstrated, that the observed deterioration of the impurity confinement due to high heating power (1.2 MW) can prevent accumulation up to densities of 9·10 19 m -3 and might -to a certain degree -also serve as an auxiliary tool to handle the impurity problem in future machines. After the installation of the island divertor in W7-AS, the transition to the HDH mode occurs beyond a power dependent threshold density and extends the range of accessible densities up to 4·10 20 m -3 .…”

Section: Discussionmentioning

confidence: 99%

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Transport of Impurity Ions in the Wendelstein 7-AS Stellarator Plasma

Burhenn

Baldzuhn

Beidler

et al. 2006

AIP Conference Proceedings

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Abstract. The impact of global plasma parameters on impurity transport in the stellarator W7-AS was investigated by laser blow-off technique. Both, density and heating power were identified to have a strong influence on impurity confinement  τ Ι ~ n e 1.2 /P ECRH 0.8. In spite of stationary conditions at lower densities, an increasing trend for accumulation was observed at plasma densities beyond 5·10 19 m -3 due to reduction of the diffusion coefficients. Up to densities of at least 9·10 19 m -3 , launching of electron cyclotron resonance heating (ECRH) power of 1.2 MW is able to counteract the impurity accumulation by deterioration of the impurity confinement with heating power according to the scaling law as given above. In neutral beam injection (NBI) heated plasmas at densities higher than 1·10 20 m -3 , long confinement times were observed, often accompanied by loss of density control and degradation of plasma energy due to increasing radiation losses. The installation of island divertor allowed a general extension of the range of accessible densities up to 4·10 20 m -3 : beyond a certain power-dependent threshold density (1.5-2.1·10 20 m -3 ), the plasma enters the High Density Hmode (HDH) regime and the impurity confinement time drops to values comparable to the energy confinement time. High density plasmas could be sustained quasi-stationary with a low level of impurity radiation. The favourable impurity behavior goes along with a reduction of the inward impurity convection in the core plasma and possible changes in the edge transport. For the characterization of the general impurity behavior in W7-AS plasmas the usual transport models for axisymmetric devices are not sufficient and additional stellarator specific processes have to be considered.

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Section: Elmfree H-mode (H*) and High Density H-mode (Hdh)mentioning

confidence: 57%

Section: Discussionmentioning

confidence: 99%

Transport of Impurity Ions in the Wendelstein 7-AS Stellarator Plasma

Burhenn

Baldzuhn

Beidler

et al. 2006

AIP Conference Proceedings

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“…In the attached divertor plasmas (linear relation of the particle flux to the separatrix density) we observe two different confinement regimes [5]. Below a separatrix density of 3 * 10…”

Section: Data Evaluationmentioning

confidence: 88%

Carbon influx from the island divertor of the Wendelstein-7AS stellarator

Wenzel

Thomsen

Grigull

2007

Journal of Nuclear Materials

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Target plate fluxes (C,H) were measured in the island divertor experiment of W7-AS by CCD cameras equipped with interference filters. Strike point patterns depend strongly on the confinement regime. Carbon fluxes are given for attached divertor plasmas under HDH and normal confinement conditions. Our studies show that the basic picture of the radiation zones which has emerged from the investigation of tokamak divertors has to be completed for the island divertor by additional radiation zones in the X-point regions.

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“…18,19 In the Wendelstein stellarator ͑W7-AS͒, 20 another improved confinement mode called the high-density H-mode ͑HDH͒ was achieved, which is characterized by flat density profiles and low impurity confinement time. 21,22 In the HDH, impurity contamination is avoided, which is also seen in the IDB-SDC mode in the LHD ͓see Sec. III D͔.…”

Section: Introductionmentioning

confidence: 99%

Superdense core mode in the Large Helical Device with an internal diffusion barrier

et al. 2007

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In reduced recycling discharges using a local island divertor in the Large Helical Device ͓O. Motojima, H. Yamada, A. Komori et al., Phys. Plasmas 6, 1843 ͑1999͔͒, a stable high-density plasma develops in the core region when a series of pellets is injected. A core region with ϳ5 ϫ 10 20 m −3 and temperature of 0.85 keV is maintained by an internal diffusion barrier ͑IDB͒. The density gradient at the IDB ͑r / a ϳ 0.6͒ is very high, and the particle confinement time in the core region is ϳ0.4 s. Because of the increase in the central pressure, a large Shafranov shift up to ϳ0.3 m is observed. The critical ingredients for IDB formation are a strongly pumped divertor to reduce edge recycling, and multiple pellet injection to ensure efficient central fueling. No serious magnetohydrodynamics activity and impurity accumulation have been observed so far in this improved discharge.

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Island divertor experiments on the Wendelstein 7-AS stellarator

Cited by 36 publications

References 21 publications

Transport of Impurity Ions in the Wendelstein 7-AS Stellarator Plasma

Transport of Impurity Ions in the Wendelstein 7-AS Stellarator Plasma

Carbon influx from the island divertor of the Wendelstein-7AS stellarator

Superdense core mode in the Large Helical Device with an internal diffusion barrier

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