Experimental study of impurity screening in the edge ergodic layer of the Large Helical Device using carbon emissions of CIII to CVI

Chowdhuri, M. B.; Morita, S.; Kobayashi, M.; Goto, M.; Zhou, Hangyu; Masuzaki, S.; Morisaki, T.; Narihara, K.; Yamada, I.; Feng, Y.

doi:10.1063/1.3103784

Cited by 40 publications

(47 citation statements)

References 31 publications

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“…Thus, the SOL impurity screening effect predicted by the 3D code remains the only explanation for the experimental observations in LHD. Indeed, the edge spectroscopic measurements on LHD have recently confirmed the SOL impurity retention effect [25]. Here, we address only numerical assessments of some essential factors affecting the impurity influx and focus on illustrating the behind physics/mechanisms associated with the common, essential geometric features of helical SOLs in non-axissymmetric devices rather than on simulating any specific discharges for a quantitative comparison with local diagnostics.…”

Section: Divertor Function On Controlling Intrinsic Impuritiesmentioning

confidence: 99%

“…Here, we address only numerical assessments of some essential factors affecting the impurity influx and focus on illustrating the behind physics/mechanisms associated with the common, essential geometric features of helical SOLs in non-axissymmetric devices rather than on simulating any specific discharges for a quantitative comparison with local diagnostics. Related experimental studies and experiment/modeling comparison results are referred to in Refs [22,25,26].…”

Section: Divertor Function On Controlling Intrinsic Impuritiesmentioning

confidence: 99%

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Comparative divertor-transport study for helical devices

et al. 2009

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Abstract. Using the island divertors (IDs) of W7-AS and W7-X and the helical divertor (HD) of LHD as examples, the paper presents a comparative divertor transport study for three typical helical devices of different machine-size following two distinct divertor concepts, aiming at identifying common physics issues/effects for mutual validation and combined studies. Based on EMC3/EIRENE simulations supported by experimental results, the paper first reviews and compares the essential transport features of the W7-AS ID and the LHD HD in order to build a base and framework for a predictive study of W7-X. The fundamental role of low-order magnetic islands in both divertor concepts is emphasised. Preliminary EMC3/EIRENE simulation results for W7-X are presented and discussed with respect to W7-AS and LHD in order to show how the individual field and divertor topologies affect the divertor transport and performance. For instance, a high recycling regime, which is absent from W7-AS and LHD, is predicted to exist for W7-X. The paper focuses on identifying and understanding the role of divertors for high density plasma operations in helical devices. In this regard, special attention is paid to investigating the divertor function for controlling intrinsic impurities. Impurity transport behaviour and wall-sputtering processes of CX-neutrals are studied under different divertor plasma conditions. A divertor retention effect on intrinsic impurities at high SOL collisonalities is predicted for all the three devices. The required SOL plasma conditions and the underlying mechanisms are analysed in detail. Numerical results are discussed in conjunction with the experimental observations for high density divertor plasmas in W7-AS and LHD. Different SOL transport regimes are numerically identified for the standard divertor configuration of W7-X and the possible consequences on high density plasmas are assessed. All the EMC3-EIRENE simulations presented in this paper are based on vacuum fields and comparisons with local diagnostics are made for low-ß plasmas.

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Section: Divertor Function On Controlling Intrinsic Impuritiesmentioning

confidence: 99%

Section: Divertor Function On Controlling Intrinsic Impuritiesmentioning

confidence: 99%

Comparative divertor-transport study for helical devices

et al. 2009

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“…Up to this day the edge impurity transport has been investigated in several magnetic field configurations, e.g., diverted tokamaks [11,12,13], non-axisymmetric tokamaks [5,14,15] and helical devices [16,17,18,19,20]. Although the edge impurity transport has been well understood based on such many previous works, the details of the edge impurity transport are still in opened question, in particular, in the combination of parallel and cross-field transports.…”

Section: Introductionmentioning

confidence: 99%

Edge impurity transport study in the stochastic layer of LHD and the scrape-off layer of HL-2A

Kobayashi¹,

Morita²,

Dong³

et al. 2013

Nucl. Fusion

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Abstract. Edge impurity transport has been investigated in the stochastic layer of Large Helical Device (LHD) and the scrape-off layer (SOL) of Huan Liuqi-2A (HL-2A) tokamak, as a comparative analysis based on the three-dimensional (3D) edge transport code EMC3-EIRENE and on the carbon emission profile measurement. The 3D simulation predicts impurity screening effect in the both devices, but also predicts different impurity behavior against collisionality and impurity source location between the two devices. The difference is caused by geometrical structures of the magnetic field lines in the stochastic layer and X-point poloidal divertor SOL, i.e., number of poloidal turns of flux tubes affecting poloidal distribution of plasma parameters and impact of perpendicular transport on parallel pressure conservation and energy transport. These processes have an influence on the impurity screening efficiency at upstream and downstream positions of field lines. The carbon emission measured in the stochastic layer of LHD clearly indicates the screening effect in high density region. The result can be qualitatively interpreted by the present modeling, although the modeling shows a slight difference in the quantitative behavior of carbon ions in the stochastic layer of LHD. On the other hand, comparison of the carbon emission profile from HL-2A with the modeling is not straightforward. It is found that the impurity distribution in the HL-2A SOL is very sensitive to the impurity source location. In order to interpret the experimental observation a further study is necessary, in particular, on the impurity source distribution in the divertor plate and the first wall.

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“…Therefore, CIII and CIV can serve as an index representing the source term, and CV can represent ions which have already experienced transport in the ergodic layer. Thus, the intensity ratio of CV to CIII + CIV represents the degree of impurity screening [16]. The results are shown in Fig.…”

Section: Edge Impurity Transportmentioning

confidence: 55%

Progress in Impurity-Related Physics Experiments in LHD

Morita

Dong

Goto

et al. 2010

Plasma and Fusion Research

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A variety of density profiles observed in the Large Helical Device (LHD) have suggested an interesting core impurity transport. The edge impurity transport in the ergodic layer formed by stochastic magnetic field lines with long connection length (10-2000 m) can also exhibit interesting phenomena in the competition of perpendicular and parallel transport. The LHD discharge is highly robust against impurity buildup, and operation is possible essentially up to the global power balance limit because current-driven instability does not principally exist. The LHD plasma has therefore provided information on many interesting physics issues closely related to impurities. Recent results of impurity-related physics experiments in the LHD are briefly reviewed. The specific contents presented here are (1) core impurity behavior with perpendicular transport, (2) edge impurity behavior with parallel transport, (3) high-Z discharges with high ion temperature, (4) impurity pellet injection with improved plasma performance, (5) impurity pellet ablation in the presence of energetic ions with high heat flux and (6) observation of magnetic dipole forbidden transitions for high-Z elements. A result from the Compact Helical System (CHS) is used only in the impurity pellet ablation study, because detailed data have not yet been obtained from the LHD. Finally, the results are summarized and future directions in these topics are noted.

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Experimental study of impurity screening in the edge ergodic layer of the Large Helical Device using carbon emissions of CIII to CVI

Cited by 40 publications

References 31 publications

Comparative divertor-transport study for helical devices

Comparative divertor-transport study for helical devices

Edge impurity transport study in the stochastic layer of LHD and the scrape-off layer of HL-2A

Progress in Impurity-Related Physics Experiments in LHD

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