A dependence of carbon impurity transport coefficients on fuel ions in hydrogen and helium plasmas of Large Helical Device

Nozato, Hideaki; Morita, S.; Goto, M.; Takase, Y.; Ejiri, A.; Amano, Tsuneo; Tanaka, K.; Inagaki, S.

doi:10.1063/1.2337790

Cited by 19 publications

(20 citation statements)

References 11 publications

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“…The experimental result can be approximately explained by the diffusion coefficient of 0.1 m 2 /s. The coefficient estimated here is very close to the values obtained from our previous experimental results in LHD [10,11]. The radial profile of the total K α emissivity is also discussed as well as the energy shift for the determination of the iron density in order to check calculation parameters.…”

Section: Results Of Data Analysis and Discussionsupporting

confidence: 68%

Determination of Iron Density at the Plasma Center Using Radial Profile of FeKα Lines in LHD

Muto

Morita

2008

Plasma and Fusion Research

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Radial profiles of K α X-ray spectra of metallic impurities have been measured using an assembly of soft X-ray pulse-height analyzers in Large Helical Device. The assembly is quipped with a radial scanning system, which gives us detailed profile information, especially in long pulse discharges. The local emissivity profiles of the Fe-K α X-ray spectra have been quantitatively obtained by an Abel inversion technique. The density profile of the iron impurity in each charge state is analyzed using the energy shift profiles of the Fe-K α spectra with the help of an impurity code analysis. As an example of the present method, the iron density of 1.2 × 10 9 cm −3 in the plasma center is obtained at the line-averaged electron density of 2.6 × 10 13 cm −3 .

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Section: Results Of Data Analysis and Discussionsupporting

confidence: 68%

Determination of Iron Density at the Plasma Center Using Radial Profile of FeKα Lines in LHD

Muto

Morita

2008

Plasma and Fusion Research

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“…The fractional abundance of the carbon ions at the plasma center is calculated using the 1-d transport code 26 assuming a diffusion coefficient of D=0.1m 2 /s, which is used to study the core transport properties of LHD plasmas 27 . Figure 3 shows the fractional abundance of carbon ions.…”

Section: Resultsmentioning

confidence: 99%

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

et al. 2009

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observed in VUV and EUV regions to study the edge carbon impurity transport in the LHD ergodic layer. Here, CIII and CIV indicate the carbon influx at the outside boundary of the ergodic layer and CV and CVI indicate the ions in higher ionization stages which have already experienced the transport in the ergodic layer. The intensity ratio of CV+CVI to CIII+CIV, therefore, represents the degree of impurity screening, which has been analyzed with different edge plasma parameters and ergodic magnetic field structures. The ratio decreases by two orders of magnitude with an increase of electron density, n e , in the range of 1810 19 m -3 . The CV and CVI emissions tend to decrease with n e , whereas the CIII and CIV emissions monotonically increase with n e .-2 -The result suggests an enhancement of the impurity screening in the higher n e range due to the increasing ion-impurity collision frequency ( Zi 1/ s =3.4×10 plays an important role in the edge impurity transport within the ergodic layer. When the ergodic layer structure is thicker, the ratio systematically decreases mainly due to a reduction of CV+CVI emissions. The ratio is also studied by changing the radial position of an externally supplied m/n=1/1 islands. When the island is positioned in the ergodic layer, the ratio indicates a remarkable change, i.e. reduction of CV+CVI and increase ofCIII+CIV. These experiments demonstrate that the modification of the ergodic magnetic field structure makes a clear change to the edge impurity transport. When the background ion species is changed from hydrogen to helium, the ratio is clearly reduced, at least at n e 4×10 19 m -3 , suggesting the enhancement of the impurity screening effect due to the increased collisionality. Finally, the experimental result is simulated using 3-dimenstional edge transport code of EMC3-EIRENE. The density dependence of the carbon ratio can be well reproduced with a simulation code suggesting that impurity screening is induced in the ergodic magnetic field layer.

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“…For low density plasmas the investigations reveal poor impurity confinement in TJ-II and LHD (n e <10 19 m -3 ) as well as in W7-AS (n e <5x10 19 m -3 ) [2,4,5]. This was attributed to either strong turbulent transport (W7-AS, LHD [6,8]) or the appearance of the electron root at low collisionality (LHD [7]). Consistently, the plasma discharges could be maintained without any accumulation of intrinsic impurities.…”

Section: Observed Trends For Impurity Confinementmentioning

confidence: 99%

“…Consistently, the plasma discharges could be maintained without any accumulation of intrinsic impurities. Additionally, a clear density dependence of the impurity confinement emerges during dedicated density scans in TJ-II, W7-AS and LHD [2,4,5] due to an increasing ratio of inwards convection V to diffusion coefficient D in the bulk plasma and thus longer confinement times at higher density as shown in FIG.1 19 m -3 , magnetic field B, electron cyclotron radiation heating (ECRH) power P ECRH ) and a density and density-gradient-dependent inwards convection velocity V in LHD [8]. In LHD, which has a large flexibility in changing the ambipolar radial electric field E r by controlling the effective ripple and the magnetic topology, the impact of E r on impurity transport was studied [7,9].…”

Section: Observed Trends For Impurity Confinementmentioning

confidence: 99%

On impurity handling in high performance stellarator/heliotron plasmas

Burhenn¹,

Feng²,

Ida³

et al. 2009

Nucl. Fusion

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Abstract. The Large Helical Device (LHD) and Wendelstein 7-X (W7-X, under construction) are experiments specially designed to demonstrate long pulse (quasi steady-state) operation, which is an intrinsic property of Stellarators and Heliotrons. Significant progress had been made in establishing high performance plasmas. A crucial point is the increasing impurity confinement at high density observed at several machines (TJ-II, W7-AS, LHD) which can lead to impurity accumulation and early pulse termination by radiation collapse. In addition, theoretical predictions for non-axisymmetric configurations predict the absence of impurity screening by ion temperature gradients in standard ion-root plasmas. Nevertheless, scenarios were found where impurity accumulation was successfully avoided in LHD and W7-AS due to the onset of friction forces in the (high density and low temperature) scrape-off-layer, the generation of magnetic islands at the plasma boundary and to a certain degree also by ELMs, flushing out impurities and reducing the net-impurity influx into the core. In both the W7-AS High Density H-mode (HDH) regime and in the case of application of sufficient ECRH heating power a reduction of impurity core confinement was observed. The exploration of such purification mechanisms is a demanding task for successful steady-state operation. Impurity transport at the plasma edge/SOL was identified to play a major role for the global impurity behaviour in addition to the core confinement.

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A dependence of carbon impurity transport coefficients on fuel ions in hydrogen and helium plasmas of Large Helical Device

Cited by 19 publications

References 11 publications

Determination of Iron Density at the Plasma Center Using Radial Profile of FeKα Lines in LHD

Determination of Iron Density at the Plasma Center Using Radial Profile of FeKα Lines in LHD

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

On impurity handling in high performance stellarator/heliotron plasmas

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