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

Abstract

Four resonance transitions of CIII (977.03 Å:2s2 S10-2s2p P11), CIV (1550 Å:2s S2-2p P2), CV (40.27 Å:1s2 S10-1s2p P11), and CVI (33.73 Å:1s S2-2p P2) have been observed in vacuum ultraviolet and extreme ultraviolet regions to study the edge carbon impurity transport in the Large Helical Device 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 in electron density ne in the range of 1–8×1019 m−3. The CV and CVI emissions tend to decrease with ne, whereas the CIII and CIV emissions monotonically increase with ne. The result suggests an enhancement of the impurity screening in the higher ne range due to the increasing ion-impurity collision frequency (νΖi≡1/τs=3.4×104 s−1 at ne=2×1019 m−3 and 1.0×105 s−1 at 6×1019 m−3 for CV). The friction force parallel to the magnetic fields 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 in CV+CVI emissions. The ratio is also studied by changing the radial position of an externally supplied m/n=1/1 island. When the island is positioned in the ergodic layer, the ratio indicates a remarkable change, i.e., reduction in CV+CVI and increase in CIII+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 ne≤4×1019 m−3, suggesting the enhancement of the impurity screening effect due to the increased collisionality. Finally, the experimental result is simulated using three-dimensional 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.