Effects of low-Z and high-Z impurities on divertor detachment and plasma confinement

Journal Article · Sat Mar 18 00:00:00 EDT 2017 · Nuclear Materials and Energy

DOI:https://doi.org/10.1016/j.nme.2017.01.027· OSTI ID:1374810

Wang, H. Q. ^[1]; Guo, Houyang Y. ^[2]; Petrie, Thomas W. ^[2]; Leonard, Anthony W. ^[2]; Thomas, Dan M. ^[2]; Watkins, Jonathan G. ^[3]

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1. Oak Ridge Associated Univ., Oak Ridge, TN (United States)
2. General Atomics, San Diego, CA (United States)
3. Sandia National Lab. (SNL-CA), Livermore, CA (United States)

The impurity-seeded detached divertor is essential for heat exhaust in ITER and other reactor-relevant devices. Dedicated experiments with injection of N₂, Ne and Ar have been performed in DIII-D to assess the impact of the different impurities on divertor detachment and confinement. Seeding with N₂, Ne and Ar all promote divertor detachment, greatly reducing heat flux near the strike point. The upstream plasma density at the onset of detachment decreases with increasing impurity-puffing flow rates. For all injected impurity species, the confinement and pedestal pressure are correlated with the impurity content and the ratio of separatrix loss power to the L-H transition threshold power. As the divertor plasma approaches detachment, the high-Z impurity seeding tends to degrade the core confinement owing to the increased core radiation. In particular, Ar injection leads to an increase in core radiation, up to 50% of the injected power, and a reduction in pedestal temperature over 60%, thus significantly degrading the confinement, i.e., with H₉₈ reducing from 1.1 to below 0.7. As for Ne seeding, H₉₈ near 0.8 can be maintained during the detachment phase with the pedestal temperature being reduced by about 50%. In contrast, in the N₂ seeded plasmas, radiation is predominately confined in the boundary plasma, with up to 50% of heating power being radiated in the divertor region and less than 25% in the core at the onset of detachment. In the case of strong N₂ gas puffing, the confinement recovers during the detachment, from ~20% reduction at the onset of the detachment to greater than that before the seeding. The core and pedestal temperatures feature a reduction of 30% from the initial attached phase and remain nearly constant during the detachment phase. The improvement in confinement appears to arise from the increase in pedestal and core density despite the temperature reduction.

• Research Organization: General Atomics, San Diego, CA (United States)
• Sponsoring Organization: USDOE Office of Nuclear Energy (NE)
• Contributing Organization: DIII-D Team
• Grant/Contract Number: FC02-04ER54698; AC05-06OR23100; AC04-94AL85000
• OSTI ID: 1374810
• Journal Information: Nuclear Materials and Energy, Vol. 12; ISSN 2352-1791
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY