Title: Impurity leakage and radiative cooling in the first nitrogen and neon seeding study in the closed DIII-D SAS configuration

Abstract

A comparative study of nitrogen versus neon has been carried out to analyze the impact of the two radiative species on power dissipation, SOL impurity distribution, divertor and pedestal characteristics. The experimental results show that N remains compressed in the divertor, thereby providing high radiative losses without affecting the pedestal profiles and displacing carbon as dominant radiator. Neon, instead, radiates more upstream than N thus reducing the power flux through the separatrix leading to a reduced ELM frequency and compression in the divertor. A significant amount of neon is measured in the plasma core leading to a steeper density gradient. The different behavior between the two impurities is confirmed by SOLPS-ITER modeling which for the first time at DIII-D includes multiple impurity species and a treatment of full drifts, currents and neutral–neutral collisions. The impurity transport in the SOL is studied in terms of the parallel momentum balance showing that N is mostly retained in the divertor whereas Ne leaks out consistent with its higher ionization potential and longer mean free path. This is also in agreement with the enrichment factor calculations which indicate lower divertor enrichment for neon. The strong ionization source characterizing the SAS divertor causes a reversalmore » of the main ions and impurity flows. The flow reversal together with plasma drifts and the effect of the thermal force contribute significantly in the shift of the impurity stagnation point affecting impurity leakage. This work provides a demonstration of the impurity leakage mechanism in a closed divertor structure and the consequent impact on pedestal. Since carbon is an intrinsic radiator at DIII-D, in this paper we have also demonstrated the different role of carbon in the N vs Ne seeded cases both in the experiments and in the numerical modeling. Here, carbon contributes more when neon seeding is injected compared to when nitrogen is used. Finally, the results highlight the importance of accompanying experimental studies with numerical modeling of plasma flows, drifts and ionization profile to determine the details of the SOL impurity transport as the latter may vary with changes in divertor regime and geometry. In the cases presented here, plasma drifts and flow reversal caused by high level of closure in the slot upper divertor at DIII-D play an important role in the underlined mechanism.« less

Authors:

Casali, L.  ^[1];

• Search DOE PAGES for author "Casali, L."
• Search DOE PAGES for ORCID "0000-0003-2924-3674"

Eldon, D.  ^[2];

• Search DOE PAGES for author "Eldon, D."
• Search DOE PAGES for ORCID "0000-0003-1895-0648"

McLean, A. ^[3]; Osborne, T. ^[2]; Leonard, A.  ^[2];

• Search DOE PAGES for author "Leonard, A."
• Search DOE PAGES for ORCID "0000-0001-9356-1074"

Grierson, B.  ^[4];

• Search DOE PAGES for author "Grierson, B."
• Search DOE PAGES for ORCID "0000-0001-5918-6506"

Ren, J. ^[5]

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+ Show Author Affiliations

1. Univ. of Tennessee, Knoxville, TN (United States); General Atomics, San Diego, CA (United States)
2. General Atomics, San Diego, CA (United States)
3. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
4. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States); General Atomics, San Diego, CA (United States)
5. Univ. of Tennessee, Knoxville, TN (United States)

Publication Date:

Tue Jan 04 00:00:00 EST 2022

Research Org.:

General Atomics, San Diego, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Fusion Energy Sciences (FES)

OSTI Identifier:

1863590

Grant/Contract Number:  

FC02-04ER54698

Resource Type:

Accepted Manuscript

Journal Name:

Nuclear Fusion

Additional Journal Information:

Journal Volume: 62; Journal Issue: 2; Journal ID: ISSN 0029-5515

Publisher:

IOP Science

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY; SOLPS-ITER; plasma drifts; momentum force balance; flow reversal; main ion flows; impurity flows; impurity leakage; SOL impurity transport; divertor closure; nitrogen and neon seeding; carbon radiation