Title: Study on divertor detachment and pedestal characteristics in the DIII-D upper closed divertor

Abstract

Abstract Experiments performed in DIII-D demonstrate that higher plasma current and heating power combined with impurity seeding facilitate the achievement of divertor detachment with a higher pedestal pressure and higher plasma performance in H-mode plasmas with a baffled closed divertor compared with an open divertor. Dedicated experiments were carried out to study the impact of power, plasma current and impurity seeding on divertor detachment with ion B × ∇ B directed into the divertor favorable for the L–H transition. With a factor of three variation in heating power and with only D2 puffing, no significant difference in the separatrix density at detachment onset was found. The higher heating power leads to higher impurity concentration and wider scrape-off layer (SOL) width, and reduces the detachment onset density to one similar to that in lower-power plasmas. Higher current requires higher pedestal and line-averaged densities to achieve divertor detachment; however, the increase in separatrix density at increasing plasma current is found to be less pronounced. Initial calculations found that both power scan and plasma current scan datasets are qualitatively consistent with theory after considering the change in impurity concentration and heat fluxmore » width. This also motivates the future extensive study of transport and divertor impurity behavior in order to have a quantitative comparison between experiment and theory. Compared with an open divertor, a closed divertor facilitates detachment onset at ∼40% lower line-averaged plasma density. Additional N2 seeding facilitates the achievement of detachment at a lower separatrix density and thus a higher pedestal temperature, which is beneficial for advanced tokamak scenarios. Higher heating power requires a higher N2 puffing rate to achieve the same degree of detachment, while a higher N2 puffing rate leads to lower detachment onset line-averaged density, both of which agree with theory. In contrast to the narrower pedestal in an open divertor approaching detachment, the pedestal density width in a closed divertor increases with density. The density gradient increases with line-averaged density at higher plasma current, but remains nearly unchanged at lower plasma current. In particular, compared with discharges with low power, at high heating power the pedestal density gradient is much weaker, while the SOL density is significantly higher and wider. At the same plasma current, both pedestal pressure gradient and temperature gradient decrease linearly with the line-averaged density but remain similar across different heating powers. Even with different plasma current and heating power, the normalized pressure gradient remains identical. As a result, achievement of divertor detachment with a higher pedestal pressure and higher plasma performance is shown in a closed divertor, which is important for improving core–edge integration as one of the critical issues for future tokamak fusion reactors.« less

Authors:

Wang, H. Q. ;

• Search DOE PAGES for author "Wang, H. Q."
• Search DOE PAGES for ORCID "0000-0003-1920-2799"

Thomas, D. M.; Leonard, A. W. ;

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

Ma, X. X. ;

• Search DOE PAGES for author "Ma, X. X."
• Search DOE PAGES for ORCID "0000-0002-7326-2146"

Guo, H. Y.; Moser, A. L.; Watkins, J. G.; Scotti, F.; Lasnier, C. ;

• Search DOE PAGES for author "Lasnier, C."
• Search DOE PAGES for ORCID "0000-0002-7109-2278"

Fenstermacher, M. E. ;

• Search DOE PAGES for author "Fenstermacher, M. E."
• Search DOE PAGES for ORCID "0000-0001-5635-7330"

McLean, A. G.; Shafer, M. W.; Grierson, B. ;

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

Ren, J.; Osborne, T. H.

Publication Date:

Tue Feb 14 00:00:00 EST 2023

Research Org.:

General Atomics, San Diego, CA (United States); Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Fusion Energy Sciences (FES); USDOE National Nuclear Security Administration (NNSA)

OSTI Identifier:

1926777

Alternate Identifier(s):

OSTI ID: 1909971; OSTI ID: 1960054; OSTI ID: 2282409

Report Number(s):

LLNL-JRNL-857630
Journal ID: ISSN 0029-5515

Grant/Contract Number:  

AC02-09CH11466; FC02-04ER54698; NA0003525; AC05-00OR22725; SC0019256; AC52-07NA27344

Resource Type:

Published Article

Journal Name:

Nuclear Fusion

Additional Journal Information:

Journal Name: Nuclear Fusion Journal Volume: 63 Journal Issue: 4; Journal ID: ISSN 0029-5515

Publisher:

IOP Publishing

Country of Publication:

IAEA

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY; plasma physics; divertor detachment; H-mode; pedestal