Title: Developing physics basis for the snowflake divertor in the DIII-D tokamak

Abstract

Recent DIII-D results demonstrate that the snowflake (SF) divertor geometry (cf. standard divertor) enables significant manipulation of divertor heat transport for heat spreading and reduction in attached and radiative divertor regimes, between and during edge localized modes (ELMs), while maintaining good H-mode confinement. Snowflake divertor configurations have been realized in the DIII-D tokamak for several seconds in H-mode discharges with heating power PNBI $$\leqslant$$ 4-5 MW and a range of plasma currents Ip = 0.8-1.2 MA. In this work, inter-ELM transport and radiative SF divertor properties are studied. Significant impact of geometric properties on SOL and divertor plasma parameters, including increased poloidal magnetic flux expansion, divertor magnetic field line length and divertor volume, is confirmed. In the SF-minus configuration, heat deposition is affected by the geometry, and peak divertor heat fluxes are significantly reduced. In the SF-plus and near-exact SF configurations, divertor peak heat flux reduction and outer strike point heat flux profile broadening are observed. Inter-ELM sharing of power and particle fluxes between the main and additional snowflake divertor strike points has been demonstrated. The additional strike points typically receive up to 10-15% of total outer divertor power. Measurements of electron pressure and poloidal beta !p support the theoretically proposed churning mode that is driven by toroidal curvature and vertical pressure gradient in the weak poloidal field region. A comparison of the 4-4.5 MW NBI-heated H-mode plasmas with radiative SF divertor and the standard radiative divertor (both induced with additional gas puffing) shows a nearly complete power detachment and broader divertor radiated power distribution in the SF, as compared to a partial detachment and peaked localized radiation in the standard divertor. However, insignificant difference in the detachment onset w.r.t. density between the SF and the standard divertor was found. The results complement the initial SF divertor studies in the NSTX and DIII-D tokamaks and contribute to the physics basis of the SF divertor as a power exhaust concept for future tokamaks.

Authors:

Soukhanovskii, V. A. ^[1]; Allen, S. L. ^[2]; Fenstermacher, M. E. ^[2]; Lasnier, C. J. ^[2]; Makowski, M. A. ^[2]; McLean, A. G. ^[2]; Meyer, W. H. ^[2]; Ryutov, D. D. ^[2]; Kolemen, E. ^[3]; Groebner, R. J. ^[4]; Hyatt, A. W. ^[4]; Leonard, A. W.  ^[4];

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Osborne, T. H. ^[4]; Petrie, T. W. ^[4]; Watkins, J. ^[5]

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+ Show Author Affiliations

1. Lawrence Livermore National Lab., Livermore, CA (United States)
2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
3. Princeton Univ., Princeton, NJ (United States)
4. General Atomics, San Diego, CA (United States)
5. Sandia National Lab. (SNL-CA), Livermore, CA (United States)

Publication Date:

2018-02-01

Research Org.:

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

Sponsoring Org.:

USDOE

OSTI Identifier:

1420291

Alternate Identifier(s):

OSTI ID: 1420278

Report Number(s):

LLNL-JRNL-730504
Journal ID: ISSN 0029-5515; TRN: US1801481

Grant/Contract Number:  

AC52-07NA27344; FC02-04ER54698

Resource Type:

Accepted Manuscript

Journal Name:

Nuclear Fusion

Additional Journal Information:

Journal Volume: 58; Journal Issue: 3; Journal ID: ISSN 0029-5515

Publisher:

IOP Science

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION; snowflake divertor; divertor; DIII-D; divertor detachment