Understanding the L-H transition isotope effect in DIII-D

ORAL  · Invited

Abstract

Recent database and gyro-kinetic analysis of DIII-D plasmas in low-confinement mode (L-mode) just before transition to high-confinement mode (H-mode) have identified decreased carbon impurity content as the dominant isotope effect responsible for increasing the L-H power threshold (PLH) in hydrogen at ITER-relevant low collisionality. Increased Zeff in deuterium, due to enhanced (mass dependent) physical and chemical sputtering of graphite (carbon) from diverter and main chamber tiles, is found to increase the ITG critical gradient, stabilizing ITG modes [1]. Gyro-fluid and gyro-kinetic simulations capture the observed impurity isotope effect and its influence on electron and ion heat transport and power loss, via the TGLF and CGYRO codes, respectively. Edge simulations identify subdominant, intrinsic main ion mass effects due to electron non-adiabaticity [2] and differences in normalized E⨉B shear, that also contribute significantly to the isotope scaling of PLH. At high collisionality, no isotope effect is observed, as previously documented [3]. Measurements of density and electron temperature fluctuations using the Beam Emission Spectroscopy (BES) and Correlation Electron Cyclotron Emission (CECE) diagnostics, respectively, were found to agree well with turbulence predictions from flux matched CGYRO simulations at ρ = 0.7, 0.9, & 0.95. This comparison was made possible via advanced synthetic diagnostics. The observed PLH reduction with low-Z impurity dilution opens the important prospect of improving H-mode access in ITER hydrogen plasmas via Ne or N seeding.

[1] S. Migliuolo 1992 Nucl. Fusion 32 1331

[2] Belli, E. A. et al 2020 PRL 125.1 015001

[3] Z. Yan et al. 2017 Nucl. Fusion 57, 126015

*This work was supported by the US Department of Energy under Awards No. DE-FC02-04ER54698, DE-SC0020287, DE-SC0019352, DE-FG02-08ER54999, DE-FG02-08ER54999, DE-AC02-05CH11231, DE-SC0018287, DE-AC02-09CH11466.

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Presenters

  • Kyle Callahan

    • University of California, Los Angeles

Authors

  • Kyle Callahan

    • University of California, Los Angeles

  • Lothar Schmitz

    • University of California, Los Angeles

  • Troy A Carter

    • University of California, Los Angeles

  • Shaun R Haskey

    • Princeton Plasma Physics Laboratory

  • Colin Chrystal

    • General Atomics - San Diego

  • Emily A Belli

    • General Atomics

  • Christopher G Holland

    • University of California, San Diego

  • Kathreen E Thome

    • GA

  • George R McKee

    • University of Wisconsin - Madison
    • UWisc. Madison

  • Arash Ashourvan

    • General Atomics

  • Guiding Wang

    • University of California, Los Angeles

  • Quinn Pratt

    • University of California, Los Angeles

  • Zheng Yan

    • University of Wisconsin - Madison

  • Matthias Knolker

    • General Atomics

  • Florian Effenberg

    • Princeton Plasma Physics Laboratory

  • Alessandro Bortolon

    • Princeton Plasma Physics Laboratory
    • PPPL

  • Filipp Khabanov

    • Wisconsin-Madison
    • University of Wisconsin-Madison
    • University of Wisconsin–Madison
    • University of Wisconsin- Madison
    • UWM