Disentangling core and edge mechanisms of the density limit in DIII-D negative triangularity plasmas

Abstract The density limit is investigated in the DIII-D negative triangularity plasmas which lack a standard H-mode edge. We find the limit may not be a singular disruptive boundary but a multifaceted density saturation phenomenon governed by distinct core and edge transport mechanisms. Sustained, non-disruptive operation is achieved at densities up to 1.8 times the Greenwald limit ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">G</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) until the termination of auxiliary heating. Systematic power scans show distinct power scalings for the core ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mi>e</mml:mi> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>SOL</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.27</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.03</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ) and edge ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mi>e</mml:mi> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>SOL</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.42</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ) density limits. The edge density saturation is triggered by the onset of a non-disruptive, high-field side radiation front and the associated cooling, which clamps the edge density below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">G</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . In contrast, the core density continues to rise until it saturates, a state characterized by enhanced core turbulence. Core transport evolves from a diffusive to an intermittent, avalanche-like state, as indicated by heavy-tailed probability density functions (kurtosis ≈ 6), increased Hurst exponents, and a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> </mml:math> -type power spectrum. These findings suggest that the density limit in the low-confinement regime is determined by a combination of edge radiative cooling and core turbulent transport. This distinction provides separate targets for control strategies aimed at extending the operational space of future fusion devices.