How turbulent transport broadens the heat flux width: local SOL production or edge turbulence spreading?

Abstract

Abstract This paper uses data from limited HL-2A Ohmic-plasma to answer the question of how turbulent transport broadens the heat flux width. A key issue in this study is the determination of the origin of scrape-off layer (SOL) turbulence. We develop the concept of the energy production ratio <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>R</mml:mi> <mml:mi>a</mml:mi> </mml:msub> </mml:mrow> </mml:math> , which compares the flux of turbulence energy across the last closed flux surface (LCFS) to the net, integrated energy production in the SOL. The flux of turbulence energy (i.e. spreading) is measured directly, using Langmuir probes. Experimental data is used to evaluate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>R</mml:mi> <mml:mi>a</mml:mi> </mml:msub> </mml:mrow> </mml:math> . Results show that usually <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>R</mml:mi> <mml:mi>a</mml:mi> </mml:msub> </mml:mrow> <mml:mo>&gt;</mml:mo> <mml:mn>1</mml:mn> </mml:math> , indicating that SOL turbulence is energized primarily by edge turbulence spreading. The exceptions—cases where <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>R</mml:mi> <mml:mi>a</mml:mi> </mml:msub> </mml:mrow> <mml:mo>&lt;</mml:mo> <mml:mn>1</mml:mn> </mml:math> —are those with relatively stronger <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mi mathvariant="bold">E</mml:mi> </mml:mrow> </mml:mrow> <mml:mo>×</mml:mo> <mml:mrow> <mml:mrow> <mml:mi mathvariant="bold">B</mml:mi> </mml:mrow> </mml:mrow> </mml:math> shear near the LCFS. The latter inhibits both turbulence spreading and local SOL production, but has greater effects on spreading. High blob fraction in the turbulence correlates with large values of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>R</mml:mi> <mml:mi>a</mml:mi> </mml:msub> </mml:mrow> </mml:math> . The implications for heat flux width physics are discussed.