Nonlinear microtearing modes in MAST and their stochastic layer formation

Abstract

Abstract First nonlinear gyrokinetic simulations of microtearing modes in the core of a MAST case are performed on two surfaces of the high-collisionality discharge used in Valovič et al (2011 Nucl. Fusion 51 073045) to obtain the favorable energy confinement scaling with collisionality, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>τ</mml:mi> <mml:mi>E</mml:mi> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mi>ν</mml:mi> <mml:mo>∗</mml:mo> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . On the considered surfaces microtearing modes dominate linearly at binormal length scales of the order of the ion Larmor radius. While the effect of electron collision frequency is moderate in linear simulations, a strong dependence on this parameter is found in nonlinear simulations at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>r</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>a</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.5</mml:mn> </mml:math> , where r and a are the surface and tokamak minor radius, respectively. The dynamics of magnetic islands generated by microtearing modes is analysed, showing that the radial extent of the stochastic region caused by islands overlapping plays an important role in determining the saturation level of the microtearing mode driven heat flux. Local nonlinear gyrokinetic simulations show that the microtearing mode driven heat flux, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi>Q</mml:mi> <mml:mi>e</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">M</mml:mi> <mml:mi mathvariant="normal">T</mml:mi> <mml:mi mathvariant="normal">M</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> , is largely dominated by magnetic flutter and depends strongly on the magnetic shear, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mi>s</mml:mi> <mml:mo stretchy="false">ˆ</mml:mo> </mml:mover> </mml:math> . Comparing two surfaces, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>r</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>a</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.5</mml:mn> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>r</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>a</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.6</mml:mn> </mml:math> , reveals that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi>Q</mml:mi> <mml:mi>e</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">M</mml:mi> <mml:mi mathvariant="normal">T</mml:mi> <mml:mi mathvariant="normal">M</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> is negligible at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>r</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>a</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.5</mml:mn> </mml:math> ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mi>s</mml:mi> <mml:mo stretchy="false">ˆ</mml:mo> </mml:mover> <mml:mo>=</mml:mo> <mml:mn>0.34</mml:mn> </mml:math> ), with the electron temperature gradient (ETG) driven heat flux, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi>Q</mml:mi> <mml:mi>e</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">E</mml:mi> <mml:mi mathvariant="normal">T</mml:mi> <mml:mi mathvariant="normal">G</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> , comparable to the experimental electron heat flux, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi>Q</mml:mi> <mml:mi>e</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">e</mml:mi> <mml:mi mathvariant="normal">x</mml:mi> <mml:mi mathvariant="normal">p</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> , while <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi>Q</mml:mi> <mml:mi>e</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">M</mml:mi> <mml:mi mathvariant="normal

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