Nonlinear two-fluid simulations of type-I edge localized mode crashes in MAST upgrade

Abstract The Type-I edge localized mode (ELM) crash dynamics is investigated for the MAST Upgrade (MAST-U) spherical tokamak plasma through nonlinear two-fluid BOUT++ simulations. Contrary to conventional peeling–ballooning paradigms, in this work the ELM crashes in MAST-U are found to be triggered by resistive ballooning modes with growth rates scaling as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>γ</mml:mi> <mml:mo>∝</mml:mo> <mml:mrow> <mml:msup> <mml:mi>η</mml:mi> <mml:mrow> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> , accompanied by formation of n = 20–25 ( n is the toroidal mode number) filamentary structures during nonlinear evolution. Parametric scans of the plasma resistivity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>η</mml:mi> </mml:mrow> </mml:math> ) reveal an inverse proportionality between the ELM size and resistivity, assuming plasma profiles independent of resistivity, aligning with prior MAST experimental observations but contrary to the classical resistive ballooning theory. Linear two-fluid simulations corroborate this anomalous scaling, while analysis of the numerical data identifies a critical stabilizing mechanism: electromagnetic-to-thermal energy exchange reduces the ELM size at high resistivity. Furthermore, nonlinear simulations demonstrate that the bootstrap current—though inactive in linear instability drives—amplifies pedestal collapse in the MAST-U scenario.