3D non-linear MHD simulations of core density collapse event in LHD plasma

Abstract A new three-dimensional, non-linear magnetohydrodynamics (MHD) model has been extended in MIPS code, incorporating parallel heat diffusivity. The model has been benchmarked against the former MHD model used in MIPS code. A preliminary study of the core density collapse event (CDC) observed in the large helical device (LHD) plasma has been performed using the developed model. The equilibrium has been constructed using HINT code for a typical super dense core discharge in LHD, with vacuum magnetic axis configuration <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>R</mml:mi> <mml:mrow> <mml:mi>axV</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mrow> <mml:mn>3.85</mml:mn> </mml:mrow> <mml:mstyle scriptlevel="0"/> <mml:mrow> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> </mml:mrow> </mml:math> and magnetic axis beta <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>β</mml:mi> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>4</mml:mn> <mml:mi mathvariant="normal">%</mml:mi> </mml:mrow> </mml:math> plasma. This configuration corresponds to a plasma with a steep pressure gradient and strong Shafranov shift, which makes the plasma potentially unstable in the LHD. The model shows preliminary characteristics of the CDC event. The plasma is destabilized by high- n ballooning modes in the low-field side region during the linear regime, eventually leading to the collapse of the pressure and density profiles, together with the stochastization of the magnetic field and a shift to low- n modes centered at the core of the plasma after the non-linear coupling at the relaxation regime.