Magnetic turbulence and pressure gradient feedback effect of the $1/2$ mode soft-hard MHD limit in Large Helical Device

摘要

The aim of this study is to analyze the feedback process between the magneticturbulence and the pressure gradients in LHD inward-shifted configurations aswell as its role in the transition between the soft-hard MHD regimes forinstabilities driven by the mode $1/2$ in the middle plasma. In the presentpaper we summarize the results of two simulations with different Lundquistnumbers, $S = 2.5 \times 10^5$ and $10^6$, assuming a plasma in the slowreconnection regime. The results for the high Lundquist number simulation showsthat the magnetic turbulence and the pressure gradient in the middle plasmaregion of LHD are below the critical value to drive the transition to the hardMHD regime, therefore only relaxations in the soft MHD limit are triggered($1/2$ sawtooth like events) [Phys. Plasmas 19, 082512 (2012)]. In the case ofthe simulation with low Lundquist number, the system reaches the hard MHD limitand a plasma collapse is observed.