Nonlinear simulations of core density collapses in large helical device

The stability of ballooning modes at the plasma edge of Stellarator fusion reactors is a major concern, considered a severe limiting factor of the device performance, particularly for low magnetic shear configurations based on quasi-isodynamic symmetry magnetic traps, restricting the maximum thermal β of the operation scenarios. The present analysis is dedicated to study the core density collapses (CDCs) observed in Large Helical Device (LHD), a hard MHD limit relaxation caused by the destabilization of high n ballooning modes at the plasma edge, leading to the collapse of the plasma pressure and the LHD performance deterioration. A set of simulations are performed using the code FAR3d to reproduce the CDC. Linear simulations calculate unstable high n ballooning modes at the plasma edge (n > 17, n the toroidal mode number and m the poloidal mode number) consistent with the ballooning modes observed in the experiment. Nonlinear simulations show the inner-middle plasma () is destabilized due to an inverse energy cascade from the saturating high n ballooning modes towards middle and low n modes as well as the thermal plasma. The energy transfer towards the thermal plasma during the high-middle ballooning modes saturation leads to the distortion of the plasma flux surfaces at the plasma periphery, inducing a flattening of the pressure profile. Likewise, the pressure gradient at the plasma core enhances causing the destabilization of low n modes, consistent with the instability inward propagation and the m = 1 perturbation measured during the CDC. The analysis indicates the deterioration of the LHD performance may be caused by an intense distortion of the magnetic surfaces by the saturating middle n ballooning modes and m = 1 perturbation between the inner-middle plasma, leading to a wide region of stochastic magnetic fields generated by reconnection processes.