Nonlinear simulations of core density collapses in Large Helical Device

The present analysis is dedicated to study the core density collapses (CDC) observed in Large Helical Device (LHD) induced by unstable ballooning modes at the plasma edge. Linear simulations using the FAR3d code find the destabilization of ballooning modes in the outer plasma region with n > 17 (n is the toroidal mode number). Nonlinear simulations find an inverse energy cascade from the saturating ballooning modes towards middle and low n modes as well as towards the thermal plasma. Plasma flux surface distortion during the ballooning mode saturation flatten the pressure gradients at the edge although enhanced at the inner plasma leading to the destabilization of low n modes at the plasma core. The simulation reproduces the inward propagation and the m=1 perturbation measured during the CDC. LHD performance deterioration could be caused by the magnetic surfaces distortion and wide stochastic magnetic field regions during ballooning modes and low n modes saturation.