Nonlinear Dynamics of Collapse Phenomena in Heliotron Plasma with Large Pressure Gradient

We have executed nonlinear magnetohydrodynamic(MHD) simulations in a heliotron-type configuration with a large pressure gradient to reveal the nonlinear dynamics of collapse phenomena. The simulation results reproduce the qualitative characteristics of the experimental observation on the so-called core density collapse (CDC) events in the Large Helical Device (LHD) plasma with the super dense core (SDC) profile. A long-term nonlinear behavior on the event, including the flushing mechanism of the core pressure, is clarified. The simulation result shows the linear growth of the ballooning-like resistive instability modes with the intermediate poloidal wavenumbers. The growth of the modes are saturated soon, and the system experiences the energy relaxation in about 1 msec. It should be noted that the linear mode structures are localized in the edge region, whereas the core pressure rapidly falls as the system reaches the relaxed state. The co-existence of the edge perturbation and the core collapse is consistent with the experimental observations. The lost pressure forms a wider tail in the peripheral region. The core pressure is remarkably reduced at a certain period, while it had withstood the disturbance before it. The most salient feature on this period is the disordering of the magnetic field structure. The system keeps the nested-flux-surface structure well in the beginnings, whereas part of them are abruptly lost at this period. Such a situation can induce a flattening of the pressure profile along the reconnected field lines. By checking the place where the plasma loss due to this mechanism occurs, such plasma outlets are found to be located mainly on the disordered region. Thus, one can conclude that the core collapse can be caused by the disturbance of the magnetic field.