Nonlinear dynamics of a collapse phenomenon in heliotron plasma with large pressure gradient

We have executed nonlinear magnetohydrodynamic simulations in a heliotron-type configuration with a large pressure gradient to reveal the nonlinear dynamics of a collapse phenomenon. The simulation results reproduce the qualitative characteristics of the experimental observation on the so-called core density collapse events in the Large Helical Device plasma with the super-dense core profile. A long-term nonlinear behaviour 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 eventually saturated, and the system experiences the energy relaxation in about 1 ms. 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. Such coexistence of the edge perturbation and the core collapse is consistent with the experimental observations. The lost pressure forms a wide base in the peripheral region. The core pressure is, on the other hand, remarkably reduced at a certain period, although it had well 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 at the beginning, whereas part of them are abruptly lost in 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.