Physics of collapse events in toroidal plasmas

A review is made of the crash phenomena in toroidal plasmas. The emphasis is placed on the physics that causes the crash of global plasma parameters. Recent progress in the measurement has provided a unified view of various crash phenomena, i.e. the sudden occurrence of the crash, the sensitivity (or probabilistic dependence) of the occurrence of the crash on the global parameters, and the abrupt excitation of a symmetry-breaking perturbation (magnetic trigger). Essential observations that describe the physics of collapse are surveyed. The theoretical study of the nonlinear plasma dynamics is overviewed. Theories of the onset and explosive growth, which are based on magnetic braiding, are discussed. As an example, a picture based on a turbulence-turbulence transition is explained. A picture based on hysteresis and bifurcation, not on the linear instability criterion, emerging from advanced measurements and recent progress, describes the basic physics of the collapse.