Plasma termination by excess pellet fueling and impurity injection in TJ-II, the Large Helical Device and Wendelstein 7-X

Plasma terminating events due to excess pellet fueling and impurity injection is studied in the Large Helical Device, Wendelstein 7-X and TJ-II. Time scales for these events range from values of a decimal fraction of typical energy confinement times to the order of the energy confinement time. The leading mechanism of energy loss appears to be radiation revealing similarities to radiation collapses when a radiative density limit is approached. Differently to tokamaks, the capability of helical devices to provide magnetic confinement in vacuum (i.e. without large inductive plasma currents) gives rise to the observation of plasma recovery even after some energy confinement times. Time-scales and the dynamics of recovery for close to marginal termination indicates some robustness in the response of helical devices to large unintended perturbations. (paper)