Analysis of the role of the ion polarization current on the onset of the neoclassical tearing mode in disrupting plasmas

In the linear theory, the stability of the mode is completely defined by the stability index connected to the change of the magnetic energy of the plasma due to the reconnetion. However, aner the magnetic reconnection, non-linear mechanisms start to play an important role so that the magnetic island grows if the destabilizing (linear and non-linear) drives overcome the stabilizing effects. The contribution of the phenomena which affects the island growth are described in the Generalized Rutherford Eqution (GRE). According with the GRE theory, the curvature of the magnetic field lines in a toroidal configuration has a stabilizing effect, while the reduction of the bootstrap current due to the flattening of the pressure profile inside the island produces the neoclassical destabilization of the magnetic tearing perturbation. In an inhomogeneous two-fluid plasma, difference in the drin motion of electrons and ions drives a parallel return current, called “polarization current”, which depends on the island width w and can be either stabilizing or destabilizing depending on the ratio ?/?& where ? and ?& are the island frequency and the ion diamagnetic frequency. The ion polarization current is thought to play an important role at the onset of the mode because of its scaling a 1/w3 that makes it a dominant term in the GRE when the island width is small. The curvature and the bootstrap terms depend on the equilibrium profiles (safety factor, pressure) and the poloidal ?, so that they change following a slower equilibrium timescale. On the other hand, the magnitude of the ion polarization current contribution also depends on equilibrium quantities, but its sign directly depends on the island frequency shin from the diamagnetic frequency, which may respond to faster temperature changes at the rational surface. This suggests that in an essentially stable discharge, a rapid change of the local temperature due to different phenomena (e.g., impurities radiation, other MHD activities…), could lead to a sudden offset of the balance of the competing bootstrap and curvature effects, leading to the onset of the mode, and eventually to disruption. The effect of the ion polarization current is evaluated with a time resolution of the available diagnostic used to estimate the ion temperature (~10ms). Firstly the analysis is performed on four JET pulses, that develop a neoclassical tearing mode in the termination phase of the plasma shot, aner a strong reduction of the temperature at the edge due to the presence of impurities (Edge Cooling). The behavior of the ion polarization current contribution is then compared to stable pulses, which exhibit an edge cooling without developing a mode. The analysis on the unstable pulses shows that the temperature fluctuation due to the edge cooling increases the destabilizing contribution of the ion polarization current. When this happens, it is easier for a resonant helical perturbation of the rational surface to overcome the stabilizing contributions leading to an unstable mode. This is confirmed by the analysis on the stable pulses showing that the destabilizing contribution of the ion polarization current does not increase aner the edge cooling. The reason why the ion polarization current contribution seems not to be affected by the edge cooling is because the reduction due to the temperature fluctuation does not reach the rational surface. This finding has been tested on a more general database of selected pulses with edge cooling. The flattening width is evaluated and compared to the position of the rational surface of every pulse in the database, showing that the mode is triggered whenever the flattening is close enough to the rational surface, producing a result which is consistent with the physical interpretation.