Blob and hole dynamics in the rotating plasma column of a simple magnetised torus in open field line configuration

Abstract

Blob and hole dynamics is investigated in the sheared rotating plasma column of a simple magnetised torus using two-dimensional time-averaged measurements and conditional averaging analysis. Experiments are performed in open field line configuration by adding a vertical magnetic field component to the toroidal one, resulting in a quasistationary equilibrium characterised by vertically elongated profiles. The combined effect of plasma rotation with the vertical field leads to a poloidally asymmetric sheared E$\times$B flow in the electron diamagnetic direction, pointing downward and upward in the low and high field side of the midplane, respectively, on both sides of the shear layer. Statistical analysis indicates that density fluctuations are dominated by non-Gaussian positive intermittent blob events in the low-density edge region, and negative hole events in the core plasma, whereas the intermediate shear layer corresponds to the blob birth zone featuring Gaussian statistics. Conditional averaging analysis reveals that, in the course of its propagation, a blob breaks off from a structure elongated in the direction of the flow due to the differential stretching in the vicinity of the shear layer. Blobs are convected out of the potential contour and ejected by the fluctuating field to the limiter, while holes stay inside and propagate back into the main plasma. Blobs move poloidally with velocities up to 3.7 km$/$s and radially up to 2.6 km$/$s. These observations confirm the picture where persistent structures provide a means for a net convection of the charged particles to the limiter, across the magnetic field and beyond the edge region of the magnetised plasmas.