Intermittency and blob dynamics in the TORPEX device

Intermittent convective transport caused by blobs is universally observed at the edge of laboratory plasmas and it is of great importance for future fusion reactors. We present recent advances in the basic study of blob formation and propagation dynamics and related universal properties in TORPEX simple magnetized plasmas. Blobs are generated from quasi-coherent interchange modes and, driven by ∇B and curvature-induced charge separation, propagate radially outwards. Depending on the regime, their velocity is determined by the available current paths (parallel or cross-field currents) to damp charge separation. In TORPEX, regimes dominated by either of the two current paths are achieved by varying the ion mass. An analytical expression for the blob velocity including cross-field ion polarization currents, cross-field ion currents due to neutral friction and parallel currents to the sheath is derived and shows good quantitative agreement with the experimental data. To confirm this interpretation, we show preliminary direct measurements of the 2D structure of the blob-induced parallel current using magnetic probes. Parallel blob dynamics are further studied with a Mach probe, revealing the convection of parallel momentum by blobs. Finally, the intermittency associated with blobs results in universal properties of plasma fluctuations, such as the skewness-versuskurtosis diagram, which is here extended to fluctuations of plasma potential and electron temperature.