The steep gradients in density and temperature at the plasma edge excite instabilities. These instabilities can be divided into two classes, exchange instabilities and drift waves. In interchange instabilities, charge separation of ions and electrons occurs due to magnetic field curvature. The resulting electric fields cause the particles to drift out. This leads to relatively strong transport. In the drift wave, the electrons respond to pressure gradients along the magnetic field. This leads primarily to propagation in the electron diamagnetic direction (perpendicular to the magnetic field and perpendicular to the radial direction). Drift waves cause little transport. At equilibrium, when the instabilities are nonlinearly saturated, strong turbulence is formed. In this situation is the low confinement mode (L-mode). The vortices occur mainly on small scales (close to the hybrid Lamor radius). Due to their high intrinsic self-rotation, these vortices mix the field faster than the instabilities can grow. The exchange instability is suppressed at small scales and has to move to larger scales. There it causes less transport. Also waves are scattered by the small vortices, so even propagation can be prevented.

relevant publications

P Manz et al. 'On the phase velocity in between weak and strong plasma edge turbulence'  Plasma Phys. Control. Fusion 60 085002 (2018)