A model for the motion of filaments in the scrape-off layer (SOL) of a tokamak is suggested. It is shown that the vacuum acceleration, g = 2(Tef + Tif)/(miR) (Te,if are filament temperatures, R is the major radius), is significantly reduced by the polarization currents in the SOL plasma. This reduction is amplified by the convergence of the magnetic field lines in the toroidal direction, especially near the X-point. For filaments with high density and temperature typical for ELMs the short-circuiting of the current through the divertor plates can be neglected since the required parallel current exceeds the ion saturation current at the plates. The predicted values of acceleration are in reasonable agreement with the filament accelerations observed on MAST and ASDEX Upgrade. In contrast, the polarization of the filaments in L-mode can be short-circuited through the divertor plates since the required parallel current is an order of magnitude smaller due to the smaller filament pressure. Hence, for L-mode filaments a constant velocity is predicted in accordance with earlier models.
X-point and divertor filament dynamics from gas puff imaging on TCV