  Mechanisms of plasmoid drift following massive material injection are studied
via 3D non-linear MHD modelling with the JOREK code, using a transient neutral
source deposited at the low field side midplane of a JET H-mode plasma to
clarify basic processes and compare with existing theories. The simulations
confirm the important role of the propagation of shear Alfv\'en wave (SAW)
packets from both ends of the plasmoid (``SAW braking'') and the development of
external resistive currents along magnetic field lines (``P\'egouri\'e
braking'') in limiting charge separation and thus the $\mathbf{E}\times
\mathbf{B}$ plasmoid drift, where $\mathbf{E}$ and $\mathbf{B}$ are the
electric and magnetic fields, respectively. The drift velocity is found to be
limited by the SAW braking on the few microseconds timescale for cases with
relatively small source amplitude while the P\'egouri\'e braking acting on a
longer timescale is shown to set in earlier with larger toroidal extent of the
source, both in good agreement with existing theories. The simulations also
identify the key role of the size of the $\mathbf{E}\times \mathbf{B}$ flow
region on plasmoid drift and show that the saturated velocity caused by
dominant SAW braking agrees well with theory when considering an effective
pressure within the $\mathbf{E}\times \mathbf{B}$ flow region. The existence of
SAWs in the simulations is demonstrated and the 3D picture of plasmoid drift is
discussed.
