  A 3D simulation of a non-relativistic, magnetically driven jet propagating in
a stratified atmosphere is presented, covering about three decades in distance
and two decades in sideways expansion. The simulation captures the jet
acceleration through the critical surfaces and the development of
(kink-)instabilities driven by the free energy in the toroidal magnetic field
component. The instabilities destroy the ordered helical structure of the
magnetic field, dissipating the toroidal field energy on a length scale of
about 2-15 times the Alfven distance. We compare the results with a 2.5D
(axisymmetric) simulation, which does not become unstable. The acceleration of
the flow is found to be quite similar in both cases, but the mechanisms of
acceleration differ. In the 2.5D case approximately 20% of the Poynting flux
remains in the flow, in the 3D case this fraction is largely dissipated
internally. Half of the dissipated energy is available for light emission; the
resulting radiation would produce structures resembling those seen in
protostellar jets.
