  We have performed 3-D numerical magnetohydrodynamic (MHD) jet experiments to
study the instabilities associated with strongly toroidal magnetic fields and
determine if such magnetic configurations in jets are as unstable as similar
situations are found to be in the laboratory and in analytical estimates. A
perfectly conducting Keplerian disc with fixed density, rotational velocity and
pressure is used as a lower boundary for the jet. The energy equation is
solved, with the inclusion of self-consistently computed heating by viscous and
magnetic dissipation. The resulting jets evolve into time-dependent,
non-axisymmetric configurations, but we find only minor disruption of the jets
by for example the kink instability. We find that magnetic dissipation may have
profound effects on the jet flow as: 1) it turns on in highly wound up magnetic
field regions and helps to prevent critical kink situations; 2) it influences
jet dynamics by re-organizing the magnetic field structure and increasing
thermal pressure in the jet; and 3) it influences mass loading by increasing
temperature and pressure at the base of the jet.
