  A set of 3D MHD simulations of magnetized jets has been performed. The jets
contain an equipartition primarily poloidal magnetic field and the effect of
jet density on jet dynamics and structure is evaluated. The jet is precessed at
the origin to excite Kelvin-Helmholtz unstable helical modes. We extensively
compare the structure in these simulations with linear stability theory. The
jet that is dense with respect to the external medium develops a high speed
core surrounded by a less dense sheath consisting of slower moving jet fluid.
These simulations suggest that extended extragalactic jets propagate to such
large distances because they are surrounded by a lobe or cocoon whose density
is less than the jet density. (Abridged abstract.)
