  Magnetic fields are believed to play an essential role in astrophysical jets
with observations suggesting the presence of helical magnetic fields. Here, we
present three-dimensional (3D) ideal MHD simulationsof the Caltech plasma jet
experiment using a magnetic tower scenario as the baseline model. Magnetic
fields consist of an initially localized dipole-like poloidal component and a
toroidal component that is continuously being injected into the domain. This
flux injection mimics the poloidal currents driven by the anode-cathode voltage
drop in the experiment. The injected toroidal field stretches the poloidal
fields to large distances, while forming a collimated jet along with several
other key features. Detailed comparisons between 3D MHD simulations and
experimental measurements provide a comprehensive description of the interplay
among magnetic force, pressure and flow effects. In particular, we delineate
both the jet structure and the transition process that converts the injected
magnetic energy to other forms. With suitably chosen parameters that are
derived from experiments, the jet in the simulation agrees quantitatively with
the experimental jet in terms of magnetic/kinetic/inertial energy, total
poloidal current, voltage, jet radius, and jet propagation velocity.
Specifically, the jet velocity in the simulation is proportional to the
poloidal current divided by the square root of the jet density, in agreement
with both the experiment and analytical theory. This work provides a new and
quantitative method for relating experiments, numerical simulations and
astrophysical observation, and demonstrates the possibility of using
terrestrial laboratory experiments to study astrophysical jets.
