  We study the interaction of relativistic jets with their environment, using
3-dimensional relativistic particle-in-cell simulations for two cases of jet
composition: (i) electron-proton ($e^{-}-p^{+}$) and (ii) electron-positron
($e^{\pm}$) plasmas containing helical magnetic fields. We have performed
simulations of "global" jets containing helical magnetic fields in order to
examine how helical magnetic fields affect kinetic instabilities such as the
Weibel instability, the kinetic Kelvin-Helmholtz instability and the Mushroom
instability. We have found that these kinetic instabilities are suppressed and
new types of instabilities can grow. For the $e^{-}-p^{+}$ jet, a
recollimation-like instability occurs and jet electrons are strongly perturbed,
whereas for the $e^{\pm}$ jet, a recollimation-like instability occurs at early
times followed by kinetic instability and the general structure is similar to a
simulation without a helical magnetic field. We plan to perform further
simulations using much larger systems to confirm these new findings.
