  The gravitational waves from the binary neutron star merger GW170817 were
accompanied by a multi-wavelength electromagnetic counterpart, which confirms
the association of the merger with a short gamma-ray burst (sGRB). The
afterglow observations implied that the event was accompanied by a narrow,
$\sim 5~$deg, and powerful, $\sim 10^{50}$ erg, jet. We study the propagation
of a Poynting flux-dominated jet within the merger ejecta (kinematic,
neutrino-driven and MRI turbulence-driven) of a neutrino-radiation-GR-MHD
simulation of two coalescing neutron stars. We find that the presence of a
post-merger low-density/low-pressure polar cavity, that arose due to angular
momentum conservation, is crucial to let the jet break out. At the same time
the ejecta collimates the jet to a narrow opening angle. The collimated jet has
a narrow opening angle of $\sim 4$-$7$ deg and an energy of
$10^{49}$-$10^{50}~$erg, in line with the observations of GW170817 and other
sGRBs.
