  We report the formation of a ring-shaped array of vortices after injection of
angular momentum in a polariton superfluid. The angular momentum is injected by
a $\ell= 8$ Laguerre-Gauss beam, whereas the global rotation of the fluid is
hindered by a narrow Gaussian beam placed at its center. In the linear regime a
spiral interference pattern containing phase defects is visible. In the
nonlinear (superfluid) regime, the interference disappears and the vortices
nucleate as a consequence of the angular momentum quantization. The radial
position of the vortices evolves freely in the region between the two pumps as
a function of the density. Hydrodynamic instabilities resulting in the
spontaneous nucleation of vortex-antivortex pairs when the system size is
sufficiently large confirm that the vortices are not constrained by
interference when nonlinearities dominate the system.
