  We present an experimental study of the saturated non-linear dynamics of an
inertial wave attractor in an axisymmetric geometrical setting. The experiments
are carried out in a rotating ring-shaped fluid domain delimited by two
vertical coaxial cylinders, a conical bottom, and a horizontal deformable upper
lid as wave generator: the meridional cross-section of the fluid volume is a
trapezium, while the horizontal cross-section is a ring. First, the fluid is
set into a rigid-body rotation. Thereafter, forcing is introduced into the
system via axisymmetric low-amplitude volume-conserving oscillatory motion of
the upper lid. After a short transient of about 10 forcing periods, a
quasi-linear regime is established, with an axisymmetric inertial wave
attractor. The attractor is prone to instability: at long time-scale (order 100
forcing periods) a saturated fully non-linear regime develops as a consequence
of an energy cascade draining energy towards a slow two-dimensional manifold
represented by a regular polygonal system of axially-oriented cyclonic vortices
that are slowly precessing around the inner cylinder. We show that this slow
two-dimensional manifold manifests a persistent slow prograde motion and a
strong cyclonic-anticyclonic asymmetry quantified by the time-evolution of the
probability density function of the vertical vorticity.
