  We present a laboratory study on the instability of internal wave attractors
in a trapezoidal fluid domain filled with uniformly stratified fluid. Energy is
injected into the system via standing-wave-type motion of a vertical wall.
Attractors are found to be destroyed by parametric subharmonic instability
(PSI) via a triadic resonance which is shown to provide a very efficient energy
pathway from long to short length scales. This study provides an explanation
why attractors may be difficult or impossible to observe in natural systems
subject to large amplitude forcing.
