  We report on a novel dynamical phenomenon in electron spin resonance
experiments of phosphorus donors. When strongly coupling the paramagnetic
ensemble to a superconducting lumped element resonator, the coherent exchange
between these two subsystems leads to a train of periodic, self-stimulated
echos after a conventional Hahn echo pulse sequence. The presence of these
multi-echo signatures is explained using a simple model based on spins rotating
on the Bloch sphere, backed up by numerical calculations using the
inhomogeneous Tavis-Cummings Hamiltonian.
