  We report the sudden and spontaneous evolution of an initially correlated gas
of repulsively interacting Rydberg atoms to an ultracold plasma. Under
continuous laser coupling we create a Rydberg ensemble in the strong blockade
regime, which at longer times undergoes an ionization avalanche. By combining
optical imaging and ion detection, we access the full information on the
dynamical evolution of the system, including the rapid increase in the number
of ions and a sudden depletion of the Rydberg and ground state densities.
Rydberg-Rydberg interactions are observed to strongly affect the dynamics of
plasma formation. Using a coupled rate-equation model to describe our data, we
extract the average energy of electrons trapped in the plasma, and an effective
cross-section for ionizing collisions between Rydberg atoms and atoms in
low-lying states. Our results suggest that the initial correlations of the
Rydberg ensemble should persist through the avalanche. This would provide the
means to overcome disorder-induced-heating, and offer a route to enter new
strongly-coupled regimes.
