  We investigate the phenomenon of bistability in a thermal gas of cesium atoms
excited to Rydberg states. We present both measurements and a numerical model
of the phenomena based on collisions. By directly measuring the plasma
frequency, we show that the origin of the bistable behavior lies in the
creation of a plasma formed by ionized Rydberg atoms. Recombination of ions and
electrons manifests as fluorescence which allows us to characterize the plasma
properties and study the transient dynamics of the hysteresis that occurs. We
determine scaling parameters for the point of plasma formation, and verify our
numerical model by comparing measured and simulated spectra. These measurements
yield a detailed microscopic picture of ionization and avalanche processes
occurring in thermal Rydberg gases. From this set of measurements, we conclude
that plasma formation is a fundamental ingredient in the optical bistability
taking place in thermal Rydberg gases and imposes a limit on usable Rydberg
densities for many applications.
