Expansion of an ultracold Rydberg plasma

We report a systematic experimental and numerical study of the expansion of ultra-cold Rydberg plasmas. Specifically, we have measured the asymptotic expansion velocities, $v_0$, of ultra-cold neutral plasmas (UNPs) which evolve from cold, dense samples of Rydberg rubidium atoms using ion time-of-flight spectroscopy. From this, we have obtained values for the effective initial plasma electron temperature, $T_{e,0} = m_{ion} v_0^2/k_B$ (where $m_{ion}$ is the Rb$^+$ ion mass), as a function of the original Rydberg atom density and binding energy, $E_{b,i}$. We have also simulated numerically the interaction of UNPs with a large reservoir of Rydberg atoms to obtain data to compare with our experimental results. We find that for Rydberg atom densities in the range $10^7 - 10^9$ cm$^{-3}$, for states with principal quantum number $n > 40$, $T_{e,0}$ is insensitive to the initial ionization mechanism which seeds the plasma. In addition, the quantity $k_B \, T_{e,0}$ is strongly correlated with the fraction of atoms which ionize, and is in the range $0.6 \times |E_{b,i}| \lesssim k_BT_{e,0} \lesssim 2.5 \times |E_{b,i}|$. On the other hand, plasmas from Rydberg samples with $n \lesssim 40$ evolve with no significant additional ionization of the remaining atoms once a threshold number of ions has been established. The dominant interaction between the plasma electrons and the Rydberg atoms is one in which the atoms are deexcited, a heating process for electrons that competes with adiabatic cooling to establish an equilibrium where $T_{e,0}$ is determined by their Coulomb coupling parameter, $\Gamma_e \sim 0.01$.