We report a systematic experimental and numerical study of the electrontemperature in ultra-cold Rydberg plasmas. Specifically, we have measured theasymptotic expansion velocities of ultra-cold neutral plasmas (UNPs) whichevolve from cold, dense samples of Rydberg rubidium atoms using iontime-of-flight spectroscopy. From this, we have obtained values for the initialplasma electron temperature as a function of the original Rydberg atom densityand binding energy. We have also simulated numerically the interaction of UNPswith a large reservoir of Rydberg atoms to obtain data to compare with ourexperimental results. We find that for Rydberg atom densities in the range$10^7 - 10^9$ cm$^{-3}$, for $n > 40$, the electron temperature, $T_{e,0}$, inthe Rydberg plasma is determined principally by the plasma environment when theUNP decouples from the Rydberg atoms at the end of the avalanche regime, andthis occurs when the plasma electrons are too cold to ionize the remainingRydberg population. The resulting electron temperature as a fraction of theinitial Rydberg binding energy is strongly correlated with the fraction ofatoms that have ionized at the end of the avalanche, and is in the range $0.7\lesssim k_BT_{e,0}/|E_{b,i}| \lesssim 3$. On the other hand, plasmas fromRydberg samples with $n \lesssim 40$ evolve with no significant additionalionization of the the remaining atoms once a threshold number of ions has beenestablished. The dominant interaction between the plasma electrons and theRydberg atoms is one in which the atoms are deexcited, a process that competeswith adiabatic cooling to establish an equilibrium where $T_{e,0}$ isdetermined by their Coulomb coupling parameter, $\Gamma_e \sim 0.01$. In thisregime, the Rydberg atoms remain localized as the UNP expands, and the plasmaand atoms decouple gradually.
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