Using Rydberg atoms to the control electron temperature in an ultra-cold plasma

In this presentation, we will describe recent results of experiments in which Rydberg atoms are embedded into an ultra-cold plasma. The plasma is created by partial photoionization of a dense, cold sample of rubidium atoms in a MOT using a Littman dye laser. After ionization, the ions have the same temperature as the parent atoms, while the initial electron temperature, $T_e$, depends on the Littman laser photon energy. At a controllable time delay (20 ns - 10 $\mu$s), neutral atoms embedded in the plasma are excited to a specific Rydberg state by a narrow bandwidth pulsed laser. In such a system, it is predicted that the plasma electrons may be cooled if the Rydberg binding energy, $E_b$, is greater than $4k_BT_e$. Our preliminary results indicate that the evaporation rate of electrons from a plasma with $T_e = 50$ K is suppressed by the addition of $36d$ Rb atoms ($E_b = 2.6 \ k_B T_e$). In addition, we find that recombination of ions and electrons into high-$n$ Rydberg states is enhanced by addition of $36d$ atoms (indicating a lower $T_e$, since $\Gamma_{TBR} \propto T_e^{-9/2}$). On the other hand, recombination is suppressed by the addition of $26d$ atoms ($E_b = 5.2 \ k_BT_e$) to the plasma, suggesting an increase in the electron temperature.