Using Rydberg atoms to increase electron coupling strength in ultracold neutral plasmas

We have experimentally demonstrated heating and cooling of electrons in an ultracold neutral plasma (UNP) with an initial electron temperature $T_{e,i}$ (determined by the frequency of the ionizing laser) by embedding Rydberg atoms in them in the first 20 ns of their evolution \footnote{Crockett {\it et al.}, {\it Phys. Rev. A}, {\bf 98}, 043431 (2018)}. We have quantified the crossover initial electron temperature, $T_{e,i} = T_{CO}$, for a plasma that is neither heated nor cooled when $N_R$ Rydberg atoms with binding energy $E_b$ are added to a plasma with initial ion number $N_{ion}$. This condition is $k_B T_{CO} \approx 2.7 \times |E_b|$ when $N_R \approx 0.2 \times N_{ion}$. Additionally, we have measured the change in the plasma expansion velocity when $E_b$ does not satisfy the crossover condition for a range of $N_R/N_{ion}$ values. These results are in good agreement with Monte-Carlo calculations. We are also pursuing similar studies, both experimental and numerical, in the regime where $N_R \gg N_{ion}$ to see if the plasma electrons can be cooled sufficiently to increase their coupling to $\Gamma_e \sim 0.5$ in the first 5 $\mu$s of plasma evolution, as predicted by by Pohl {\it et al.} \footnote{T. Pohl {\it et al.}, {\it Eur. Phys. J. D}, {\bf 40}, 45 (2006)}.