Achieving Higher Γ in an Ultracold Neutral Plasma
Ultracold neutral plasmas are generated by photoionizing laser-cooled atoms close to threshold.The ion strong coupling at early times after the plasma is created is high, due to the very low initial temperature (1 mK) of the ions. However the strong coupling parameter Γ is limited at times later than 100 ns by an ultrafast, nonequilibrium relaxation of the ions. This process is called “disorder-induced heating” (DIH) and it limits Γ in our plasmas to order unity. In this talk I will describe an experiment in an ultracold neutral calcium plasma that shows that electron screening slows the ion equilibration rate and reduces the equilibrium ion temperature, but also limits the effective (screened) ion strong coupling Γ^ to approximately 2. I will also present recent results from an experiment designed to increase the strong coupling of an ultracold neutral plasma by promoting the plasma ions to the second ionization state. Simulations predict that the maximum value of Γ depends on the time at which the second ionization laser pulses arrive relative to the DIH phase of the plasma. Using laser-induced fluorescence we map out the ion velocity distribution of the Ca+ ions in a partially doubly ionized plasma and show that the heating due to the second ionization depends on the timing of the second ionization laser pulses, as predicted by MD simulations. I will also discuss the future of this project and other projects underway that are designed to increase Γ in an ultracold neutral plasma.
Host: Trey Porto