Ultracold Neutral Plasmas Well into the Strongly Coupled Regime

Abstract

Abstract Ultracold neutral plasmas (UNPs) are plasmas generated through a rapid photoionization process of a laser-cooled atomic gas. Because of the very low initial ionic temperature ( T i (0) ∼mK), UNPs are extremely strongly coupled. Following the formation of correlations, UNPs settle into a coupling regime with Γ ∼ 1, where Γ is the usual Coulomb coupling parameter. The observation of a wider range of plasma phenomena requires experimental control over the details of this process. We describe the generation and diagnosis of UNPs in the strongly coupled plasma regime with Γ ≥ 1 using calcium in a magneto-optical trap. We discuss four avenues to achieve such couplings, including the use of electron screening, multiple ionization to higher ionization states, Rydberg atom dynamics, and direct laser-cooling of the ions. Electron screening mitigates the initial Coulomb repulsion, but also impacts the final effective coupling. We illustrate this by calculating the structural properties of UNPs for different strengths of electron screening for typical values of Γ. Molecular dynamics (MD) simulations are used to reveal the dynamical impacts of electron screening, and show that the final Γ is readily increased whereas the effective coupling remains of order unity. Similarly, we perform MD for a double ionization process in which the second ionization is timed carefully to correspond to a minimum in the time evolution of g ( r , t ). In addition to their intrinsic interest, UNPs can provide a platform for exploring basic plasma physics relevant to a wide range of seemingly disparate plasmas, including fusion-class plasmas.