The limit of strong ion coupling due to electron shielding

TL;DR

This paper demonstrates that electron shielding in ultracold neutral plasmas limits the maximum ion coupling strength, with potential energy ratios capped around 2 unless electrons are externally heated, supported by experimental and theoretical analysis.

Contribution

It provides new experimental measurements of ion velocity distributions and theoretical insights into the limits of ion coupling due to electron screening in ultracold plasmas.

Findings

Ion-ion potential energy is limited by electron screening.

The ratio of potential to kinetic energy is approximately capped at 2.

Experimental results align with Yukawa system simulations.

Abstract

We show that strong coupling between ions in an ultracold neutral plasma is limited by electron screening. While electron screening reduces the quasi-equilibrium ion temperature, it also reduces the ion-ion electrical potential energy. The net result is that the ratio of nearest-neighbor potential energy to kinetic energy in quasi-equilibrium is constant and limited to approximately 2 unless the electrons are heated by some external source. We support these conclusions by reporting new measurements of the ion velocity distribution in an ultracold neutral calcium plasma. These results match previously reported simulations of Yukawa systems. Theoretical considerations are used to determine the screened nearest-neighbor potential energy in the plasma.