Ion Temperature Evolution in an Ultracold Neutral Plasma

TL;DR

This study investigates the long-term ion temperature evolution in an ultracold neutral plasma, revealing adiabatic cooling effects, heat exchange limitations, and potential additional heating mechanisms during plasma expansion.

Contribution

It provides the first detailed measurement of ion temperature dynamics in an expanding ultracold plasma, including a model that accounts for ion-electron heat exchange and Coulomb coupling.

Findings

Ion temperature decreases by an order of magnitude during expansion.

Heat exchange with electrons limits cooling.

Coulomb coupling increases under certain conditions.

Abstract

We study the long-time evolution of the ion temperature in an expanding ultracold neutral plasma using spatially resolved, laser-induced-fluorescence spectroscopy. Adiabatic cooling reduces the ion temperature by an order of magnitude during the plasma expansion, to temperatures as low as 0.2 K. Cooling is limited by heat exchange between ions and the much hotter electrons. We also present evidence for an additional heating mechanism and discuss possible sources. Data are described by a model of the plasma evolution, including the effects of ion-electron heat exchange. We show that for appropriate initial conditions, the degree of Coulomb coupling of ions in the plasma increases during expansion.