Influence of Electron Evaporative Cooling on Ultracold Plasma Expansion

TL;DR

This paper investigates how electron evaporative cooling affects the expansion dynamics of ultracold neutral plasmas at low densities, combining experimental data with modeling to identify conditions where evaporation significantly influences plasma behavior.

Contribution

It introduces a model accounting for electron evaporation effects on plasma expansion and provides a calibration technique linking detector signals to plasma particle numbers.

Findings

Electron evaporation significantly impacts plasma expansion at densities below 10^8 /cm^3.

The model shows good agreement with experimental data on plasma expansion.

A calibration method relates detector signals to total plasma particles.

Abstract

The expansion of ultracold neutral plasmas (UCP) is driven primarily by the thermal pressure of the electron component and is therefore sensitive to the electron temperature. At lower densities (less than 10$^8$ /cm$^3$), evaporative cooling has a significant influence on the UCP expansion rate. We studied the effect of electron evaporation in this density range. Owing to the low density, the effects of three-body recombination were negligible. We modeled the expansion by taking into account the change in electron temperature owing to evaporation as well as adiabatic expansion and found good agreement with our data. We also developed a simple model for initial evaporation over a range of ultracold plasma densities, sizes, and electron temperatures to determine over what parameter range electron evaporation is expected to have a significant effect. We also report on a signal calibration technique, which relates the signal at our detector to the total number of ions and electrons in the ultracold plasma.