The ion wave formation during the ultracold plasma expansion

TL;DR

This paper uses direct simulations to analyze ion wave formation during ultracold plasma expansion, revealing a self-similar velocity profile and a charged boundary layer that influences the expansion dynamics.

Contribution

It provides new self-similar solutions and detailed insights into the charged layer formation during ultracold plasma expansion, supported by comparison with experimental data.

Findings

Charged boundary layer retains electrons during expansion.

Ion velocity becomes self-similar and exceeds sonic speed.

Expansion dynamics depend on plasma parameters and match experiments.

Abstract

We present the results of direct simulation of the expansionof a two-component ultracold plasmafor various numbers of particles, densities, and electron temperatures. A description of the expansionprocess common to all plasma parameters is given. After the escape of fast electrons from the plasmacloud, the excess positive charge is localized at the outer boundary, in a narrow layer. This layerhas a characteristic front shape with a sharp drop in the charge concentration. The charged layerretains the remaining electrons during the entire expansion process. As the plasma expands, thespeed of movement of the charged layer becomes constant and significantly exceeds the sonic speedof ions. In addition, the dependence of the radial velocity of ions on the radius acquires a self-similarcharacter long before the final stage of expansion. On the basis of the calculation results, equationsand self-similar solutions are obtained. General dependences on plasma parameters are determined,which are compared with experimental data.