Wave Steepening and Shock Formation in Ultracold Neutral Plasmas

TL;DR

This paper reports the observation of wave steepening and potential shock formation in ultracold neutral plasmas, demonstrating how steep density gradients can lead to shock-like phenomena in these systems.

Contribution

It provides the first experimental evidence of wave steepening and conditions near shock formation in ultracold neutral plasmas, highlighting a new research pathway.

Findings

Wave steepening observed during plasma expansion

Significant ion heating at steep density gradients

Ion velocities near Mach number 1 indicating shock conditions

Abstract

We present observations of wave steepening and signatures of shock formation during expansion of ultracold neutral plasmas formed with an initial density distribution that is centrally peaked and decays exponentially with distance. The plasma acceleration and velocity decrease at large distance from the plasma center, leading to central ions overtaking ions in the outer regions and the development of a steepening front that is narrow compared to the size of the plasma. The density and velocity change dramatically across the front, and significant heating of the ions is observed in the region of steepest gradients. For a reasonable estimate of electron temperature, the relative velocity of ions on either side of the front modestly exceeds the local sound speed (Mach number $M \gtrsim 1$). This indicates that by sculpting steep density gradients, it is possible to create the conditions for shock formation, or very close to it, opening a new avenue of research for ultracold neutral plasmas.