Expansion of Ultracold Neutral Plasmas with Exponentially Decaying Density Distributions

TL;DR

This study investigates the expansion dynamics of ultracold neutral plasmas with exponential density profiles, revealing hydrodynamic behavior and deviations due to local non-neutrality at certain conditions, and compares it to Gaussian profile expansion.

Contribution

It introduces a detailed analysis of plasma expansion with exponential density distributions and compares it to Gaussian cases, highlighting new insights into early-time non-neutrality effects.

Findings

Hydrodynamic expansion model fits well for most conditions.

Local non-neutrality observed at low density and high electron temperature.

Similar scaling laws apply to exponential and Gaussian density profiles.

Abstract

We present a study of the expansion of an ultracold neutral plasma (UCNP) with an initial density distribution that decays exponentially in space, created by photoionizing atoms shortly after their release from a quadrupole (or biconic cusp) magnetic trap. A characteristic ion acoustic timescale is evident in the evolution of the plasma size and velocity, indicating the dynamics are reasonably well described by a model of hydrodynamic expansion of a quasi-neutral plasma. However, for low plasma density and high initial electron temperature, excess ion kinetic energy in the vicinity of the central density peak suggests significant local non-neutrality at early times. Observations are compared to the well-understood self-similar expansion of an UCNP with an initial Gaussian density distribution, and a similar scaling law describes the evolution of plasma size for both cases.