Ultracold Plasma Expansion in a Magnetic Field

TL;DR

This study investigates how ultracold plasma expands across magnetic field lines, revealing a B$^{-1/2}$ scaling of expansion velocity explained by a nonlinear ambipolar diffusion model.

Contribution

It provides the first measurement of ultracold plasma expansion across magnetic fields and introduces a nonlinear ambipolar diffusion model to explain the anisotropic expansion behavior.

Findings

Expansion velocity scales as B$^{-1/2}$

Ion distribution imaging reveals anisotropic diffusion

Early expansion dominated by Coulomb explosion

Abstract

We measure the expansion of an ultracold plasma across the field lines of a uniform magnetic field. We image the ion distribution by extracting the ions with a high voltage pulse onto a position-sensitive detector. Early in the lifetime of the plasma ($< 20$ $\mu$s), the size of the image is dominated by the time-of-flight Coulomb explosion of the dense ion cloud. For later times, we measure the 2-D Gaussian width of the ion image, obtaining the transverse expansion velocity as a function of magnetic field (up to 70 G). We observe that the expansion velocity scales as B$^{-1/2}$, explained by a nonlinear ambipolar diffusion model with anisotropic diffusion in two different directions.