Shell-Structure Phase of Magnetically Confined Strongly Coupled Plasmas

TLDR

We report the observation of shell structure in ^9Be⁺ nonneutral plasmas confined in a Penning trap. Ion clouds of up to 15,000 ^9Be⁺ ions (~10⁸ cm⁻³) were laser‑cooled to ~10 mK and imaged via laser‑induced fluorescence over Coulomb coupling constants Γ≈20–200 to reveal shell formation. Under these conditions the ions are strongly coupled and exhibit liquid‑like and solid‑like behavior through concentric shell formation, which matches theoretical predictions.

Abstract

We report the observation of shell structure in $^{9}\mathrm{Be}^{+}$ nonneutral plasmas (ion clouds) confined in a Penning trap. Clouds containing up to 15000 ions (density \ensuremath{\cong} ${10}^{8}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) were laser cooled to temperatures of about 10 mK. Under these conditions, the ions are strongly coupled and exhibit liquidlike and solidlike behavior through the formation of concentric shells. The shells were observed by direct imaging of the laser-induced ion fluorescence for values of the Coulomb coupling constant $\ensuremath{\Gamma}$ ranging from about 20 to 200. The shell structure is compared with theoretical predictions.