Coulomb clusters of dust particles in a cusp magnetic trap under microgravity conditions

Abstract

We have performed experimental and theoretical investigation of the formation and behavior of Coulomb clusters of charged diamagnetic particles in a cusp magnetic trap under microgravity conditions aboard the International Space Station. Graphite particles of 100-400 μm in size were used in experiments due to the highest specific magnetic susceptibility. We have observed the formation of clusters in the shape of an oblate ellipsoid of revolution and their oscillations after dynamical action by changing the magnetic field. Observing the excitation and damping of oscillations we have made some estimations. Molecular dynamics simulations of the observed processes have been made. Their results are in reasonable agreement with experiments. Some differences are evidently due to some unaccounted-for reasons.

Structure of a Coulomb cluster in the cusp magnetic trap under microgravity conditions

“…Its thickness 2 c along the symmetry axis z is about 1.2 cm and its diameter 2 a in the plane x − y is about 2.4 cm. The relation c / a = 1/2 corresponds to the inverse ratio between the axial and radial components of the cusp trap force and is in agreement with the theoretical result in the approximation of continuous medium . The position of the cluster on the z ‐axis depends on the relation between the currents in the coils.…”

“…This means that the auto‐adhesion counteracts the Coulomb repulsion to a greater extent than magnetic confinement. If the charged particles were separated and held only by magnetic forces, the cluster would have become more oblate; for the cluster of charged particles in a cusp trap, c / a = 0.3 . But the particles do not form a dense structure; they occupy only 1/10 of the cluster volume.…”

“…It is described in detail in Refs. . A cusp magnetic field is produced by two electromagnet coils located on the same axis with currents circulated in opposite directions.…”

“…The main idea of the experiment “Coulomb crystal” aboard the International Space Station (ISS) is the formation, confinement, and investigation of stable structures of charged particles in a non‐ionized medium at room temperature. This is an alternative to the plasma‐dust structures in gas discharges usually used for the study of Coulomb structures .…”

“…However, the obtained clusters consisted of fewer than 10 particles in the magnetic field B ≈ 10 4 G with ∇ B ≈ 10 5 G/cm. In the next step, to create Coulomb clusters containing thousands particles, we have used the cusp magnetic trap aboard ISS under microgravity conditions . In this case, the fields B ≈ 10 2 G with ∇ B ≈ 10 2 G/cm were enough.…”