Dusty plasma — in which dust particles mix in with plasma — is naturally found in places such as planetary rings, comet tails and the interstellar medium. The dust particles can form hexagonal crystal lattices, which happens when the mutual electrostatic repulsion between the charged dust particles is much larger than the particles’ thermal energy.
In Physics of Plasmas, authors describe how an external magnetic field induces a phase transition in this crystalline structure. The coupling between the magnetic field and the background plasma causes the structure to rotate around the collective center at different rates and generate enough shear to melt the crystal.
In the new experiments, the researchers measured the spatial ordering, or correlation, between the dust particles as the strength of the magnetic field increased. When the magnetic field was relatively weak (up to 5,000 times as strong as the Earth’s magnetic field), the dust particles circulated like a rigid crystal. The spatial orientations of the dust particles did not change during rotation, but when stronger magnetic fields were applied, those particles at outer portions of the rotating dust crystal could not rotate as quickly as the central portions.
Above about one tesla (or 10,000 times the Earth’s magnetic field), the crystalline arrangement of the dust disintegrates, and the dust behaves more like a liquid. Comparing these new observations to theoretical models, the researchers suggest either electric fields at the outer edge or collisions with neutral gas may cause differential rotation and dust crystal melting.
This kind of experiment moves researchers towards studying dusty plasmas as they occur in nature, said Edward Thomas, one of the work’s authors.
Source: “Effect of magnetic field on the phase transition in a dusty plasma,” by S. Jaiswal, T. Hall, S. LeBlanc, R. Mukherjee, and E. Thomas, Physics of Plasmas (2017). The article can be accessed at https://doi.org/10.1063/1.5003972.