Rigid and differential plasma crystal rotation induced by magnetic fields

Observations show that plasma crystals, suspended in the sheath of a radio- frequency discharge, rotate under the influence of a vertical magnetic fiel d. Depending on the discharge conditions, two different cases are observed: a rigid-body rotation (al the particles move with a constant angular veloc ity) and sheared rotation (the angular velocity of particles has a radial d istribution). When the discharge voltage is increased sufficiently, the par ticles may even reverse their direction of motion. A simple analytical mode l is used to explain qualitatively the mechanism of the observed particle m otion and its dependence on the confining potential and discharge condition s. The model takes into account electrostatic, ion drag, neutral drag, and effective interparticle interaction forces. For the special case of rigid-b ody rotation, the confining potential is reconstructed. Using data for the radial dependence of particle rotation velocity, the shear stresses are est imated. The critical shear stress at which shear-induced melting occurs is used to roughly estimate the shear elastic modulus of the plasma crystal. T he latter is also used to estimate the viscosity contribution due to elasti city in the plasma liquid. Further development is suggested in order to qua ntitatively implement these ideas.