Electrostatic shocks excited by velocity modulation of an ion beam in a plasma.

Nonlinear evolutions of electrostatic shocks excited by a velocity-modulated ion beam along a magnetized plasma column are investigated by computer simulation for a Q-machine experiment. In the case of a beam velocity modulation, the perturbations grow spatially with subsequent saturation due to an ion bunching of the beam. With an increase in modulation ratio an electrostatic shock is formed, accompanied with a steepening of the propagating front. In the case of a beam density-modulation, however, the initial density jump decays simply. The velocity modulation method is quite effective for an excitation of electrostatic shocks in the Q-machine plasma with finite Landau damping. The simulation results using velocity modulation are consistent with the experimental results.

Rotation of a two-dimensional Coulomb cluster in a magnetic field.

A coordinated study of a laboratory experiment, a computer simulation, and a theoretical analysis reveals a structure and its dynamic motion of a Coulomb cluster trapped in a plasma with a confining potential in the presence of gravitational and magnetic fields. Charged dust particulates are found to form a circle in a horizontal plane with a radius determined by a balance of a restoring force due to a confining potential and the screened Coulomb force between dust particulates. The dust particulates in a circle show angular rotation with the structure intact, while they oscillate radially around the equilibrium orbit. The analytical study reveals the oscillatory rotational nature of dust particulates as a result of coupling between the Lorentz force and the harmonic oscillation.