Effect of Cusp Magnetic Field on Ion Acoustic Wave Propagation

Surprises are galore when quiescent plasma is perturbed by small periodic voltages. For that, the achievable quiescence level in plasma is very important to start with. Multi-pole cusp configuration is found to be an ideal configuration in which the field is B~0 in the center as well as at the boundary magnetic field curvature is good for plasma confinement. The filamentary produced argon plasma confined in this configuration with six electromagnets is found to be very quiescent (<0.1%).

The perturbations are applied at ion-acoustic (IA) frequencies. Since the MPD has electromagnets and the plasma confined by these magnets can be micro-controlled using those magnets, the IA wave studies are done with varying cusp magnetic field strengths. The IA has launched in the field-free region where the ions are unmagnetized and hence the IA propagation is expected not to be affected by the magnetic field in the cusp regions. But since the electrons also participate in the IA wave propagations and the cusp magnetic field affects them. Since the cusp magnetic field confines the hot energy electrons by mirror effect, the increase in cusp field provides various IA wave damping scenarios including the finite Larmor radius effect of electrons. These phenomena have been studied in detail by varying the cusp magnetic field strength and will be discussed.