Probing Plasma Density using Magnetosonic Waves in the Big Red Ball.*

The nature of the magnetosonic wave is investigated in the Big Red Ball (BRB) at the Wisconsin Plasma Physics Laboratory (WiPPL). BRB is a spherical multi-cusp plasma confinement vessel that generates plasmas to study magnetic reconnection. The Helmholtz coils in the BRB produce a magnetic field with uniform magnetic pressure and then the field is reversed using the drive coils to force reconnection. As the drive coils are turned on, there is a reduction in the magnetic pressure allowing plasma to move outward. As the plasma moves outward, the wavefront separating the outward-moving plasma and the stationary plasma moves inward toward the center. Measuring the velocity of this wave allows us to calculate the initial plasma density distribution which in turn allows us to calculate the density distribution at later times using ideal MHD. In our experimental data, along with a primary wave, we also see many secondary waves that can tell us more about the density evolution in time. The analysis of these secondary waves is in progress. The identification of the wave allows us to determine the plasma density distribution in a single stroke. It also allows us to calculate temperature, boundary conditions imposed on the reconnection layer, and other plasma properties.