Laboratory studies of the transition region from a magnetized to an unmagnetized plasma

This dissertation presents the results of a series of new and unique laboratory studies that explore the physics of the temporal and spatial transition from a plasma with magnetized electrons to unmagnetized electrons. Understanding this transition region has implications for several areas of plasma physics, including nonlinear whistler wave-packet propagation, magnetic field energy conversion to electron heat, and magnetic field line reconnection. The experiments are performed in a plasma where the bulk electron population is magnetized but the ion population is unmagnetized. The two main studies are the lack of magnetic helicity conservation of a whistler mode wave packet transmitted through a magnetic null point and magnetic field line reconnection/annihilation of a reversed magnetic field topology.; The first set of experiments presented in this dissertation show that a whistler wave packet propagating in a magnetized plasma has a non-zero helicity and that it is a conserved quantity. When transmitted through a magnetic neutral region, however, the helicity changes sign and is not conserved. This is the first measurement of such a helicity reversal. The detailed magnetic structure of the whistler wave packet before, during, and after the transmission indicates how the helicity reversal occurs. These results have implications for the many theories and simulations of dynamical plasma systems that employ conservation of helicity.; The second set of experiments show the free relaxation of a closed B-line region with three-dimensional (3-D) magnetic null points and a two-dimensional (2-D) null layer. It is found that the relaxation time is fast compared to the classical diffusion time but slow compared to convective time scales; B-lines annihilate at the 2-D null layer (not at the 3-D null points); magnetic energy converts to electron thermal energy; and various microinstabilities are produced including ion acoustic noise and plasma waves. This is the first experiment of this kind performed in this parameter regime and it has applications to the field of magnetic field line reconnection.; Also included (as an appendix) is the first set of detailed measurements of the structure of field lines near 3-D magnetic null points in a plasma. The presence of plasma currents cause the field lines to form a large variety of shapes. The elements of the linear expansion matrix of B at the null point are used to quantitatively categorize the different null point structures. These structures naturally arise in a large variety of plasma systems and their role in solar flare evolution is currently being investigated.