| A new experiment of magnetic field diffusion through plasma was performed with static, time varying, quadrupole magnetic fields created by a dual coil device. The experiment was designed to answer some of the questions raised by the process of plasma energization at magnetic neutral points. Prior experimental and theoretical work offers controversial views regarding the possibility of efficient plasma energization in these conditions. Thus, the investigation concentrated on the evaluation of the effectiveness of plasma energization (heating and acceleration) when starting from a hydrostatic initial condition, and the identification of actual mechanisms of energy transfer and dissipation. The possibility that the energy transfer process involves magnetic field line reconnection was also discussed.; Parallel to the experimental work, an unsteady analytic model was proposed. The analysis presented two views of the process: (a) a fluid model describing the energization of fully ionized plasmas, and (b) a kinetic model adequate for describing the energization of weakly ionized plasmas. Experimentally, the investigation was limited to the case of weakly ionized plasma. The experimental data showed good agreement with the model and with some results from past experiments. Thus, the experiment demonstrated that the diffusion of variable magnetic fields through (weakly ionized) plasma leads to one or two orders of magnitude increase in the ionization fraction, to plasma heating, and to plasma motion. A distinct result of the MDE experiment is the detection of low frequency (kHz range) ion acoustic waves. The analysis shows that the generation of ion acoustic waves is the main mechanism of energy dissipation, in the plasma, during the slow decline of the magnetic field.; In terms of energy transfer, the experiment showed that the weak coupling between the magnetic field and the low conductivity plasma is directly responsible for a small energy conversion efficiency. It is concluded, however, that the process may prove useful in engineering applications (involving plasma ionization and acceleration), if certain conditions are met. The data also showed that, in the configuration tested, magnetic field line reconnection is an unlikely mechanism for plasma energization. |
