| The Plasmoid Thruster Experiment (PTX) is a pulsed inductive thruster design using a conical theta pinch coil to form and accelerate plasmoids. Magnetized or Compact Toroid (CT) plasmoids have closed magnetic field structures making them naturally detached from the nozzle of a space craft as well as making them candidates for fusion fueling and confinement. In order to optimize usage of plasmoids, it is important to study the formation processes.;In this study, a method for integrating a magnetic bias field with PTX was investigated. With a magnetic bias field in place at the time of ionization of the plasma propellant, the propellant would ionize on the field lines, "freezing" the field lines into the weakly ionized gas. At that point, the main capacitor bank would be discharged into the conical theta pinch coil and the rapidly increasing main magnetic field would compress and heat the ionized gas into a fully ionized plasma. As the ringing capacitor discharge changes direction, the field would rapidly reverse, forcing the frozen field lines in the plasma to tear, either controlled at the locations of x-points, or uncontrolled (resistive tearing) and reconnect with the bias field, forming a plasmoid with a self-contained field. It would then be accelerated away from the coil by the Lorentz (J x B) force. The purpose of adding an external magnetic bias field is to increase the trapped magnetic flux in order to study how this would affect the formation process and final quality of the formed plasmoids.;There are various designs that could be attempted in order to produce the magnetic bias field. The main design studied herein puts the bias field coil directly underneath the main coil using loosely wrapped wires to form the bias coil. Through Finite Element Analysis (FEA) and troubleshooting of the experimental setup, it was determined that such a design interferes with the main field to the point where any plasmoid formation becomes very difficult. Additionally, the bias coil wires are so close to the magnetic probes used to characterize formed plasmoids that interpreting the probe data becomes non-trivial due to the sharp magnetic field gradients close to the probes. The gradients stem from interaction between the main coil field, the bias coil field and the material conductivity of the bias wires.;A preliminary study of an alternate magnetic bias design was carried out in which the problems with plasmoid formation and probe interference have been solved at the expense of a much weaker bias field. |
