Effects of inductive coil geometry in the conical theta pinch faraday accelerator with radio frequency assisted discharge

The effects of inductive coil geometry on current sheet velocity in a conical theta pinch acceleration stage were explored through experiments and calculations in order to obtain insight into optimizing the coil geometry for the theta pinch Faraday accelerator with radio frequency assisted discharge (CΘP-FARAD). The experiment consisted of carrying out static measurements of the mutual inductance between a set of conical inductive coils of varying lengths and half-cone angles and their associated currents sheet analogs made out of metallic foil. The measured dependences of mutual inductance on the axial distance between the coil and the sheet were used as input to an analytical circuit model of inductive current sheet acceleration. This allowed the inference of some of the effects of coil geometry on the performance of the accelerator (the final sheet velocity). It was found that shorter driving coils, and to a lesser extent larger half-cone angles (whose effects are less pronounced for longer coils), are more favorable for vigorous current sheet acceleration. The trends were explained by considering that the uniformity of the magnetic field within the volume of a coil increases for longer, more solenoidal coils, and therefore would present a reduced magnetic field gradient to a current sheet translating away from the coil, thus reducing the rate of the change of inductance with distance and consequently decreasing the final sheet velocity.