Precision electron flow measurements in a disk transmission line

In large z-pinch drivers, it is ideal to minimize the electron flow near the load while maintaining a low inductance as to not suffer a decrease in system efficiency. While inductance and electron flow are normally competing requirements, transmission lines with a spatially varying gap in the direction of power flow allow for an inductive profile to be built directly into the geometry of the electrodes. If the transit time of the transmission line is small compared to the pulse width of the forward wave, then the spatially varying electric and magnetic fields associated with the power pulse are capable of altering the distribution of the electron current in the direction of power flow. Some previous designs assumed that the field emitted electrons within the vacuum line were incapable of returning to the cathode surface, unable to reduce the electron flow, and thus determined an optimally uniform radial current. Recent experiments and simulations, however, suggest that the re-trapping of electrons is possible and could result in even lower inductive machines. An experimental study was devised to determine the feasibility of tailoring the radial profile of the electron current within a magnetically insulated transmission line with the intent of further reducing the overall inductance of the system. This was approached through the development of a theoretical model which exploits the possibility that electrons are capable of returning to the cathode surface through a continual gain in their total energy. The inductive profile was derived from the theoretical model\u27s prediction of a radially reducing electron flow and built directly into the curvature of the transmission line\u27s electrodes. This model was validated for strongly insulated electron flows, V \u3e 5 MV, using particle-in-cell simulations which had been used extensively on the large z-pinch driver located at Sandia National Laboratories. Low voltage tests, V \u3c 1 MV, were experimentally tested and provided a benchmark for the use of the PIC simulations to model marginally insulated electron flows. Experiments also allowed the development of a new diagnostic for measuring the electron current directly at the load. This would provide a unique method for studying strongly insulated flows which have historically been difficult to measure