Re-trapping of vacuum electron current in magnetically insulated transmission lines

Inductive Voltage Adders (IVA) such as RITS [1] use a self magnetically insulated transmission line (MITL) along the centerline of the induction cells to add the cell voltages and transport the power to the load. As the voltage increases along the MITL the vacuum flow or sheath current becomes a larger fraction of the total MITL current and the more useful (for radiographic applications) cathode or boundary current becomes a smaller fraction of the total current. When the load impedance at the end of a MITL is less than the MITL impedance based on parapotential flow at minimum current [2], the MITL behavior is divided into a vacuum precursor transient, a magnetic insulation phase, and a retrapping phase. This paper discusses the theoretical basis for and the experimental observation of this retrapping wave. Parapotential flow theory predicts that as the load impedance is decreased below the MITL matched minimum current impedance the rate of change of the MITL boundary current with load impedance is infinite. The vacuum current can thus be re-trapped with only a minimal decrease in voltage at the load compared to the matched minimum current impedance case. The re-trapping experiments on RITS-3 at 4 MV with the original lower impedance MITL and the more recent 5.25 MV experiments with the high impedance MITL will be summarized and compared to particle-in-cell simulations and the simpler analytical model. All the experiments demonstrate that significant re-trapping can occur with only a minor decrease in the matched output voltage.