Characteristics of Switch Prefire Generated Fault Voltages Transmitted in a Four-Stage LTD Module

Future large-scale pulsed-power accelerators are commonly designed based on linear transformer driver (LTD) topology. With the number of gas switches being significant (~ 100000), gas switch prefire might be an inevitable malfunction in an LTD accelerator. The transmission characteristics of switch prefire and its influences on major components in a multicavity LTD module remain to be investigated. In this article, the mechanism of fault voltage generation by switch prefire and inductive coupling effect of the magnetic core were thoroughly understood. It was found that switch prefire drove the cores to magnetize reversely toward negative saturation state ( $- B_{s}$ ), raising the voltage on switches in adjacent stages. A circuit model reflecting both initial states and magnetization dynamics of cores was developed for numerical analysis of switch prefire. From previous tests of a four-stage LTD module with deionized water insulated transmission line, unexpected discharges were captured and recognized by D-dots installed in the internal electrode of the transmission line. Test results indicated that an evident manifestation of switch prefire was the polarity reversion of D-dot voltage pulses. Both theoretical and experimental analyses revealed that single brick prefire generated a peak voltage of 70 kV across the transmission line under the charge voltage of ±70 kV. Circuit simulations were in good accordance with experimental data of D-dot signals in the main pulse. However, discrepancies between simulations and experimental results implied that the model was not precise enough in the magnetization dynamics under saturated state.