Modelling of vacuum breakdown in the MegaAmpere X-ray Installation (MAXI) output line

Two MegaAmpere X-ray Installations (MAXIs) have been developed to provide flash X-ray radiographic sources for hydrodynamic experiments. The MAXI generates a power pulse that propagates into a vacuum output line terminating with a diode. The output line is designed to be magnetically insulated and forms a Magnetically Insulated Transmission Line (MITL). During tests with a Large Area Diode (LAD) vacuum breakdown of the output line led to machine downtime. The electromagnetic Particle-In-Cell code Large-Scale Plasma (LSP) [1] has been used to model the MAXI output line to evaluate a hypothesis to explain the cause of the vacuum breakdowns. The model includes possible electrode plasma generation. Modelling results of a failed MITL with electrode plasma generation qualitatively agree with vacuum breakdown measurements. The model indicates plasma filling of the vacuum gap and significant current density increase during the pulse, implying that an arc discharge would have been feasible. Recommendations are made to ensure each output line assembly gives a minimum vacuum gap of (12.5 ± 0.5) mm over a range of azimuthal positions, to use a refined LAD setup for electron energy deposition, and to upgrade this modelling work to the Chicago code [2].