Plasma decay around control grid apertures of medium voltage direct current circuit breaker with a thermionic hollow cathode

We report recent progress on modeling the interruption phase in a medium voltage direct current (MVDC) circuit breaker with a hollow thermionic cathode. When switching to the interruption phase, a negative bias is applied to the control grid, discharge is not sustained any more at the anode side and plasma starts decaying. Plasma decay is governed by the formation of an ion matrix sheath in the grid apertures, competing with ongoing ionization. Prior work indicates maximal interruption current density j$_{\mathrm{max}}$ \textasciitilde 5 A/cm$^{\mathrm{2}}$ but for thermionic cathode, the expectation is that a higher density \textasciitilde 15 A/cm$^{\mathrm{2}}$ can be achieved. Our model can help find optimal grid dimensions that don't compromise breaker operation in the closed phase and guarantee its successful opening phase. A two-dimensional asymmetric model of plasma decay around the control grid aperture was implemented into the 3D particle-in-cell code WARP. It has been modified to simulate particle collisions needed for plasma modeling and validated against previously developed 1D EDIPIC code and experimental data.