MVDC circuit breaker hollow cathode thermal and magnetic field modeling

We will present recent results of coupled thermal/magnetic field model of hollow thermionic cathode for MVDC circuit breaker. The effects of cathode geometry and material work function on cathode thermals and therefore device performance and life will be discussed. The model results indicate that shorter cathodes are required to limit the axial thermal gradient and support an increase in cathode life. The uncertainty in the cathode work function results in ~25 C variation in the predicted temperature, but is still within the temperature measurement error.  We will also show the importance of accounting for Schottky correction in the emission current calculations. According to the model, the Schottky correction results in an additional 30% of electron emission from the cathode. We will also describe the magnetic field modeling of various options to uniformly distribute the plasma across the control grid. We will compare the magnetic field lines and strength obtained for designs with either a permanent magnet or solenoid.