Performance and life measurements of LaB₆ thermionic hollow cathodes

We are developing a fast inline medium-voltage direct current (MVDC) circuit breaker based on a gas discharge tube (GDT). The GDT breaker is an enabling technology for development of multi-terminal MVDC and ultimately a meshed MVDC grid. A long-life, low forward-voltage drop thermionic hollow cathode is an essential part of the GDT. We will give an overview of the hollow cathode design considerations. We will present results of performance and life measurements of LaB₆ hollow cathodes of select geometries in various gases and operating conditions.  Plasma voltages below 25V were measured for cathode current densities in the 1-5 A/cm² range (discharge currents of 5A to 100A). At similar conditions (discharge current and gas pressure), the plasma voltage was found to be lower for gases with lower ionization potential. Measured material loss rates were consistent with the literature values of thermionic emission coefficients and LaB₆ vaporization rates.