Predicting the end of service life for a vacuum electron device

The subject matter described herein generally relates to apparatus, systems, methods and associated computer instructions for predicting the end of service life of a space charge limited vacuum electron device. The device produces an electron beam current and has a cathode and a filament powered by an adjustable voltage power supply providing a voltage between a first low voltage and a second higher voltage to heat the cathode to an electron emitting temperature. The process includes periodically, while the device is in operation, adjusting the voltage provided by the power supply while monitoring the beam current, determining a knee-point in the voltage where the beam current begins to decrease as the voltage is decreased, and calculating, based on the determined knee-point and a predetermined voltage vs service life remaining relationship, the amount of service life left in the device.