Distribution of desorption products on interior surfaces of scandate cathode test vehicle

Scandate cathodes have shown a higher emitted current density than any of its predecessors, which is hypothesized to stem from chemical complexes on the W matrix surface. Therefore, it is crucial to examine the cathodes' desorption behavior to understand how surface chemistry evolves during cathode lifetime. In this work, the distribution of desorption products of scandate cathode activation and operation mapped with XPS and EDX are reported. The compositional studies were conducted on the interior of the glass envelope wherein the cathodes were activated and operated, as well as the anode resting directly atop the cathodes. The results from the glass envelope indicate that Ca is expelled from the surface during pre-activation, not activation. Results from the anode yield insight into the composition and volume of material expelled throughout cathode activation and operation. Lastly, within the detection limits of XPS and EDX, no appreciable Sc was found on the anode nor the glass envelope, suggesting it is not evaporating in great amounts at any stage of cathode lifetime.