## Methods and Materials
### Surface Compositional Analysis  
A K-Alpha X-ray Photoelectron Spectroscopy system (Thermo Fisher Scientific, Hillsboro, OR, USA) was used to analyze surface composition. Both interior and exterior surfaces of the envelope were studied, and only the external surface of the glass exterior control sample was cleaned in any fashion. The glass envelope and anode were studied in two separate but similar XPS experiments. The samples were secured to the sample stage with copper pins. An Al K\(\alpha\) micro-focused monochromator with a spot size of 400 \(\upmu\)m was used, and provided a probe sampling depth of ~10 nm. An electron flood gun was used to maintain charge neutrality of insulating samples. For each point studied on the glass samples, a full spectrum survey composed of 20 individual survey scans as well as targeted surveys for Ba, Ca, Sc, Al, W, Si, C, and O were recorded. Measurement conditions were the same for anode samples, except that the flood gun was not used and a targeted scan for Mo was substituted for Si.  
Calibration of XPS results for sample regions inside the glass envelope was accomplished by shifting the binding energies so that the Ba 3d peaks matched known positions of 780.0 eV and 795.3 eV.  
Figure 1: (**a**) Photographs of the scandate cathode test vehicle, comprised of a glass envelope containing the cathode assembly, and (**b**) an “unrolled” schematic highlighting different views around the circumference of the envelope.  
Figure 2: A photograph of the side of the anode facing the cathode components. During testing, the small pinholes at the center of each discolored ring are directly in line with the cathodes.  
The latter peak (present due to spin-orbit effects) directly overlaps with Co2p peaks; however, the ratio of the primary 3d peak and the spin-orbit peak is 3:2 for Ba, which was observed for the present samples [16]. Traditionally, binding energies obtained via XPS are calibrated by shifting to align the adventitious carbon peak. However, as a range of carbonates (which complicate identification of the adventitious C peak) may be present inside the envelope and Ba is present in all tested interior samples, energies were shifted according to Ba. Results from the envelope's exterior, obtained to determine baseline glass composition, were calibrated with the adventitious C peak.  
Quantitative analysis of peak heights is presented in the form of signal-to-noise (S/N) ratios to allow comparison between scans. Noise levels were calculated as the standard deviation of measured counts in a binding energy range without peaks for each spectrum. Typical noise levels were \(\sim\)2000 measured counts with maximum peak intensities typically two orders of magnitude greater than the noise. Background-corrected counts for each peak were divided by the noise level to give S/N ratios in multiples of noise level. In subsequent discussion, peak intensity is considered to be \"weak\" or \"trace\" if the peak height as a multiple of the computed noise level (S/N ratio) is less than 3, \"moderate\" if it is in a range of 4 to 15, and \"strong\" if it exceeds 16.  
Additional cross-sectional transmission electron microscope (TEM) analysis was conducted on a glass sample taken from the Ba-Al flash region and an anode sample taken from a heavily deposited region opposite a cathode emitting surface. The two corresponding lift-out samples were fabricated with a dual-beam focused-ion beam (FIB) and imaged with scanning electron microscopy (SEM), utilizing an FEI Helios NanoLab 660 (Thermo Fisher Scientific, Hillsboro, OR, USA). Elemental mapping of the lift-out samples was conducted with an FEI Talos F200X TEM (Thermo Fisher Scientific, Hillsboro, OR, USA) equipped with a Super-X EDS system (which is comprised of four confocal detectors that simultaneously collect X-rays from the same location). The whole anode deposit area was characterized with SEM and elemental mapping obtained with the Oxford X-Max EDS 80 mm\({}^{2}\) detector (Oxford Instruments, Abingdon, UK)-equipped FEI Helios NanoLab 660.