Emissivity of Lanthanum Hexaboride Thermionic Electron Gun Cathode

The emissivity of the thermionic electron gun cathode material lanthanum hexaboride (LaB (_6) ) at an operating temperature of 1622 K has been measured for wavelengths from 550 nm to 2400 nm. The emissivity is calculated from scanning monochromator spectral measurements calibrated with a tungsten lamp and the published emissivity of 0.82 at 1600 K for LaB (_6) at 650 nm. The results show a higher emissivity at shorter wavelengths (up to 0.86 at 729 nm) and a lower emissivity in the near-IR to mid-IR (down to 0.41 at 2146 nm). These measurements were motivated by the need to make fast, accurate optical measurements of the temperature of the LaB (_6) cathode in our thermionic microwave gun, and to select the wavelength most readily absorbed by the cathode for manipulation of its surface temperature as a means of counteracting back-bombardment heating.