Photon emission from an alumina insulator in vacuum subjected to DC electrical fields

Radial direct current electric fields were applied to a cylindrical alumina (94% Al 2 O 3 ) insulator, which was operated under ultra-high vacuum (pressure <10 -8 mbar), with current magnitudes limited to pre-breakdown values of less than 50 mu A. A high-resolution monochrome charge-coupled device camera was used to capture images of photon emission from the alumina; the spectrum of the emitted light was quantified using a high-speed, high-resolution scanning monochromator with a photo-multiplier tube detector. Optical images of the observed luminescence are presented, together with wavelength spectra recorded at fixed values of direct current bias voltage. An observed time-dependence of the photon emission intensity is correlated with an apparent time-variation in the direct current-voltage characteristic of the bridged alumina insulator. The features of the recorded spectra are discussed in terms of solid-state photon-emission processes, arising from radiative electron recombination at impurity and structural defect centres within the surface layers of the alumina ceramic.