An apparatus to investigate the effects of hot electrons in the aging of polyethylene cables

It is now established that the aging of organic insulators due to DC or AC voltage stresses involves the interaction of hot electrons within the dielectric and/or at the electrodes. In order to study the details of this aging process in industrial devices, the authors have recently developed an apparatus to probe charge accumulation induced by hot (i.e., low-energy) electrons of very well-defined energies near the surface of thin film samples of cables or other products made of organic dielectric materials. In an ultrahigh vacuum chamber, a monochromatic (ΔE≈60 meV) pulsed electron-beam (10^-14-10^-12 C/pulse) of variable energy (0-20 eV) impinges on a sample film of about 100 μm thickness. Both electron transmission through, and charge accumulation into, the film are monitored as a function of incident electron energy. An ultraviolet source allows discharge of the sample. The apparatus is described and preliminary results of such measurements are reported for samples cut from polyethylene cables