An Apparatus for the Measurement of Field Emission From Insulators
Raimi Clark, Texas Tech University
In the presence of a high electric field at a surface, electrons can be liberated from a material into vacuum. This process is reasonably well quantified for electron emission from metals and depends primarily on the work function. For insulators, however, the band structure and related physics (band bending at interfaces, for instance), complicates the emission processes. Amorphous polymers, in particular, are expected to have very complicated band structures with a variety of trap levels, further complicating an analytical description of electron transport. Subsequently, reports of field emission from insulators to date have been anecdotal in nature. Given increasingly stringent high voltage insulation requirements for stockpile stewardship technology, improving the understanding of insulator field emission is crucial. Identification of emission thresholds, current-voltage characteristics, and material-dependent or surface treatment-dependent effects will help enable superior insulation design for vacuum interfaces.
To this end, an apparatus is presented for the measurement of field emission from dielectrics. In vacuum, a ~30 ns, fast-rising negative pulse is applied to a cathode which is entombed behind a thin section (~ 1 mm) of a hollow insulator, such that the macroscopic field on the vacuum side of the insulator is ~500 kV/cm. Field-emitted electrons are accelerated across a ~1 mm gap towards perforations in the anode. An electron multiplier tube, EMT, is positioned behind the perforations to measure a combination of primary electrons, x-rays, and secondary electrons generated by collisions of primary electrons with the anode. Design challenges associated with achieving high-sensitivity measurement of relatively energetic electrons and isolating field emission as the physical phenomena of interest, as opposed to breakdown or flashover, are discussed.
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