Measurement and analysis of advanced field emission cold cathodes

Abstract

Summary form only given. Field emission from cold copper knife-edge cathodes was experimentally studied by measuring emission currents on the Madison cathode experiment (MACX) in UHV with a base pressure of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> Torr. The cathode was machined first using EDM to produce raised ridges or knife-edges of 0.13 mm width and 0.75 mm depth. The knife-edge width was further reduced by chemically etching it to 15 ?m. The anode-cathode gap is continuously variable from 0-10 mm with 0.50 mm accuracy. In addition, the cathode can be moved in the 2-D transverse direction with an accuracy of 0.004 mm. The MACX facility also provides the ability to heat the cathode from 300-600 K while accurately measuring electron emission currents from hundreds of mA to ~2 nA. Matching of the experimental results to the transfer matrix method (TMM) model has been utilized to determine the work function and knife edge surface field enhancement factor. The results indicate a modest discrepancy in the intermediate field region, which we hypothesize is due to the variation of effective fractional emission area with applied electric field. A point-by-point matching of the TMM simulation with experimental data yields an inferred variation of emission area with applied field. Further emission current distribution measurements were done in the high field (Fowler-Nordheim) emission regime. There is a substantial increase (x140 at 10 KV) in the total field emission current observed with the thinner knife-edge, chemically etched cathode. Using a Faraday cup with a micro-current measurement aperture of 0.5 mm diameter, we measured the spatial distribution of emission current by scanning the lateral dimension across the cathode. The cathode was moved transversally by a vacuum micro-positioner such that the knife-edge was moved past the 0.5 mm aperture. At voltages of 7 kV and above, two (unequal) peaks in emission can be seen, at lateral displacement locations of 1.08 and 2.6 mm. This spacing of 1.5 mm of the two emission peaks corresponds to the distance between the knife-edges on the cathode confirming that the high field enchancement (?) regions on the cathode dominate the emission current in the field emission regime. We will present results that further examine the spatial variation of the micro-current distribution with applied field and temperature, enabling an experimental investigation of the local field enhancement and effective emission area variations.