Field emission of electrons is the major cause of electrical breakdown in high voltage systems in vacuum. The highest hold-off electric field of the carefully polished and cleaned stainless steel cathodes was increased to 70MV/m. Thin silicon monoxide, SiOX, cathode coatings reduced field emission and increased the hold-off field further. Coating the stainless steel cathodes with 2/mu m/ SiOX reduced the field emission current by at least two orders of magnitude at field of 50MV/m and increased the breakdown field to 140MV/m, doubling the breakdown voltage. The increase in hold-off voltage with SiOX coatings is discussed in terms of electron transport within the coating. Measurements indicate that current in SiOX at high fields is controlled by Frenkel- Poole electron emission from deep centers located about 1eV below the conduction band. Field emission current is limited at the coating-vacuum interface due to an accumulation of filled electron traps. A figure of merit, /gamma, for SiOX cathode coatings is given by/gamma=[Vb(coated)/over Vb(uncoated)] = [βm[ EC/ coating]/overβc[ EC/ metal]]Based on this model the characteristics of an ideal cathode coating are described.
- Pub Date:
- October 1998
- Engineering: Electronics and Electrical, Physics: Electricity and Magnetism