Emission Properties and Dielectric Strength of a Nanostructured Tungsten Field-Emissive Cathode

Abstract

Research of the tungsten “fuzz” emission properties and dielectric strength was carried out. A “fuzzy” sample was made through He+ ion exposure of an initially polished tungsten surface using an inductively coupled plasma (ICP) facility. The eventual He+ ion fluence was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 1.7\times 10^{25}\,\,\text{m}^{-2}$ </tex-math></inline-formula> . The induced nanostructured layer thickness was estimated to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 1~\mu \text{m}$ </tex-math></inline-formula> . The measurements were performed with an automated experimental setup developed. The setup was operated under the control of the programmable DAC/ADC module “LCard E20-10.” As a result, the untypical behavior of a nanostructured cathode in comparison with an ordinary field emissive cathode was registered. Instability of the W “fuzz” emission properties was demonstrated. Typical values of the electric field enhancement factor were found to be in the range of ~120–200 both before a single breakdown and after it. These values are in agreement with the estimations of the enhancement factor performed with taking into account the geometry of protrude cathode structures. Stabilization of the emission properties after the breakdown was observed. The conditioning effect of the nanostructured cathode was found to take place after approximately five breakdowns. After conditioning, an asymptotic value of a breakdown electric field strength was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 8.5\times 10^{7}$ </tex-math></inline-formula> V/m, which is in agreement with the one for an ordinary tungsten cathode. The conditioning effect and the emission current stabilization are believed to be due to the destruction of the nanostructure under the impact of the breakdowns.