Investigation of Volt-Ampere Characteristics of Dark Current in Vacuum Pre-breakdown Phase

During the vacuum pre-breakdown phase, field emission serves as the primary physical process preceding the formation of plasma. This phenomenon, which cannot be observed optically, is referred to as dark current. The transition from electron emission to breakdown occurs in the dark current stage, highlighting its pivotal role in the development of breakdowns. However, a comprehensive understanding of dark current remains elusive. The objective of this paper is to elucidate the electrical characteristics of dark current, and thus identify the influence of field emitter parameters on electron emission. An experimental platform was established to record dark current in the pre-breakdown phase, facilitating the measurement of volt-ampere curves under various cathode structures. Subsequently, physical parameters of field emitters, including the field enhancement factor $\boldsymbol{\beta}$ and effective emission area $\boldsymbol{A}_{\mathbf{f}}$, were determined using the Fowler-Nordheim theory. The results reveal a significant impact of dark current on $\boldsymbol{\beta}$, with an approximate fourfold increase in $\boldsymbol{\beta}$ resulting in a dark current increase of at least three orders of magnitude. Furthermore, $\boldsymbol{\beta}$ is influenced by factors such as the macroscopic shape of the cathode, the roughness of the cathode surface, and the presence of dielectric films. Additionally, there exists a positive correlation between $\boldsymbol{A}_{\mathbf{f}}$ and the intensity of emitted electron. In addition, the intensity of the dark current is limited by space charge effects, which weaken the electric field near the cathode surface, consequently reducing electron emission efficiency. Consequently, dark current does not entirely conform to the exponential growth trend predicted by field emission theory.