Transition characteristics in an indium tin oxide hollow cathode discharge

Hollow cathode discharges can be used to efficiently produce stable, high-density plasmas, which are crucial for a variety of plasma-related applications. However, the metallic cavity structure of traditional hollow cathodes limits the observation and study of plasma transition characteristics. In this work, a conductive and transparent material, i.e., indium tin oxide, is used to fabricate a hollow cathode to overcome diagnostic difficulties. This allows us to successfully capture frontal and lateral projection images of the discharges. A 2D fluid dynamics model is also developed to investigate the plasma physics mechanisms depicted in discharge images. The experimental results indicate that the negative glow area gradually constricts toward the cavity with increasing pressure, whereas increasing current enhances both the radiation intensity and length within the cavity. The differences in discharge images primarily result from variations in the spatial distribution of different ionic and atomic excited states. The spontaneous emission spectra of these states cause differences in the grayscale values captured by the camera's red, green, and blue channels. This study provides fundamental insights into the regularities and radiation characteristics of hollow cathode discharge under low-current and low-pressure conditions.