Self-organized pattern formation in helium dielectric barrier discharge cryoplasmas

Abstract

Self-organized patterns appear in many biological, chemical and physical systems, including electric discharges. Under certain conditions, self-organized patterns also form in plasmas generated below room temperature. These so-called cryoplasmas have also shown promise for low-damage materials processing; however, the underlying mechanisms and experimental conditions that lead to either uniform discharges or those containing self-organized patterns are still not understood completely. Here, we investigated the formation and dynamics of self-organized patterns in dielectric barrier cryoplasmas generated at plasma gas temperatures ranging from 264 down to 7 K at a constant gas density ρ = 5 × 1019 cm−3. The electrode gap was 0.15 mm and the cryoplasmas were generated at voltages between 0.8 and 1.5 kV, at frequencies ranging from 20 to 30 kHz. The discharges were characterized by time-resolved imaging, optical emission spectroscopy and current–voltage measurements. For temperatures down to 250 K, the discharges are uniform, whereas between 250 and about 140 K, self-organized, bright filamentary patterns form. Below that temperature, the discharge regime changes again to a uniform glow and for temperatures below 20 K, different types of discharges—uniform, but also self-organized dark solitons and bright stripe patterns—are observed. The cryoplasmas show current–voltage characteristics that are similar to atmospheric pressure glow discharges and the different types of uniform or self-organized discharges are suggested to be caused by the disappearance of impurities in the plasma as the temperature is lowered, and changes in the mobilities of ion species and surface charges.