The HelixJet is a capacitively coupled radio-frequency (RF) plasma source operating at atmospheric pressure. The RF power is applied to two double helix electrodes. The electrodes are placed outside a quartz tube fed by the working gas. The HelixJet has unique features highly relevant for practical applications. The innovative double helix electrode design enables extremely stable and homogeneous plasma conditions at low gas flow rates. This plays a crucial role for the quality and reproducibility of several applications, e.g. PECVD of SiO₂ with a large footprint diameter or plasma microprinting of polymeric profiles (e.g. polyamide).