Characteristics of the plasma sheath in helium discharge within dielectric tubes

To understand the characteristics of the plasma sheath within small tubes, a 2D numerical model of He discharge within dielectric tubes is developed. During plasma propagation for a tube diameter of 0.05 mm, the sheath thickness in the plasma head is almost equal to the tube radius. It decreases rapidly to several micrometers at an axial distance of 0.05 mm behind the plasma head, and then slightly increases and saturates at the axial position far behind the plasma head. A plasma-gas sheath surrounding the central plasma column is observed for a tube diameter equal to or greater than 0.8 mm. It is replaced by a plasma-wall sheath for smaller tubes. With the decrease in the tube diameter, the sheath thickness far behind the plasma head decreases while the ion flux increases significantly. However, when O₂ gas with a proportion of 2% is added, both the sheath thickness and ion flux decrease.