Simulation of the discharge propagation in a capillary tube in air at atmospheric pressure

This paper presents simulations of an air plasma discharge at atmospheric pressure initiated by a needle anode set inside a dielectric capillary tube. We have studied the influence of the tube inner radius and its relative permittivity ε_r on the discharge structure and dynamics. As a reference, we have used a relative permittivity ε_r = 1 to study only the influence of the cylindrical constraint of the tube on the discharge. For a tube radius of 100 μm and ε_r = 1, we have shown that the discharge fills the tube during its propagation and is rather homogeneous behind the discharge front. When the radius of the tube is in the range 300-600 μm, the discharge structure is tubular with peak values of electric field and electron density close to the dielectric surface. When the radius of the tube is larger than 700 μm, the tube has no influence on the discharge which propagates axially. For a tube radius of 100 μm, when ε_r increases from 1 to 10, the discharge structure becomes tubular. We have noted that the velocity of propagation of the discharge in the tube increases when the front is more homogeneous and then, the discharge velocity increases with the decrease in the tube radius and ε_r. Then, we have compared the relative influence of the value of the tube radius and ε_r on the discharge characteristics. Our simulations indicate that the geometrical constraint of the cylindrical tube has more influence than the value of ε_r on the discharge structure and dynamics. Finally, we have studied the influence of photoemission processes on the discharge structure by varying the photoemission coefficient. As expected, we have shown that photoemission, as it increases the number of secondary electrons close to the dielectric surface, promotes the tubular structure of the discharge.