The Effect of Tube Diameter on an Atmospheric‐Pressure Micro‐Plasma Jet

Abstract

The micro-plasma jet is a unique way of transporting active species of plasma to small targets for applications such as a single cancer cell treatment, and endoscopic surgeries. In this letter, the authors present numerical studies on a single cylindrical electrode micro-plasma jet. The effect of tube diameter on sustaining voltage, electron density, and the density of reactive species are investigated. The diameter of the dielectric tube was varied in the range between 0.1 and 1mm. A pulsed direct current voltage with rising phase of 50 ns was used to ignite the plasma jet. The sustaining voltage monotonically increased with the decreasing diameter of the tube. More electrons wall losses happened due to the faster drift movement to the inner wall of smaller tube diameter, therefore, the decreasing pre-avalanche electron density need increasing ignition voltage to facilitate the avalanche-streamer transition as the tube diameter decreased. The increasing ignition voltage increased the chemical reactivity of plasma jet. The peak density of O 2 Ã and O increased more than 20 times as the tube diameter decreased from 1 to 0.1 mm.

The effects of the tube diameter on the discharge ignition and the plasma properties of atmospheric‐pressure microplasma confined inside capillary

“…In addition to the diffusion loss, in the negative cycle of applied voltage, the electron loss due to the electron drifting to the wall may play an important role, as predicted by the numerical simulations. [16,21] However, due to the unknown electric field in the transversal direction from the microplasma to the wall, it is unable to give a quantitative analysis of the effects of the electron drifting loss to the wall. From the numerical results in a 1 mm-sized He plasma jet, the electric field in the transversal direction is estimated at over 10 kV cm −1 .…”

“…[21] Another explanation for the increasing ignition voltage is related to seed electrons that could be provided from the residual electrons and the Ar metastables by the photonionization or step-ionization. [16,41] As the tube diameter decreases, the diffusion process becomes faster, and the Ar metastables and the residual electrons will reach the tube wall more quickly. For the Ar metastables, their stored energy will lose by the collisions with the wall.…”

“…However, the plasma properties may be changed as the tube diameter decreases. To understand the effects of the tube diameter, several research groups have made great efforts in previous works . As the tube diameter decreases from several mm to 80 µm, the significant increase of the ignition voltage, the current density, the electric field, and the electron density is observed .…”

“…To understand the effects of the tube diameter, several research groups have made great efforts in previous works. [14][15][16][17][18] As the tube diameter decreases from several mm to 80 µm, the significant increase of the ignition voltage, the current density, the electric field, and the electron density is observed. [17,19,20] Meanwhile, the gas temperature is weakly dependent on the tube diameter.…”

“…[17] The numerical simulation results reveal that the increase of the ignition voltage with the reduction of the tube diameter is caused by the decrease of the seed electron density. [16] Moreover, using a high-speed imaging technique, as the tube diameter is further reduced to 70 µm, the propagation of the helium (He) microplasma plume converts from a discrete "plasma bullet" mode to a continuous plasma column mode. [20] In the front view of the plasma volume, when the tube diameter decreases, the discharge transits from a donut-shaped mode to a contracted mode.…”