On the gas heating effect of helium atmospheric pressure plasma jet

TL;DR

This study investigates the heating mechanisms of helium atmospheric pressure plasma jets, revealing that gas expansion significantly contributes to temperature increases, and demonstrates the ability to produce low-temperature plasma jets.

Contribution

It is the first detailed analysis showing that helium gas expansion, not just plasma discharge, causes heating in atmospheric pressure plasma jets.

Findings

Gas expansion significantly raises gas temperature in helium plasma jets.

Low-temperature plasma jets (<40°C) are achievable at low helium flow rates.

Temperature increases with distance from the outlet, especially at low flow rates.

Abstract

Plasma medicine is an emerging research field which has been driven by the development of plasma sources suitable to generate low temperature plasmas. In many cases, such devices can operate without a gas flow, producing a plasma discharge from the ambient air. However, the most common case is the use of a working gas at a given flow rate to produce a plasma jet. Helium (He) is one of the gases commonly used as the carrier gas to generate cold atmospheric pressure plasma jets (CAPPJs) due mainly to the easiness to ignite a gas discharge with it. However, in this work it was observed that most of the heating of a He CAPPJ can come just from the expansion of the He gas. This was found through measurements of gas temperature ($T_{gas}$), using fiber optic temperature (FOT) sensors, and thermal output, using both FOT and infrared imaging with the He flow impinging on a copper (Cu) plate. Such findings were achieved through comparisons of $T_{gas}$ and the temperature on the Cu ($T_{Cu}$) plate in the conditions with and without discharge ignition, as well as comparing $T_{gas}$ in the free gas/jet mode with and without discharge ignition. It was verified that the $T_{gas}$ values increased as the distance from the gas outlet was enlarged, especially at low He flow rates, even without discharge ignition. Despite the temperature increase with distance, it is possible to produce plasma jets with temperatures lower than 40 $^\circ$C at low He flow rates.