# Time Behaviour of Helium Atmospheric Pressure Plasma Jet Electrical and Optical Parameters
## 1 Introduction  
Atmospheric pressure plasmas based devices are suitable solutions to generate transient or stationary plasmas, with various applications. Plasma jets are a distinct category of plasma sources, with specific transport of active species, able to be integrated in production lines or to be used in single, specific applications. Over the past half-century, atmospheric pressure plasma jets (APP) were widely used for technological applications, starting with thermal plasma jets used for propulsion, cutting and welding devices or materials deposition [1]. Subsequently, non-thermal plasma jets were developed in various configurations, as response to the increasing industry demands to process heat sensitive materials. Recently, this opened the possibility to accurately study the plasma jets facing natural biological architectures or living organisms. These are unique natural molecular structures, presenting a hierarchical organization with many levels, able to respond to external stimuli, to self-repair and to replace damaged molecules or sub-assemblies, often showing an increasing resistance to various formsof stress factors. Plasma medicine and plasma agriculture represents nowadays distinct research topics in many plasma laboratories. The advancement of the plasma community in this field and bridging with new communities, such as biologists and medical doctors, led to numerous applications of plasma physics in life sciences during the last decade. They involve sterilization and decontamination of liquids, medical surfaces, implants or skin; influences over cellular processes: wound healing, dermatological diseases, cancer cells targeting; dentistry and related applications and treatment of seeds or vegetal tissues [2]. Cancer treatment is certainly one major topic of research, due to the fact that preliminary results of different groups indicate that selectivity may be achieved under specific plasma treatment conditions [3,4]. In order to distinguish between the different studied plasma jets, a series of parameters may be used to classify these plasma sources: thermal equilibrium, geometry and gas environment, electrical field configuration, excitation method, stationary versus transient processes [1]. It is thus possible to identify local thermodynamic equilibrium (LTE) plasma jets (based on arc and torches) and non-LTE plasma jets (based on barrier, corona, radiofrequency or microwave discharges). The working gas is usually helium or argon, but we may also find studies on nitrogen or air plasma jets. The geometrical parameters of these plasma jets and the materials used to assembly the device are very numerous [5]. Thus sometimes the biological effects of plasma jets are strongly conflicting, being even opposite in certain studies [6, 7, 8, 9, 10]. The barrier discharge is a frequently used experimental solution to generate the APPJ in various gas mixtures, at atmospheric pressure. A dielectric cylinder (made usually of quartz, alumina or plastic), having internal diameter in mm or sub-mm range, is used as flow channel for the working gas, injected continuously at flow rates that maintains laminar flow inside the tube and outside for a limited distance. External conductive tape electrodes or other electrode configurations, including internal tip or needle electrodes are used to generate transient electric fields, with imposed geometry, in order to induce gas breakdown. For appropriate numbers of electronic avalanches, followed by individual streamer development, a volume discharge is generated by streamers coupling, having the aspect of a millimeter sized plasma column. The discharge is sustained for a limited period of time, from nanosecond to microseconds depending mainly on the rise time of the applied voltage pulse and the characteristics of the dielectric materials. The intensity of electric current in the plasma column and arc transition are limited by the dielectric material.