# Time Behaviour of Helium Atmospheric Pressure Plasma Jet Electrical and Optical Parameters
## 1 Introduction
Gas heating is avoided and the plasma gas temperature is maintained around room temperature, this being one of the main advantages of using the APPJ in direct contact with living tissues, biomolecular architectures or other thermolabile materials. It is interesting to note that miniaturisation of some plasma jets is possible using capillary dielectric tubes [11,12]. For example plasma jets with diameter values in submillimeter range can be generated and used to induce controllable effects on proteins and amino acids, for local etching of biomolecular films, local chemical functionalization, patterning, and local modification of physical parameters such as roughness or optical properties [13]. Due to very low dimensions of this plasma jet we foresee that single cell exposure to plasma action is also a topic to be studied in the following period, since single cell-experiments represent cutting edge studies in modern cell biology. Concerning direct applications of plasma jets on living animal tissues, it is interesting to analyse the physical and chemical agents delivered at the interphase. When trying to identify the factors able to induce modification of biological molecules or generate a biological response, one will find a list of agents already used independently in various medical practices. We will emphasise here: increase of local temperature (used in thermotherapy methods), pulsed charge transport (i.e., ac or pulsed electrical current, used in many electotherapy methods), charge accumulation and related electric fields (used in electroporation techniques), photon flashes in various spectral ranges (i.e., ultraviolet (UV), visible (VIS), near infrared (NIR), used in phototherapy methods), chemical agents in form of oxygen and nitrogen reactive species (e.g., O, OH, O\({}_{2}\)(\({}^{1}\)\(\Delta\)g), O\({}_{3}\), H\({}_{2}\)O\({}_{2}\), NO, NO\({}_{2}\), delivered by various forms of drugs). Very often, synergy occurs and plasma treatment is more efficient than the individual use of physical or chemical agents. Characterization of plasma jets is usually achieved by means of electrical, optical and spectroscopic diagnostic techniques. For example, applied voltage and discharge monitoring are indispensable for quick check of plasma operation regime, while emission spectroscopy is widely used to get information on excited atomic and molecular species in plasma jet volume, or to estimate plasma characteristic temperatures [14; 15]. Rotational spectra analysis for molecular ions and neutrals, using Boltzmann plot method or numerical simulation, in order to calculate the real gas temperature has only limited applicability for transient dielectric barrier discharge plasmas, giving the maximum temperature occurring during discharge. This methods overestimates the gas actual temperature values, as obtained using others physical principles to return the gas temperature value. For example using alternative methods based on fiber optic temperature sensors, immune to electromagnetic interferences, returns an gas temperature increase of only few tens of degrees Celsius for helium dielectric barrier discharges [16; 17]. During the last years, mass spectrometry studies of atmospheric pressure plasma jets became an appropriate technique to identify atomic and molecular species present in the plasma volume, either neutral or charged states. Time resolved studies or ion energy distribution are supplementary data that can be obtained from mass spectrometry investigations. Sampling of plasma species is usually done by differential pumping, ensuring a collision free molecular beam, in order to measure with high accuracy the same species as the ones present in plasma volume. A high variety of nitrogen and oxygen neutral molecules and molecular ions are observed in the discharge volume [18; 19; 20]. This paper aims to present fundamental aspects related to the physics of atmospheric pressure helium plasma jet, generated using the barrier discharge principle.