I. Introduction
The plasma needle is a novel design of a plasma device with a single-electrode configuration and known to produce nonthermal atmospheric pressure glow discharge plasma to achieve a nonthermal plasma jet [1]–[3]. The basic advantages associated with the plasma needle are that it can be operated in open air, outside a vessel. The plasma that is generated with the plasma needle is small (about one millimeter) and nonthermal; the temperature of the neutral particles and ions is in about room temperature and suitably can be used to treat living biological cell without damaging the cell [2], [4]–[9]. Wide range of frequencies including radio frequency [1], [10], ac [11]–[13], and dc [2] are used to operate the plasma needle. Important properties of this type of plasma are that it operates near room temperature, allows treatment of irregular surfaces, and has a small penetration depth. These characteristics give the needle great potential for use in the biomedical field. Several experiments have shown that the plasma needle is capable of bacterial decontamination, localized cell removal without causing necrosis to the treated or neighboring cells and ablation of cancer cell [4]–[7], [9], [14]–[19]. It is believed that plasma particles, such as radicals and ions, and also emitted UV light, interact with the cell membranes and cell adhesion molecules and therefore cause detachment of the cells [4], [20]. Due to its nonequilibrium nature, the electron temperature of the plasma needle is quite high (on the order of electonvolt), while the temperature of the heavy plasma particles such as ions and neutrals are in room temperature. The high-temperature electrons can interact with living bacteria, microorganism, or microbes and can destroy them without effecting the living cell/human body due to its low energy (due to low mass of electron). Thus, it is realized that the quantitative knowledge on the characteristics of the electron temperature is necessary for proper implementations, understanding, scaling, and optimizing atmospheric pressure plasma for specific biomedical investigation.