# Time Behaviour of Helium Atmospheric Pressure Plasma Jet Electrical and Optical Parameters
## 3 Results and Discussion
### Plasma Jet Warm-Up Period  
Discharge monitoring immediately after ignition, using the FastFrame (TM) acquisition mode launched every 15 s, shows the existence of a specific settling time to reach a quasi-stationary working regime, as observed for the time behaviour of all studied electrical parameters (Figure 3). For example, \(I_{1}\), the gap current, measured in the external circuit, is the result of electrical phenomena occurring inside the dielectric tube. During the first 10 min from plasma ignition, we observed a decrease of \(I_{1}\) current peaks, for both rising and falling edges of the HV pulses, followed by an increase and a transition to a steady state value. This can be attributed to few significant factors: a decrease of electric field applied to the flowing gas, due to charge accumulation on tube inner surface over multiple discharge events; generation of a conduction channel, that provide an electrical connection with outside plasma jet; built-up of excited and metastable species in the helium channel over multiple discharge events. Using time-resolved local electric field measurements, a similar behaviour was observed for a plasma jet impinging on a dielectric surface, in different configuration and very different time scale [29]. This was called initialization of the plasma jet and a net transition from branching streamers to guided streamer was pointed [29; 30]. Contrary, the \(I_{2}\) (plasma jet current at nozzle exit) peak presents very low values, around 1 mA and we observed an increase of about 33% of initial value. This can be attributed to the flow stabilization outside the dielectric tube and air molecules mixing with the rare gas. By turning on the gas flow 10 min prior to HV pulses launch (red symbols in Figure 3), we observe that parameters of the plasma jet current at nozzle exit are overlapping with the same parameters for the case when the gas flow and the HV pulses are turned on simultaneously (black symbols in Figure 3). Notably, the total transported charge during discharge events saturates quickly, after approximately 5 min (Figure 3, right side column). Similar behaviours was found for all characteristic times of current pulses (ignition time, peak current time and extinction time) during the monitoring period (Figure 4). The duration of current peaks was found to be comparable (between 7.5-10 \(\upmu\)s) for \(I_{1}\) and \(I_{2}\) current peaks observed for the rising edge of HV pulses. A slight increase of \(I_{1}\) was observed from 7.3 to 8.5 \(\upmu\)s and for \(I_{2}\) from 8.0 to 9 \(\upmu\)s. A higher duration (10 \(\upmu\)s) of discharge events during falling edge of HV pulses was observed for \(I_{1}\). No significant variations of these parameters where observed in the gas flow in turned on 10 min prior to HV pulses launch (red symbols in Figure 4), excepting the start-up time of \(I_{1}\) observed for the rising edge of HV pulses. Figure 4: Influence of elapsed time on ignition (\(t_{ON}\)) and extinction (\(t_{OFF}\)) time and current pulse duration (\(\Delta t=t_{OFF}-t_{ON}\)) (pulse parameters and graphical details are the same as in Figure 3). Figure 3: Influence of elapsed time on peak value of the current (\(I_{max}\)), the time value corresponding to peak current (\(t_{Imax}\)) and total transported charge during a single discharge event (HV pulse parameters: 8 kV, 2 kHz, 50 \(\upmu\)s, positive polarity). Dots represent the mean value of all parameters, while the error bars represent the standard deviation (SD) of normal distributions. Black dots: gas flow and HV pulses turned on simultaneously; Red dots: gas flow turned on for 10 min prior the HV pulses. Transitions between electronic states of atomic species (e.g., He, H and O) are observed as lines in the jet's emission spectrum, while spectral bands are originating from rovibrational transitions of small molecules such as N\({}_{2}\), N\({}_{2}\)\({}^{+}\) and OH.