# Time Behaviour of Helium Atmospheric Pressure Plasma Jet Electrical and Optical Parameters
## 3 Results and Discussion
### Plasma Source Quasi-Stationary Working Regime  
After a certain characteristic time, all monitored electrical and temporal parameters reach a quasi-stationary value. This working regime can be used to rationally explore the discharge current scaling with plasma source operational parameters. Nevertheless, this returns a large data set of numerical value for all monitored electrical and temporal parameters, as they nonlinearly depend on HV pulses amplitude, duration, frequency and polarity (explored intervals are mentioned in Table 1, average values from 512 discharge events). For both positive and negative polarity HV pulses, the following general trends were observed: the increase on the HV amplitude induces an increase in intensity for all currents and the total transported charge as well; plasma jet current tends to saturate; shorter HV pulses induces higher currents; higher frequency of the HV pulses strongly reduces the total transported charge, the gap current (\(I_{1}\)) and increases the plasma jet current (\(I_{2}\)). The time corresponding to the peak current and ignition time decreased with both HV pulse frequency and amplitude increase, while the extinction time remains almost constant. For most parameters the SD of  
Figure 5: Influence of elapsed time on selected bands and lines integral intensity (HV pulse parameters: 8 kV, 2 kHz, 50 \(\upmu\)s, positive polarity). Black dots: gas flow and HV pulses turned on simultaneously; Red dots: gas flow turned on for 10 min prior the HV pulses. normal distribution is larger for \(I_{2}\) peak current, due to dynamic regime of gas flow and mixing with air impurities outside dielectric tube. Numerically, the extreme values of all measured parameters during the variation of all operational parameters (Table 1) are presented in Table 2.  
The total transported charge agrees very well with similar experimental observations of the actual charge deposited on a liquid or solid target [31; 32]. The average values of power dissipated during the plasma jet development for a single current pulse are of watt order, for all parameters studied here, while the energy dissipated during a single current pulse is of few tens of microjoules order. These values are in agreement with previous computational results [33] which shows that around 70% of the total energy is dissipated by plasma electrons, while most of the remaining energy could be dissipated by plasma ions. Electric field measurements using spectroscopic measurements returns values of around 20 kV/cm along the along the outside plasma jet [34], consistent with the time resolved measurements using Pockels effect based probes [35]. The delay observed between the peak current for \(I_{1}\) and \(I_{2}\) can be used to calculate the value of the ionization front propagation speed [36; 37]. For example, using the time value corresponding to peak current (\(t_{1 max}\), 50 \(\upmu\)s duration of HV pulses, positive polarity), we obtain the following: 8 \(\pm\) 0.5 km/s for 4 kV, 16 \(\pm\) 2 km/s for 6 kV and 8 kV; there was no statistically significant variation function of HV pulse frequency, in the investigated range. It should be mentioned that these are spatially averaged values, over the current probe width (i.e., 25.4 mm). Nevertheless, these values match very well with the streamer head propagation speed, as obtained from the ICCD imaging of guider streamer head (Figure 6), in the nozzle exit region. The values returned by both methods are much larger than the gas flow velocity, i.e., 4.18 m/s for our geometry and flow conditions. The gas flow rate and the consequent gas flow velocity in fixed geometry is a parameter that correlates with the length of the plasma jet, impinging the free air [38; 39].