# Time Behaviour of Helium Atmospheric Pressure Plasma Jet Electrical and Optical Parameters
## 3 Results and Discussion
### Plasma Source Quasi-Stationary Working Regime
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Current Type**} \\ \cline{2-4}
**Parameter** & \(\mathbf{I_{1}}\) & \(\mathbf{I_{1}}\) & \(\mathbf{I_{2}}\) \\
earrow\)** & **HV Falling Edge \(\searrow\)** & **HV Rising Edge \(\
earrow\)** \\ \hline Peak Current (\(I_{max}-\)mA) & 4.5–17 (\(\pm\)0.02) & \(-\)2—10 (\(\pm\)0.9) & 0.2–1.3 (\(\pm\)0.05) \\ \hline Time value correspondingto peak current (\(t_{1 max}-\upmu\)s) & 4.8–10.6 (\(\pm\)0.04) & 54.9–59.3 (\(\pm\)0.63) & 6.9–15.2 (\(\pm\)0.25) \\ \hline Charge (\(q-\)nC) & 7.83–16.1 (\(\pm\)0.04) & 7.6–15.6 (\(\pm\)0.15) & 0.38–1.32 (\(\pm\)0.08) \\ \hline Ignition time (\(t_{ON}-\upmu\)s) & 4.05–9.5 (\(\pm\)0.27) & 53.2–56.0 (\(\pm\)1.09) & 6.0–13.4 (\(\pm\)0.3) \\ \hline Extinction time (\(t_{OFF}-\upmu\)s) & 12.0–15.5 (\(\pm\)0.69) & 63.6–64.1 (\(\pm\)0.41) & 13.5–18.1 (\(\pm\)1.22) \\ \hline Current pulse duration (\(\Delta t-\upmu\)s) & 3.36–10.9 (\(\pm\)0.7) & 7.9–10.6 (\(\pm\)1.35) & 4.1–8.5 (\(\pm\)1.15) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The influence of operational parameters on discharge current characteristics and maximum standard deviation. The advantage of ICCD imaging was the possibility to study the guided streamer head propagation over relatively long distances from nozzle exit, with a high spatial resolution. We were able to point in this way that, for constant flow rate conditions and fixed electrode arrangement, the length of the outside jet depends on the frequency and amplitude of the driving high voltage pulses (Figure 6). Supplementary, it has been demonstrated in a number of recent studies, that this characteristic dimension of the outside plasma jets in helium is also influenced by the electrodes dimensions and constructive details [40,41], gas flow rate [42,43,44, 38], dielectric tube diameter [45,46,47]. For all studied parameters, we observed an acceleration of the streamer head after leaving the dielectric tube, followed by an acceleration free region and a deceleration in the plasma jet tip region. These three distinct phases of the guided streamer head, during its travel through ambient air, may be also considered as launching (with increasing velocity), propagation (roughly constant velocity) and ending/landing (in mid air or upon a surface, with decreasing velocity) [48, 49, 50]. The main processes in weakly ionized plasmas are the charge-neutral interactions, as all other electrostatic interactions have a minor contribution. More specifically, electron impact processes (e.g., electron impact excitation, dissociation or ionization) are dominant reactions in the plasma volume and at interfaces with soft materials. Thus electron density and gaseous environment are of major importance for the physics and chemistry of plasma jets. Based on the [PERSON]'s law for the electron current in the plasma column generated in similar plasma devices, an estimation of the electron density can be achieved [51, 52, 53]. This is valid for a conductor with homogenous distributed charge over transversal cross section (i.e., negligible radial charge transport) and constant diameter (\(D\)) in the evaluated region. Actually, if the diameter of the plasma jet charge region is considered to be equal to the luminescent region, we can only accurately estimate the product between the maximum electron density (\(n_{e}\)) and electron drift velocity (\(v_{d}\)):  
Figure 6: Plasma jet length scaling with the frequency and amplitude of the HV pulses (positive polarity, 50 \(\upmu\)s duration and 3 L/min helium flow rate); the origin of Ox axis is considered the top of the dielectric tube. Error bars indicate the standard deviation of three independent measurements. \[n_{e}v_{d}=\frac{4I_{max}}{e\pi D^{2}} \tag{1}\]  
where \(e\) is the electron charge. Using Equation (1) and assuming that value of the axial electron drift velocity can be roughly approximated by the streamer head velocity, for \(I_{2}\) values in range 0.2 to 1.6 mA, electron density values between 0.6 to \(2\times 10^{11}\) cm\({}^{-3}\) are obtained.