# Time Behaviour of Helium Atmospheric Pressure Plasma Jet Electrical and Optical Parameters
## 3 Results and Discussion
### Influence of a Third Electrode  
The neutral gas composition of the helium plasma jet and the positive or negative ions populations were studied using molecular beam mass spectrometry. In order to sample the plasma jet phase, the jet assembly axis was aligned with mass spectrometer axis, keeping a constant distance of 10 mm between the dielectric tube nozzle and the first cone of the mass spectrometer, containing the sampling orifice. This metallic component, biased function of the selected analysis mode, acts as a third electrode and becomes part of the equivalent electrical circuit. Thus, we observed significant variations in the peak values of \(I_{2}\) current, for the selected parameters to operate the plasma jet during mass spectrometry analyses (i.e., HV pulse amplitude 6.5 kV, 4 kHz frequency, 50 \(\upmu\)s duration, positive and negative polarity). More precisely, for positive polarity high voltage pulses the peak value of \(I_{2}\) current during the rising edge is now around 10 mA (compared with 0.96 mA for the free plasma jet) and we were also able to detect an \(I_{2}\) current of around \(-\)5.5 mA peak value during the falling edge. For negative polarity high voltage pulses we observed an \(I_{2}\) current of around \(-\)7.5 mA peak value during the falling edge and the peak value of \(I_{2}\) current during the rising edge is now around \(-\)9.6 mA (compared with 0.93 mA for the free plasma jet). This very different behaviour of free plasma jets and plasma jets impinging on conductive targets was argued by some groups, including us [58; 59; 60; 61; 62; 63].  
Figure 7 shows the mass spectra of the plasma source under investigation, using both residual gas analysis mode and ions analysis modes. The residual gas analysis spectrum reveals the presence of air traces in the plasma source effluent, signatures around the following mass numbers being found in the residual gas analysis (RGA) spectra of the plasma jets, driven by both positive and negative polarity HV pulses: \(m/z\) = 4 (He), \(m/z\) = 18 (H\({}_{2}\)O), \(m/z\) = 28 (N\({}_{2}\)), \(m/z\) = 32 (O\({}_{2}\)), and even \(m/z\) = 44 (CO\({}_{2}\)).  
The negative ions spectrum is dominated by oxygen species, in atomic form, \(m/z\) = 16 (O\({}^{-}\)) or homonuclear molecules form, \(m/z\) = 32 (O\({}_{2}\)\({}^{-}\)) and \(m/z\) = 48 (O\({}_{3}\)\({}^{-}\)). Heteronuclear molecules of oxygen with H, N and C are observed at the following mass numbers: \(m/z\) = 17 (OH\({}^{+}\)), \(m/z\) = 30 (NO\({}^{-}\)), \(m/z\) = 33 (O\({}_{2}\)H\({}^{-}\)), \(m/z\) = 46 (NO\({}_{2}\)\({}^{-}\)), \(m/z\) = 47 (HNO\({}_{2}\)\({}^{-}\)), \(m/z\) = 60 (CO\({}_{3}\)\({}^{-}\)), \(m/z\) = 61 (HCO\({}_{3}\)\({}^{-}\)), \(m/z\) = 62 (NO\({}_{3}\)\({}^{-}\)) and \(m/z\) = 78 (O\({}_{2}\)CO\({}_{2}\)\({}^{-}\)). At relatively high mass numbers, up to 150 amu, water based clusters around OH\({}^{-}\), O\({}_{2}\)\({}^{-}\), O\({}_{3}\)\({}^{-}\), NO\({}_{3}\)\({}^{-}\), CO\({}_{3}\)\({}^{-}\), HCO\({}_{3}\)\({}^{-}\) core ions are observed.  
The positive ions spectrum is less abundant in species and contains signatures at \(m/z\) = 1 (H\({}^{+}\)), \(m/z\) = 4 (He\({}^{+}\)), \(m/z\) = 14 (N\({}^  
Congruent results, concerning the ambient impurities present in the discharge effluent, were also reported by other research groups for various types of atmospheric pressure plasma jets [18; 19; 20; 64; 21]. This rich ion chemistry is only due to air impurities diffusion in the rare gas channel and it may be modified for desired applications if small amounts, less than 1% of additional gases (e.g., \(\mathrm{O_{2}}\), \(\mathrm{N_{2}}\), \(\mathrm{CO_{2}}\) or \(\mathrm{H_{2}O}\)), are mixed with the main working gas.  
Only small changes are visible in the time averaged mass spectra by changing the HV pulses polarity. For example, if positive polarity HV pulses are used to drive the plasma jet, the negative ions mass spectrum shows contributions at mass numbers higher than 120 amu, associated with a large number of water molecules present in the hydrated shell.