# Time Behaviour of Helium Atmospheric Pressure Plasma Jet Electrical and Optical Parameters
## 2 Experimental Set-Up and Methods
Taking this into account ignition time \(t_{ON}\) for \(I_{2}\) current is defined where the current has a minimum between the Imax current and the maximum signal related to the ground line current. Independent measurements were performed for currents observed during the rising and the falling edges of the HV pulses. Frequency count was computed for all returned data and we found that all parameters follow a normal distribution (Figure 2). The mean value and standard deviation (SD) of all parameters were used to describe the statistical behaviour of the discharge events, as function of elapsed time from turning on the plasma jet or selected operational parameters. The above described experimental methodology was also employed to study the influence of selected plasma source operational parameters on discharge current statistical behaviour. All experiments were carried out 20 to 30 min after switching on the helium flow and the HV amplifier output (quasi steady state regime). Extensive measurements of all operational parameters were \begin{table}
\begin{tabular}{c c} \hline Gas pressure (atm) & 1 \\ He flow rate (L/min) & 3 \\ Waveform & Rectangular pulse, positive and negative \\ Amplitude p–p (kV) & 4–8 \\ Duration (\(\upmu\)s) & 50–500 \\ Frequency (kHz) & 0.5–8 \\ \hline \end{tabular}
\end{table} 
Table 1: Operational parameters of the helium plasma jet. performed by averaging the voltage and current waveforms on 512 frames using the digital scope and storing the results for further analyses. For all above described electrical measurements, displacement currents were recorded for all studied experimental conditions, in the absence of helium flow, and were subsequently subtracted from the total current to obtain the discharge current (i.e., gap current). Beside electrical measurements, optical emission spectroscopy was used to study emission spectra of the plasma jet using a monochromator equipped with 600 grooves/mm and 2400 grooves/mm scanning gratings (Horiba Ltd., Kyoto, Japan, model Triax 550 with Symphony CCD detector). By placing the optical fiber on the axial position of the plasma jet, the integral intensity of selected spectral bands and lines was monitored during plasma jet operation with a 1 s time step, using fixed grating spectrometer (AVS-USB2000 from Avantes BV, Apeldoorn, The Netherlands). Furthermore, the integral light emitted by the plasma jet can be monitored using a photomultiplier (PMT, Hamamatsu Photonics K.K., Hamamatsu, Japan, model R955) or a gated ICCD camera (Hamamatsu Photonics K.K., Hamamatsu, Japan, model C8484-05G), using a Canon (Canon Inc., Tokyo, Japan) EF-S 18-55 mm f/3.5-5.6 lens. Using 30 ns gate width and 100 ns time step, dynamics of the ionization wave outside the dielectric tube was studied. To avoid hydrodynamic instabilities, due to the convection currents in the room, the jet was surrounded by a supplementary 30 mm diameter quartz tube. The HPR-60 Molecular Beam Sampling Mass Spectrometer System (Hiden Analytical Ltd., Warrington, UK, with 2500 amu upper mass range) was used to study the atmospheric plasma jet chemistry's composition. This quadrupole-based mass filter system consists of three differential pumping stages, separated by three aligned skimmer cones and is able to extract neutrals or ions from atmospheric pressure, ensuring a collision free molecular beam to its detector. To extract positive or negative charged species from plasma volume, the aligned cones were properly biased, tuned for a maximum signal and carefully adjusted so as to not induce additional ionizations processes. The first  
Figure 2: Details of estimated parameters for the currents \(I_{1}\) and \(I_{2}\) observed during the rising edge of the high voltage (HV) pulses (**left side**) and typical histograms of all parameters for 1419 consecutive frames, with superimposed normal distribution curves (**right side**).