# Time Behaviour of Helium Atmospheric Pressure Plasma Jet Electrical and Optical Parameters
## 2 Experimental Set-Up and Methods  
Figure 1 presents a schematic of a common technological solution to generate the helium APPJ. Aluminium foil adhesive tape electrodes are applied to a quartz tube (4 mm inner diameter, 6 mm outer diameter), hosting a 4.6 helium channel at 3 L/min flow rate, maintained constant by a digital flow meter (MKS 1179A, MKS Instruments, Inc., Andover, MA, USA). The separation distances, starting from the nozzle are: 7 mm gap, 4 mm width grounded electrode, 10 mm discharge gap, 10 mm width powered electrode. A vertical arrangement, with the plasma jet oriented upwards, was used in this study to minimize buoyancy effects. The relative humidity of ambient air was continuously monitored and no significant changes were observed during all experiments (60 \(\pm\) 1% RH). Figure 1: Sketch of the experimental set-up (**left side**, **not drawn to scale**); typical signals for positive polarity high voltage pulses, \(I_{1}\), \(I_{2}\) currents and photomultiplier (PMT) signal (**right side**). Rectangular high voltage (HV) pulses are delivered to the powered electrode, using a voltage amplifier (TREK, Inc., Lockport, NY, USA, Model PD07016) and a digital waveform generator (Tektronix, Inc., Beaverton, OR, USA, model AFG3022B). The operational parameters of the plasma jet used in this study are detailed in Table 1. The rise/fall time (10% to 90%) of the high voltage pulse was ranging from 6.5 to 9.2 \(\upmu\)s function of all studied parameters of the HV pulses: frequency, width and amplitude. Plasma operation monitoring was ensured by external electrical measurements of applied voltage and electrical currents using probes: Tektronix P6015A (Tektronix, Inc., Beaverton, OR, USA) and two Pearson 6585 (Pearson Electronics, Palo Alto, CA, USA) (to simultaneously measure the current on the ground line, \(I_{1}\) and the plasma jet current, \(I_{2}\)) connected to digital scope (Tektronix, Inc., Beaverton, OR, USA, model TD55034, 350 MHz bandwidth, 5 GSample/s sampling rate). The signal measured by the high voltage probe was used to trigger the oscilloscope, at a level of maximum 2.5% from the maximum voltage amplitude. Thus, with a very good approximation, the origin of time (\(t=0\)) is considered to be the HV pulse rise start point for positive polarity HV pulses or the pulse fall start point for negative polarity HV pulses. In order to test the transient response of the plasma jet, due to gas flow stabilization and mixture with air, outside the dielectric tube, the following experimental approach was used: immediately after switching on the helium flow and the HV amplifier output, acquisition of all electrical parameters was performed at a 15 s time step. Using the FastFrame(TM) Segmented Memory acquisition mode of the scope it was possible to capture bursts of 1419 frames, i.e., consecutive individual discharge events. In this way during all experiments we stored more than 3105 individual current traces. All data were subsequently analysed using a custom code, in order to obtain from the \(I_{1}\) and \(I_{2}\) traces simultaneously recorded by the two current probes, the following parameters for each discharge event: ignition (\(t_{ON}\)) and extinction (\(t_{OFF}\)) time, current pulse duration (\(\Delta t=t_{OFF}-t_{ON}\)), peak value of the current (\(I_{max}\)), the time value corresponding to peak current (\(t_{Imax}\)) and the integral of the current signal from \(t_{ON}\) to \(t_{OFF}\) (i.e., total transported charge during a single discharge event). The ignition time \(t_{ON}\) for current \(I_{1}\) and extinction time \(t_{OFF}\) for \(I_{1}\) and \(I_{2}\) currents are defined where the current amplitude reach 3% of \(I_{max}\) current. As can be observed in the typical current traces in Figure 2, the second current probe, used to monitor the plasma jet current (\(I_{2}\)) measures also a small signal related to the ground line current due to the small separation distance between the ground electrode and the dielectric tube nozzle.