# Water Modification by Cold Plasma Jet with Respect to Physical and Chemical Properties
## 2 Experimental Setup and Materials  
### Plasma Setup and Diagnostics  
Figure 1 depicts the concept of the experimental setup used for the plasma production and characterization. In the caption of the same figure, the distinct components are described. Figure 1: Main components of the experimental setup. 1: plasma jet reactor; 2: carrier gas reservoir; 3: mass flow controller; 4: signal generator; 5: power amplifier; 6: step-up transformer; 7: high voltage divider; 8: oscilloscope; 9: current transformer; 10: specimen; 11: specimen holder; 12a: optical thermometer circuitry; 12b: optical fiber; 13a: spectrometer; 13b: CCD camera; 13c: optical matcher; 13d: micro-slit; 13e: optical fiber; 13f: optical probe; 14: optical breadboard. The top-right inset illustrates the plasma-DDW interaction. Swirling-induced agitation is distinguishable, whereas the yellowish color is due to the reaction between HE and O\({}_{2}^{\bullet}\), which forms 2-OH-E\({}^{\ast}\). The design of the plasma jet reactor is based on a coaxial DBD configuration. This reactor has been efficiently employed in the past for bacterium [25] and cancer cell [26] inactivation, human skin treatment [27], liposome disruption [28], and other studies in the field of biomedicine. It comprises: (i) a capillary alumina tube (\(\mathcal{O}_{\text{out}}\) = 2.5 mm; \(\mathcal{O}_{\text{in}}\) = 1.14 mm; length of 76 mm) acting as the dielectric barrier; (ii) a filamentary internal electrode (made of tungsten; \(\mathcal{O}\) = 0.125 mm) inserted into the tube and terminated 20 mm behind the tube orifice; (iii) a cylindrical hollow external electrode (made of brass; \(\mathcal{O}_{\text{out}}\) = 10 mm; length of 10 mm) tightly fixed around the tube. Its outer edge is placed 20 mm behind the tube orifice forming thus a 10 mm in length coaxial DBD with the internal electrode. The internal electrode is the driven one and the external is grounded directly. The power supply is designed by PlasmaHTec(r) (plasmahtec.com, accessed on 20 November 2022), and it consists of a signal generator (based on direct signal synthesis; DDS), a power amplifier of low harmonic distortion, and a step-up, ferrite-core, high voltage transformer of special design. The specifications refer to sinusoidal waveform, rated voltage 12 kV peak-to-peak, rated power 210 W, and frequency range 1-20 kHz. The working gas is high purity argon (Ar; 99.999%) and it is introduced into the dielectric tube through a digital mass flow controller (0-5 slm). The voltage and current waveforms are continuously monitored during the experiments for consistency reasons (voltage divider: 1000:1, DC - 75 MHz; current monitor: 400 Hz-250 MHz; oscilloscope: 400 MHz, 5 GSamples s\({}^{-1}\)). Two identical current transformers are used for recording both the DBD and the plasma jet currents, as it is shown in Figure 1. At the same time, the mean power delivered to the system is measured as the mean value of the voltage-to-current product over successive periods. Thus, the DBD and the plasma jet consumed mean powers are distinguished and quantified. More concretely, the instantaneous power \(v(t)\times i(t)\) is numerically integrated over 3 voltage cycles \(T\) and 30 such samples are averaged. The current \(i(t)\) stands for either the DBD current \(i_{\text{DBD}}(t)\) or the jet current \(i_{\text{JET}}(t)\). Formulas (1) and (2) summarize the process. \[\overline{P_{\text{DBD}}}=\frac{1}{30}\sum\limits_{1}^{30}\left[\frac{1}{3T} \int\limits_{0}^{3T}v(t)\times i_{\text{DBD}}(t)\text{d}t\right] \tag{1}\]  
\[\overline{P_{\text{JET}}}=\frac{1}{30}\sum\limits_{1}^{30}\left[\frac{1}{3T} \int\limits_{0}^{3T}v(t)\times i_{\text{JET}}(t)\text{d}t\right] \tag{2}\]  
Figure 2 provides representative waveforms of the corresponding quantities. The physical interpretation of these signals has been reported previously [29].