## 3 Results and Discussion  
### Electrical Characterization of the Discharge  
To ignite a plasma between the metal electrodes and the surface of the liquid, we used a commercially available high voltage source usually used to operate gas discharge lamps [30]. This device provides two outputs with sinusoidal high voltages. As depicted in Figures 1 and 3, these voltages are phase-inverted to each other, meaning that the voltage on one electrode increases whereas the voltage on the other electrode decreases and vice versa. This leads to a sufficient potential difference between one electrode and the other with the liquid and the gas gaps in between. The discharge configuration could also work with grounded liquid, but this was not necessary in our configuration. Typical voltage and current waveforms for the two-electrode configuration with tap water as the liquid are shown in Figure 3. When the discharge occurs, the voltages at the electrodes (black and grey lines) collapse and the current (red and green lines) flows for some tens of nanoseconds. In these commercially available devices, high voltage is provided by use of leakage transformers to ensure short-circuit resistance. This means that the magnetic coupling between the primary and the secondary coil of the high-voltage transformer is weak, and thus, the output voltage collapses when the output is loaded and, consequently, the output current is limited. No additional current limiters like dielectric barriers or resistors are needed.  
In Figure 4, the influence of the liquid on the electrical parameters of the discharge is shown. For clearance, only the data for one electrode are presented. Generally, discharges occur very erratically, with varying breakdown voltage and different numbers of discharges per period. The presented waveforms are the most common. It was determined that for tap water (conductivity \(\sigma\) = 0.648 mS/cm) and saline solution (\(\sigma\) = 16.23 mS/cm) the voltage drop and the discharge current was significantly higher than for distilled water (\(\sigma\) = 0.013 mS/cm). The higher discharge current was the result of the higher initial conductivity of tap water and saline solution compared to distilled water (Figure 7). It also means a higher electrical load of the secondary coil of the output transformer, resulting in a higher magnetic stray flux, leading to a lower output voltage.  
The significant influence of the conductivity of the liquid was also studied by [PERSON] et al. (2017). There, it was shown that a conductivity of 0.5-1 mS/cm leads to the highest current compared to larger and smaller conductivities. This was also confirmed in our experiments with the highest current using tap water (\(\sigma\) = 0.648 mS/cm).  
Because of the erratic behavior of the discharges, the electrical power dissipated into the plasma was difficult to quantify. For a preliminary assessment of the efficiency of the process, the plug power (in this case the power dissipated into the high voltage source) was measured by a commercially available power meter. It was less than 50 W for the 4-electrode configuration, regardless of the treated liquid. This resulted in the consumption of less than 50 Wh for the production of 1 L plasma-treated water (equaling an expense of presently \(\approx\) 0.02 \(\upmu\) per liter for electricity and water).  
Figure 3: Electrical characterization of one discharge event with tap water as liquid ([PERSON], 2017): Driving voltage at the electrodes (black and grey lines) and discharge current (red and green lines), discharges take place at both electrodes simultaneously.