## 3 Results and Discussion  
### Plasma Diagnostics  
Typical current and voltage waveforms, optical-emission spectra, and photo of the plasma used in the present work transferred to the surface of DI water using a discharge gap of 8.5 mm are presented in Figure 2.  
Figure 2a shows that discharge was repulsing at a frequency of about 60 kHz, with voltage varying in the range of 700-2700 V and current in the range of 0-19 mA. In the case of arc discharge, plasma is sustained by thermal electron emission from the cathode resulting in low voltages (below 100 V) during discharge, when the current is typically in order of tens or hundreds of amperes (more than 5 A even for microarc discharges) [61; 62]. In the case of glow-discharge electrons, which sustain plasma, they are formed in the cathode fall area resulting in voltages across the discharge gap in order of hundreds or thousands of volts and currents of few tens of mA [62; 63]. From the current and voltage waveforms, the discharge current dropped to 0 if voltage was below the threshold (about 700 V), and the current started to increase if voltage was above the threshold, suggesting that the electrons are produced by a certain electric field. Considering the peak current of 25 mA and peak voltage of 3000 V (which is too large for normal glow), the discharge have burnt in a subnormal glow regime. Moreover, with further increase of input power from 28 W used in the present work to 35 W, the average current starts to increase when the average voltage starts to decrease (data not shown), which is typical behavior for subnormal glow discharge.  
Additional evidence that discharge burns in a glow regime was the presence of a N\({}_{2}\) second positive emission band in the optical emission spectrum (Figure 2b), which typically originates from the positive column of glow discharge [56]. Additionally, no continuum emission from black-body radiation typical for arc discharges could be observed in the spectra (mostly OH and N\({}_{2}\) emission on  
Figure 2: Typical (**a**) current and voltage waveforms, (**b**) optical emission spectrum, and (**c**) photo of plasma transferred to surface of deionized (DI) water.  
Figure 2b), which is another piece of evidence that discharge in the present work was not arc discharge. The structure of the discharge (Figure 2c) ignited between two Cu rod electrodes (using identical conditions) recorded by high-speed camera features strong cathode emission, dark space, emission from plasma bulk, and strong anode emission which is a typical structure of glow discharge (cathode emission, dark Faraday space, positive column, anode emission) [63].  
Initially, discharges similar to AAGD were called a gliding arc (in the case of sliding discharge between the electrodes) or low-current arc (in the case of stationary discharge); however, current and voltage waveforms together with optical emission spectra are suggesting that this type of discharge burns in a subnormal or normal glow regime. Similar observations and conclusions were reported elsewhere; therefore, in the present work, we use \"ambient-air glow discharge\" (AAGD) instead of \"ambient air low-current arc\" used in previous work [64; 65; 19; 66; 67].