## 3 Results and Discussion  
The spectrum obtained with an optical emission spectrometer is shown in Figure 5. The spectral resolution of an optical emission spectrometer (ratio between the wavelength of the measured light and the full width at half maximum (FWHM) of the analysed spectral peak) was a limiting factor when analysing emission lines or monitoring plasmas. Overlapping spectral lines of different species or multiple lines with wavelengths close to the expected wavelength posed a problem in the accurate identification of emission lines. Therefore, it was important to use a high-resolution spectrometer, as we used in our experiment, and spectral line tables with transition probabilities between different energy levels [54]. The generated plasma was dominated by emissions of argon and oxygen. Due to the high radiation intensity, a spectral line of emission of the hydrogen atom was observed at 656.3 nm, corresponding to H\({}_{\alpha}\), the first line of the Balmer series at the transition \(n=3\to 2\) (\(n\) is the principal quantum number of the electron). The radiation intensities of the oxygen spectral lines were significantly lower than the radiation intensities of the argon spectral lines, so that only argon lines were considered for the estimation of the electron temperature using the Boltzmann plot (Table 1). The Boltzmann plot (Figure 6) shows two groups of points that are related to the energies at which ionisation takes place and the degree of ionisation: Ar I and Ar II. In transient plasmas, which receive energy from short high-voltage pulses, the density and temperature of the electrons change rapidly. Although high-voltage electrical discharges due to ionisation do not occur continuously in space and time, assuming a partial local thermodynamic equilibrium, it can be concluded that the temperature of a closed plasma system corresponds to the value resulting from the Boltzmann relation and the Saha equation. Despite the spatial inhomogeneity of the plasma, small volumes are used under laboratory conditions, and the assumption of partial local thermodynamic equilibrium is valid. There are always enough fast, high-energy electrons in the tail of the Maxwell distribution to cause ionisation, even at temperatures characteristic of low-temperature plasmas. The radiation intensities of the spectral lines of Ar I were higher than the intensity for Ar II, and the system reached a temperature reflecting that it is a complex system of  
Figure 5: Spectrum obtained with optical emission spectroscopy. electrons, ions and neutral atoms. The addition of energy to the system supported ionisation and the plasma state, but recombination and cooling occurred rapidly. The temperature of the ions and neutral atoms, which were not ionised, remained low at 300-350 K. The use of non-thermal plasma technology can increase the temperature of the system to a lesser extent, but not enough to cause thermal damage or change the properties of the system to which it is applied. Non-thermal technologies work at temperatures up to 350 K, but non-thermal plasma technology works at temperatures close to room temperature. The Boltzmann plot in our study with experimentally obtained spectral data gave the electron temperature \(T_{e}=(24\pm 2)\cdot 10^{3}\) K and the electron thermal energy \(k_{B}T_{e}=(2.1\pm 0.2)\) eV. In optical emission spectroscopy, the signal was averaged over the observed interval to increase the signal-to-noise ratio (SNR). However, laboratory-generated plasmas are almost always inhomogeneous, which should be taken into account when making assumptions to fulfil the thermodynamic equilibrium conditions: local thermodynamic equilibrium or partial local thermodynamic equilibrium. The requirement for the complete local thermodynamic equilibrium can be expressed as follows:  
\[\frac{n_{e}}{\mathrm{m}^{-3}}\geq 1.4\cdot 10^{20}\left(\frac{E_{i}-E_{j}}{ \mathrm{eV}}\right)^{3}\left(\frac{k_{B}T_{e}}{\mathrm{eV}}\right)^{1/2} \tag{6}\]  
\(E_{i}\), \(E_{j}\) are the energies of the two states \(i\) and \(j\).