# Temporal Stability of Modified Surfaces of Insulating HV Devices Using Atmospheric Pressure Plasma
## 3 Experimental Conditions of Plasma Surface Treatment of Test Samples  
A radio-frequency (RF) slot plasma nozzle with an active plasma width of \(w_{\mathrm{g}}=150\) mm was used for the surface treatment of the tested polymer insulation samples, as shown in Figure 2. The flow rate of Ar through the plasma nozzle was consistently \(w_{\mathrm{Ar}}=4\) m\({}^{3}\)/hour (67 L/min). Depending on the selected working conditions, nitrogen gas (N\({}_{2}\)) at a flow rate of \(w_{\mathrm{N}}=1.5\) L/min or oxygen (O\({}_{2}\)) at a flow rate of \(w_{\mathrm{O}}=1\) L/min was added to the argon. The power supplied from the RF harmonic signal generator \(f=13.56\) MHz to the plasma nozzle ranged for individual surface treatments approximately around \(P_{\mathrm{out}}=500\) W (slight reflected power up to \(P_{\mathrm{ref}}=10\) W). The mouth of the plasma nozzle was \(d=10\) mm above the surface of the samples. The samples were placed on a mica composite plate, which moved on a conveyor belt at a speed of \(v=22\) mm/s during two passes under the active part of the plasma of the plasma nozzle. Plasma exposure was performed on only one side of the testing samples. The samples were untreated from the factory and they were not purely planar, but slightly wavy. The discharge can sometimes reach under the samples, likely creating inhomogeneities in the surface treatments when the samples are treated on both sides, as was found in previous experiments [14]. Diagnostics of plasma discharge properties, setting of its parameters and interaction with material samples were previously published [6; 24; 25]. In the publication [24], the behavior of the discharge was described in detail with recordings by a fast camera at the exit of the slit plasma nozzle and on the surface of the material for argon plasma and admixtures of N\({}_{2}\) and O\({}_{2}\) in the argon plasma. The experiment demonstrated the channel character of the discharge, with individual channels oscillating and moving rapidly along the slot of the plasma nozzle mouth. This allows for the creation of very good surface finish homogeneity on the surface of the material. In the publication [25], the behavior of the discharge during interaction with sample edges of different heights (5-30 mm) is presented and the influence of edges on the surface finish homogeneity in their vicinity is determined. Publication [6] contributes to the issue of the efficiency of surface treatment of a slot plasma nozzle on a thermoplastic (polypropylene) including the assumed reaction processes and diagnostics of the material surface (XPS method). Publication [6] also presents in detail the optical emission spectroscopy of the discharge and includes typical optical emission spectra emitted by the plasma (for pure argon and argon with an admixture of N\({}_{2}\)). Other related parameters of the discharge setting and its behavior are available, for example, in works [26; 27]. For the radio-frequency detection of the plasma spectrum, the spectrum analyzer TinySA ULTRA 100 KHz-5.3 GHz (SA) and a spectrum evaluation application for MS WIN designed for tinyULTRA were used and a fiber rod antenna of length \(l_{\mathrm{a}}=500\) mm was placed at a distance \(D=2000\) mm from the mouth of the plasma nozzle. The antenna factor was included in the SA setting, as shown in Figures 2 and 4. The methodology for exposing samples and measuring them at time intervals was adapted for the set measurement conditions (similar to work [14]), as shown in Figure 5.  
The experiment was carried out in laboratories with plasma apparatus at CEITEC BUT in Brno. A plasma slit nozzle was used under the conditions described above. At the beginning, the spectral background was measured in the laboratory without switching on the RF generator or other operating equipment.