# An Investigation into the Behavior of Cathode and Anode Spots in a Welding Discharge
## 2 Materials and Methods
The films were sprayed at a residual gas pressure of \(p\) = 10 8 Pa and a substrate temperature of T = 70 C. We subjected all substrates to double pretreatment: first, we chemically etched them in a chromium mixture solution (Na\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\)--100 g, H\({}_{2}\)SO\({}_{4}\)--50 g, and distilled water--up to a liter), followed by plasma chemical purification using the Nano Clean \"Model 1070\" installation (Fischione Instruments, Export, PA, USA). Microscopic and gravimetric methods determined the thickness of the deposited films. We studied aluminum films using ready-made samples from aluminum mirrors. The research employed a reverse-polarity direct current welding arc with a current strength ranging from 2 to 60 A, burning vertically between a tungsten nonmelting electrode (2.4 mm in diameter) and a film cathode in argon shielding gas environment. The arc burned steadily. The total arc burning time was 0.03-1 s. During arc burning, cathode spots form on the surface of the film cathodes, the film evaporates under the action of heat, and a thermal imprint remains on the surface of the glass (autograph)  
Figure 1: The scheme of the experiments: (**a**) general, (**b**) the type of plate and electrode, (**c**) a fragment of the film from the work [49]. (Figure 2). The conditions of the experiment correspond to the real technological process of welding with a nonmelting electrode (tungsten) in a protective gas environment (argon) at a direct current of reverse polarity (minus on the product), which is used in industry for welding products made of steel, aluminum, copper, and titanium. The arc-burning process was recorded by a digital SLR camera (a Sony 350). To record the first series of experiments of sequential images during the arc burning act, high-speed cameras were used: RCO.1200 hs, RCO.120 s, NIKON AF-S DX Zoom lens-Nikkor ED, CMOS matrix, pixel size: 12 \(\upmu\)m 12 \(\upmu\)m, resolution: 1280 1024 pixels, exposure time range: 1 Gorenje ns-5 s, interframe viewing time: 75 ns, dynamic range: 59.6 dB, spectral sensitivity: 290-1100 nm. At a maximum resolution of 1280 1024, the maximum shooting speed is 501 fps. At a minimum resolution of 1280 16 pixels, the maximum shooting speed is 32,000 fps. To record the second series of experiments, a high-speed Phantom V711 camera with a CMOS (CMOS) resolution of 1280 800 pixels and a shooting speed of 7530 fps was used. The bandwidth is 7 Gpix/sec with a resolution of 1 megapixel, and the pixel size is 20 microns. The maximum shooting speed is 1,400,000 fps. The minimum exposure time is 300 ns. The visualization technique and processing of the obtained images are described in detail in [44; 49; 50]. In addition, methodological recommendations for image evaluation and the results of various authors' work in this area of research were taken into account [23; 24; 25; 32; 33; 34; 35; 36; 37]. We used the Tektronix TDS-1012 B digital (Tektronix, Beaverton, OR, USA) oscilloscope to measure current and voltage. The method of shooting was as follows: We used a front trigger to set up the data logger, triggering it either by voltage or current. Registration started when the set voltage and current values were reached. In the process of measuring the state of the data logger, the interval and time for measuring voltage and current during welding were set to 1 microsecond and 50 ms-1 min, respectively. We connected the data logger's output trigger terminal to the input terminal of a high-speed video camera. When the trigger signal was received from the data logger, the high-speed video camera immediately began shooting. The time delay between the data recorder and the high-speed video camera was 2 microseconds. The survey was conducted at an angle of ~7-15 with respect to the cathode, depending on the interelectrode gap. To accurately determine the position of the spots in the immediate vicinity of the electrodes, superminiature lamps (CMN 10-55-2) were placed in such a way that their image fell into each frame.