# Multimodal Comparison of Cold Atmospheric Plasma Sources for Disinfection
## 3 Plasma Sources  
Three different plasma sources relevant for biomedical applications have been selected for this work: a micropulsed source, which can operate with either helium or argon; a radiofrequency source, which similarly can operate with helium or argon; and a DBD-like source, operating in air. For each source, after a preliminary sampling of chemicals production in different power supply configurations, a set of electrical parameters has been selected such that they all produce similar amounts of reactive species. Considering the three sources and their different working gases, a total of five configurations are investigated. The first source (Figure 2a) is a micropulsed plasma jet, previously reported in the literature as Plasma Coagulation Controller (PCC) [31; 32]. It consists of a tungsten electrode, covered with a closed capillary that acts as dielectric, and excited with a positive microsecond pulse (0.45 uS FWHM) at 7.4 kV\({}_{\text{peak}}\), repeated at a frequency of 5 kHz. The power absorbed by the full system, including the plasma and the transformation circuit, is measured to be 8.8 \(\pm\) 0.3 W. A glass nozzle is inserted coaxially with the electrode (internal diameter at the tip 3 mm), and between the capillary and the nozzle a noble gas flows at 1 L/min. An external ground ring acts as the grounding reference. This configuration results in the formation of a plasma plume, generated inside the nozzle and extending into the air. The characteristics of the plume differ for the two gases: while the helium plume is more diffuse and uniform, the argon plume appears filamentary. The second source (Figure 2b) is similarly a plasma jet, but operating in the radiofrequency regime. The central electrode, made of tungsten, is covered in alumina except for the last 10 mm, which are exposed. A glass nozzle is positioned coaxially with the electrode (internal diameter at the tip 2.7 mm), and either helium or argon is flown at 1 L/min. The electrode is excited with a radiofrequency signal, 8.8 MHz at 2.5 kV\({}_{\text{ptp}}\), modulated with a 10% duty cycle at 2.0 kHz to limit gas heating. The averaged power absorbed by the plasma is measured to be \(6\pm 1\) W. Similar to the previous source, the plume is generated inside the nozzle and expands into the surrounding atmosphere. With helium, the plume appears to be diffuse and uniform but relatively short, not exceeding a few millimetres in length. With argon, instead, the plume is longer and it is characterised by a single central channel surrounded by a diffuse, weaker glow. The last source, called Miblet (Figure 2c), is an original prototype developed in our laboratories, designed to operate directly in air without the need of an external gas supply. It consists of a tube of alumina, with internal diameter 19 mm, in which air flows at 0.5 L/min. Inside the alumina tube a grounded stainless steel cylindrical grid is placed; outside, a band of copper tape is excited with positive pulses 26 us wide and 5.5 kV\({}_{\text{peak}}\) high, repeated with a 1 kHz frequency. The average power absorbed by the plasma, measured using Lissajous figures method [33], is estimated to be \(3.4\pm 0.3\) W. Plasma is ignited like a surface DBD in the holes of the inner grid, and the air flow collects the species which are delivered to the substrate. Contrary to the previous two sources, in this case the plasma is not directly in contact with the substrate, which instead is treated with the afterglow. However, it is considered of interest to analyse the performance of this source, which does not require an external gas supply and instead works with ambient air. Figure 2: Plasma sources used in this work. (**a**) Micropulsed source; (**b**) radiofrequency source; (**c**) Miblet. The plasma sources in operation during the treatment: micropulsed source operated in (**d**) helium and in (**e**) argon; radiofrequency source operating in (**f**) helium and in (**g**) argon; (**h**) Miblet.