## 3 Results and Discussion  
### Basic Electrical and E-Field Characterization of Plasma Source  
The voltage and current curves for helium and argon operation show a periodic signal with a dominant symmetry between the positive and negative voltage period (Figure 2). For identical applied voltage, the current shape under argon generates a main discharge current up to 4.2 mA with a duration of about 6 \(\upmu\)s and a second discharge current of 4.5 mAand a duration of 3 to 6 \(\upmu\)s. The helium cap shows discharge conditions of 2 mA amplitudes and a discharge duration of 12 \(\upmu\)s. The estimated electrical power for the Ar-CAP, using the traditional applied voltage multiply by discharge current, revealed a mean value of 0.7 to 1 W, while for He-CAP 0.4 to 0.96 W.  
While the geometry is rather simple, the operation in argon especially is scarce in literature and comparison hence not easy. Considering the works in a similar frequency range for argon, an investigation on argon metastable density distribution with a capillary wrapped by the grounded electrode and an internal powered electrode correlated phase resolved imaging with the main current peak with the discharge inside the capillary between both electrodes and the second peak with the generation of the guided streamer exiting the capillary [61,62]. Another recent work with a close geometry focused on the atomic auroral oxygen line emission rather than the discharge current [63]. The described phase should be similar, while the geometric properties and the flow range differ. However, the current was only measured on the input line and an overall high displacement current is included. The helium signals for voltage and current are comparable to characteristics in other studies indicating the jet current signal relates to the guided streamer propagation [64,65]. Our previous works with a similar basic geometry showed discharge peaks of shorter duration [57,66,67]. When comparing images of the different systems, the effluent differs from the previous system with a filamented effluent and the present system with a rather diffuse discharge (as in discharge photos from Figure 1). **Electric field diagnosis**  
The presented voltage and current characteristics in Figure 2 are for the case without a surface. Meanwhile, the signals measured when placing the electrical field probe represent the operation with a dielectric target placed in front (Figure 3). As it is observed, the basic characteristics are similar, with comparable time scales and slightly increased amplitudes. The symmetry between negative and positive half phase of the voltage period remains as well. The main change is, however, the occurrence of more consecutive discharge peaks after the first, increasing in amplitude and number in accordance with the applied voltage. Operation in argon generates up to three peaks per half cycle while helium goes up to two peaks, respectively. Consecutive discharge peaks like these were already reported to correlate with weaker but dominantly repeating discharge dynamics [31]. In a previous study for an in principle identical electrode setting, we reported the generation of an inverse current pulse following the primary discharge pulse which correlated with the bullet propagation and charge exchange processes [66,67]. While voltage settings and electrode geometries equal the present setup, changes in capillary dimension, implying flow condition changes, dielectric properties and electrode dimensions are present. Figure 2: The applied voltage and discharge current for the Ar (**left**) and He (**right**) CAP in mid air (no surface). The electrical field probe shows an increase of electrical field amplitudes with increasing applied voltage. The signal correlates with events observed on the current signal. An enhanced electrical field amplitude from 25 kV/m up to 320 kV/m in argon and from 50 kV/m up to 300 kV/m in helium is achieved (see Figure 4). For Ar-CAP and He-CAP operation, a plateau dominates each discharge cycle.