## 3 Results
### The Influence of the Degree of Electrodes Asymmetry on the NP Synthesis with the UDP  
The UDP, which occurs when the ballast resistor is inserted in the discharge circuit, was characterized by approximately twice less amplitude and duration comparable to one half-wave of the ODP. Thus, the total electric charge passing through the spark gap in a single discharge pulse was about 7 times less for the UDP than for the ODP. For this reason, the electrical erosion wear of the electrodes during the several hours of the experiment was not sufficient to form on the cathode a significant area parallel to the anode, even at the angle of 10\({}^{\circ}\). Therefore, experiments on the electrical erosion of electrodes and NP synthesis by the UDP were carried out with unchanged electrode shape at the macroscale. It was found that, in processes of NP synthesis with the UDP, in cases of parallel arrangement of electrodes' ends or with a small slope of the cathode surface (3\({}^{\circ}\)), an electroerosive instability of the spark process occurred. It was manifested in the localization of spark channels to some response areas of electrodes end faces and was random in time. On the anode end face, a protrusion from the electrodes material had been appearing at the location of the spark channel. The protrusion grew over time and reduced the gap between electrodes. Reaching the cathode, it formed a metal bridge between electrodes, short-circuiting the interelectrode gap and ceasing spark discharges and NP synthesis. The start of the protrusion's rapid growth from the anode to the cathode is clearly indicated by the sudden change in functions of the capacitor charging voltage and the frequency of the capacitor discharges. This is represented in Figure 9 by black and red lines. The time of the beginning of the metal protrusion formation on the anode is longer for the cathode end face orientation at the angle of 3\({}^{\circ}\) than for the case of parallel arrangement of electrodes' ends. In the case of the cathode end face oriented at an angle, NP deposition on the anode reduces as it becomes easier for NPs to leave the interelectrode gap. Obviously, the protrusion is formed by NPs synthesized from the cathode material and transferred to the anode. The time dependences of modal sizes and concentrations of NP agglomerates synthesized in the SD with the UDP at different arrangements of the electrode ends are shown in Figure 10. From the comparison of these dependences, one can see an increase in the initial modal size of agglomerates with an increase in the angle between electrodes' end faces. This can be explained in the analogy with the case of the ODP by the localization of SDs in the area of the shortest distance between electrodes that leads to the increased current density in the SD and to the synthesis of large NPs. Figure 9: The dependences of (**a**) the capacitor charging voltage and (**b**) the frequency of capacitor discharges for the case of synthesis with the UDP for electrodes’ ends arranged in parallel and at the angles of 3\({}^{\circ}\) and 10\({}^{\circ}\). Since, in the case of the UDP, a significant part of the capacitor's energy was dissipated on the ballast resistor, the energy spent on the electrical erosion of the electrode material was much lower than in the case of the ODP, and the synthesized NP agglomerates had smaller dimensions. Herewith, for the arrangement of electrodes ends at the maximum angle (10\({}^{\circ}\)), an asymptotic approximation of the dependence of modal sizes of NP agglomerates from time to the average size of about 130 nm is observed. In the process of NP synthesis with the UDP, the anode end face is covering with an array of dendrite-like NPs. The cathode end face is modified by electrical erosion, followed by the formation of a set of overlapping microcraters. Such microcraters are formed as a result of evaporation and spilling of metal heated by the SD at places where it occurs on the cathode.