## 4 Discussion
The observed effect of the energy efficiency decreasing or of the increasing in the equivalent working area of electrodes' ends initially arranged at an angle correlates qualitatively with the theoretically predicted effect of reducing the energy efficiency of NP synthesis with an increase in the diameter of the electrodes used [21]. Previously, it was noted that a higher mean temperature of electrodes increases mass production [23; 36; 40] and can lead to an increased production of large particles. This is realized in our experiments at the initial moments of the discharge process between electrodes ends arranged at an angle, when the equivalent working area of electrodes is small and the average density of the discharge current is high. In experiments with the UDP, even when a cathode end had the angle of 10\({}^{\circ}\), the synthesis of parasitic particles (more than 40 nm) was not observed, and the agglomeration of NPs was not significant. This occurred because, in the case of the UDP, the energy released in the spark gap per pulse was almost an order of magnitude less than in the case of the UDP. Therefore, there was no high energy release density even when the discharge was localized between beveled electrodes. NP arrays on electrodes' end surfaces, as shown in Figure 4e, appeared in the case of an asymmetric arrangement of electrode end faces with the ODP. These surfaces, covered with NPs and not covered with discharge craters, indicate the deposition of a significant part of synthesized NPs back to the electrode and, probably, the transfer from the opposite electrode. Earlier in [23; 36], the authors also noted the transfer of large particles to the opposite electrode and the transfer of NPs synthesized from the cathode material on the cathode [41; 42]. Our experiments on the NP synthesis by the SD with the UDP, in which the mass of the anode increased (Table 2), clearly indicate the transfer of a significant proportion of synthesized NPs from the cathode to the anode. It is known that in the experiments with the UDP, each electrode works mainly as anode or as cathode and the electrical erosion of the cathode material turns out much higher than that of the anode, which was more accurately  
Figure 14: (**a**) The electrodes’ mass loss rate, (**b**) energy efficiency, and (**c**) power released in the spark gap for the three cases of the degree of the electrode asymmetry with the ODP. measured for larger interelectrode gaps in [43; 44]. The width of the interelectrode gap of about 1 mm in our work contributed to an increased transfer of synthesized NPs to the opposite electrode. Since the erosion wear of the anode is much less than that of the cathode under these conditions, the material transferred from the cathode compensates the mass loss of the anode and can even increase it. The origin of protrusions of NP clusters at the anode occurs due to the increased transfer of synthesized NPs from the cathode. These protrusions contribute to the electric field localization and, consequently, increase the probability of an electric breakdown of the gas in front of such protrusions. The growth of such protrusions is unstable, since the highest of them will more likely increase the local electric field and cause subsequent SD from the cathode in its direction. It is this single protrusion that grows and localizes the entire discharge process from the opposite region of the cathode. The growth of such a protrusion can be considered as the unstable process of the SD developing with a positive feedback. Moreover, the discharge process is stopped when the growing protrusion reaches the cathode and forms a conductive metal bridge between electrodes. To avoid instability due to the protrusion growth, it is necessary to create conditions for the removal of synthesized NPs from the interelectrode gap with higher efficiency. To do this, the rate of removal of NPs from the interelectrode gap must be greater than the rate of their synthesis.