# Diversity of Physical Processes: Challenges and Opportunities for Space Electric Propulsion
## 2 Variety of Physical Processes to Integrate into a Single Design
Vacuum arc thrusters [84, 85] are another good example of the conversion of a device initially developed for initiation of arc discharge in technological setups [86, 87]. The arc systems vary in design and type of mass supply (via material ablation or using gas supply) and they are relatively simple devices when compared to other thruster types [88]. The device basically consists of two electrodes. The arc discharge is sustained between these two electrodes to produce the plasma out of electrode material and accelerate it within the same discharge. Despite the simplicity, they are still among the promising candidates for powerful thrust systems capable of powering large spacecraft needed for large-scale exploration of remote planets, and possibly Mars colonization, due to their relatively high impulse and ability to produce high levels of thrust (see Figure 1). Figure 4: Initial design scheme of the practical thruster based on the Rotamak concept (under development). The design consists of three axial coils and two holders joined with cross-plates to form the main frame, where the plasma vessel is installed. Additional coils, together with gas supply pipes and insulating inserts, are fixed within the frame. Reprinted with permission from Ref. [83]. Copyright 2021, IOP. However, design simplicity does not necessarily imply the simplicity of the involved physical and chemical processes. Even in such a simplest design, the wide variety of intertangled processes should be integrated. Due to its design simplicity, the vacuum arc thruster is a good illustration of the complexity of problems that can emerge even in such small plasma setups (Figures 5 and 6). The first problem is arc discharge ignition, which is far from being solved despite many years of development and a very simple design. Currently, at least three main schematics have been proposed to trigger the discharge. These include independent triggering [89], resistance triggering [90], and triggerless schematic [91]. The first two methods rely on an auxiliary pulse generator, and a resistor included into a circuit of a trigger electrode [92], respectively. For the independent triggering, the device includes an annular trigger electrode mounted around a thin cathode and an anode of metal grid with somewhat larger aperture. Figure 5: Triggering methods for various types of arc plasma sources, for potential application in space propulsion systems. This scheme illustrates only one aspect of plasma generation in arc plasma sources, while the operation of thrusters producing an accelerated plasma jet involves a wide variety of the intertangled processes, including processes of ion acceleration and other key factors that underpin thruster physics. Reproduced with permission from Ref. [26]. Copyright 2019, Springer-Nature. For the resistance triggering, the voltage between the cathode and anode should be relatively low, thus leading to the generation of a much weaker axial electric field, as compared to the independent triggering, so the transition from the radial to axial expansion of the flux of plasma electrons is relatively slow. Moreover, the independent triggering ignites more symmetric discharge with respect to the setup axis of symmetry. The last method, i.e., triggerless, was initially proposed for the vacuum arc deposition setups, where a part of the material evaporated in the arc discharge is deposited on a surface of the igniting electrode, namely in the gap between the cathode and anode [91]. At the next ignition cycle, the deposited layer is heated resistively, and evaporates at the electrical breakdown of the gap. To analyse this problem, readers are encouraged to examine a general schematic of a wide variety of the intertangled processes that reflects only a limited number of aspects of plasma generation, without taking into account processes involved in ion acceleration and other key factors that underpin these thrusters [26].