# Diversity of Physical Processes: Challenges and Opportunities for Space Electric Propulsion
## 3 Physical Processes and Metamaterials: New Horizons for Space Propulsion
Progress in material science is particularly relevant for the development of more efficient and functional space assets of the CubeSat standard, where these have transformed from relatively simple single-function systems to systems with increased complexity and functionality needed for more advanced space exploration missions [116]. Figure 7: (**a**,**b**) Smart metamaterials may be used for various aerospace technology and many other applications. Reprinted from Ref. [107] under terms and conditions of CC BY License. Copyright 2019, the Authors. Of particular interest are materials that display complex functionality, adaptivity and self-driven behaviour, such as self-cleaning and self-healing. The metamaterials, along with the materials that arise from their unique structure and can increase device efficiency [117, 118], comprise hierarchically organized physical elements that span multiple scales [119, 120]. Some of these features have already been realized, with reported significant improvements in material lifetime and performance, whereas the possibility of self-healing in materials used for thrusters is still at the concept stage and would require significant effort and investment before it can be realized practically. Metamaterials are likely to find applications across all facets of space propulsion technology [15, 23], and more broadly, space exploration technologies. However, the process of plasma interaction with metamaterials is very complex, and special tools and instruments need to be developed to implement this technique in future space propulsion systems. An interesting example of a hierarchical hybrid system that involves novel materials and advanced design is a small Hall thruster with a hybrid magnetic system consisting of permanent magnets and magnetic coils (Figure 9). Figure 8: Plasmas and metamaterials: interrelated processes and properties create new capabilities. Different pathways are possible to enhance the characteristics of space propulsion systems via metamaterial-inspired technologies. Plasma can be used directly to create the novel metamaterials for space applications; plasma can interact with metamaterials to produce novel effects, e.g., a type of virtual antenna; and plasma itself could be a type of metamaterial. For this design, the samarium-cobalt permanent magnets were tested. Samarium-cobalt magnets are applied in various branches of industry, e.g., in the computer numerical control units or magnetron heads of DC motors [121], because the magnets produce strong magnetic fields at a relatively small size. At the same time, magnetic fields are an essential feature of plasma thrusters such as Hall and arc thrusters, and the configuration and strength of the field is critical for the performance of the thrusters. For thruster operation, the strength varies in the range of a few mT to a few T (for arc thrusters in pulsed mode). Obviously, the need for flexible control of thruster operation promotes the application of electromagnets, where the direction and strength of the magnetic field can be varied by simply changing the terminals of the power source and the direction of the current passing through the coils of the electromagnet. Unfortunately, electromagnets undergo extensive heating with the increase of the coil current, which is the reason a magnetic field of 1 T is necessary to operate a miniaturized arc thruster can only be obtained in the pulse mode. Additionally, electromagnets significantly increase the weight of the device, and require a power supply. Thus, due to the large range of the applied magnetic field and the necessity of flexible operation of the thrusters, a combination of permanent magnets that are able to generate a strong but constant magnetic field, and electromagnets that can adjust the magnetic field strength, is considered. Figure 7a presents a schematic of a typical Hall-type thruster, while a hybrid magnetic system [122] of a small Hall thruster (23 mm diameter) is shown in Figure 7b.