# Diversity of Physical Processes: Challenges and Opportunities for Space Electric Propulsion
## 3 Physical Processes and Metamaterials: New Horizons for Space Propulsion  
Synthesis of novel materials for the aerospace industry has always been a challenge since the properties of the materials greatly affect a device's operation. Indeed, high energy fluxes of particles, high temperatures, the aggressive environment of other planets, dusty particles and so on can affect the surfaces of spacecraft. As a results, cracks, dents and other discontinuities in the material structure occur, thus causing device failure. Similarly, in plasma propulsion devices, erosion of structure elements is the critical factor that greatly affects the thruster's lifetime. Since the material should withstand external forces, its function can be described as passive. However, there is another group of materials that play a very important role in the space industry in general, and in plasma thrusters in particular. These are active functional materials such as those used in emitters of cathode-neutralizers to produce an electron flux, or magnetic materials used to generate magnetic fields. As in the development of thruster design, newly adopted technical solutions are employed in this domain. Metamaterials represent a new class of functional materials that could boost space propulsion in various ways. However, the application of metamaterials also requires the integration of a complex set of physical processes. For the interaction with cold plasma when the electron temperature does not exceed a few tens of electron-volts, three possibilities can be considered to boost space propulsion technologies via advanced metamaterials (Figure 7). Several possible approaches to this are illustrated in Figure 8. The first way depends on the large diversity of plasma species, their mobility, their extraordinal chemical activity and their flexibility towards the control means. Due to its ability to be guided by pressure gradients, friction forces, and electric and magnetic fields, plasma is a perfect tool to modify materials through sputtering, etching and other ballistic and thermal effects. As a result, complex hierarchical micro- and nanostructures are synthesised on various substrates, thus forming a unique platform for future applications [67; 105]. The second possible application arises due to the effect of the generation of patterned spatial distributions of electromagnetic fields and charged particles, thus creating the complex plasma structures that can be implemented in space propulsion. For example, metamaterial composed of split ring resonators (SRR) produces shifted SRR resonance when interacting with low-pressure argon plasma [106]. The third application arises from the cases when plasma itself behaves like a metamaterial [108]. There are several examples: An extraordinary wave transition is obtained by the combination of a metamaterial with negative permeability and a plasma array [109]; broadband reduction of a radar cross-section (RCS) is produced by implementation of a combined coating of metamaterial and plasma [110]; and a frequency selective filter can be designed using plasma generated between coupled dielectric resonators (DRs) [111]. Illumination of the single-layer metamaterial by microwave (MW) pulses produces effects that are potentially useful in the development of high-power MW systems [112]. Several comprehensive review articles outline the physics and applications of plasma when it takes on the role of a metamaterial [113]. For example, the featherlight plasma horn antenna could be considered as a system with metamaterial properties [114]. Plasma acting as a metamaterial could be used as a non-linear media for cloaking, for example. [115]. In this system, a set of thin metallic wires generates the plasma that produces a radiation very similar to that produced by the metallic horn. Miniaturisation could also be discussed from the point of view of emerging and advanced engineering materials that are playing an increasingly important role in driving the progress of space technologies [2,15].