Figure showing particle-in-cell simulations of the density fluctuation spectrum near the boundary of a He-Xe plasma [W6]. Theory indicates that the amplification is associated with an ion-ion two-stream instability.
Neutral gases, like air, are highly collisional media that support only a few types of collective wave motions, such as sound waves. In contrast, plasmas support a rich variety of different types of waves because the interaction between charged particles and electromagnetic fields couples the plasma fluid to electromagnetic waves. Furthermore, plasmas are often weakly collisional and, as a result, they are commonly far from equilibrium. This leads to a common situation where free energy in the form of non-equilibrium plasma kinetics can amplify plasma waves. These are called plasma instabilities. Often times the material properties of a plasma, such as the transport of heat and particles, is dominated by these collective wave motions (sometimes leading to turbulence) rather than by collisions. This is a primary reason why plasma behavior can be difficult to predict.
Our research in this area develops new methods for describing how nonequilibrium plasmas can excite instabilities, and how these instabilities can act back on the plasma in terms of influencing particle and energy transport. Part of this is developing new kinetic theories that account for wave-particle interactions (e.g., see [W1,W4]), and part is solving these kinetic (microscopic) theories to determine how the waves influence macroscopic behaviors. New types of plasma instabilities are still being discovered, and we have contributed to this area as well. We use particle-in-cell simulations to gain a deeper understanding of these interactions, and we also work closely with experimental colleagues to test and validate theoretical predictions.
Often times these studies are connected to particular applications. Much of our work in this area is connected to instabilities that arise when plasmas interact with boundaries (see sheaths/boundary layers page), but we have also studied other devices such as thrusters [W8-W10].