Plasma Science: From Fundamental Research to Technological Applications (1995)
Chapter: Magnetic Reconnection and Dynamo Action
Chaos, Turbulence, and Transport
Most plasmas of interest are nonuniform in density and temperature, which results in the excitation of turbulent waves and fluctuations. These fluctuations in turn produce the transport of particles and energy that tend to drive the plasma toward a more uniform state. Very generally, turbulence and turbulent transport are not understood. The recent renaissance in nonlinear dynamics and studies of phenomena such as chaos provide new tools with which to attack these problems, and in fact, plasmas offer a convenient and often unique medium in which to study turbulent phenomena. Many of the scientific issues are now clear, and the plasma applications are many. For example, turbulent transport is the dominant mechanism for energy and particle transport in tokamak fusion plasmas. Turbulent transport is frequently the dominant particle and energy loss mechanism in low-temperature plasmas. It is important in the Earth's magnetosphere, in stellar convection zones, and in astrophysics in settings such as the interstellar medium.
Plasma Sheaths and Boundary Layers
Understanding the boundaries of plasmas (called sheaths) is a well-defined problem with many practical consequences. In magnetically confined fusion plasmas, the hot plasma cannot be allowed to contact the material walls. The result is that there must be large gradients in plasma temperature and density in the plasma and, frequently, non-equilibrium particle distributions. The precise character of these boundary layers can greatly influence the character of the bulk plasma and the rate at which wall damage occurs. Plasma sheaths are important in the plasma processing of materials. This sheath is adjacent to the material surface to be processed; therefore, the properties of this layer determine the characteristics of the plasma-matter interaction. A similar phenomenon occurs in space plasmas, where a plasma sheath separates a satellite from the surrounding space plasma, and the properties of this sheath determine the interaction of the plasma particles with the satellite. This is relevant for considerations such as surface damage and electrical phenomena. A type of boundary layer called a "double layer" can separate regions of plasma with distinctly different properties. Double layers are known to occur both in laboratory and in space plasmas, where they play an important role in determining the global configuration of the plasma. Related electrode-sheath phenomena are the least well understood aspects of lighting plasmas, and learning to control them better would lead to more efficient and longer-life products.
Magnetic Reconnection and Dynamo Action
The behavior of a magnetized plasma is determined largely by the configuration of the magnetic field in the plasma. Currents and plasma flows can induce
