Plasmas: plasma-surface boundary

The properties of the plasma, and the way the plasma interacts with the wall of the container or with any surface immersed in it (such as a sample) are described by a number of important parameters. A short review of some parameters is given hereafter, and typical values are reported in Table 1. In a first order approximation, parameters in the volume of the plasma control the formation of the active species and the chemical reactions in the gas phase, parameters at the plasma surface boundary control how these species interact with the surface. As described below this description is far too simple, and an important feedback exists between the plasma-surface interaction and the gas phase chemistry. 

Surface Modification. Plasma surface modification can include surface cleaning, surface activation, heat treatments, and plasma polymerization. Surface cleaning and surface activation are usually performed for enhanced joining of materials (see Metal SURFACE TREATMENTS). Plasma heat treatments are not, however, limited to high temperature equiUbrium plasmas on metals. Heat treatments of organic materials are also possible. Plasma polymerization crosses the boundaries between surface modification and materials production by producing materials often not available by any other method. In many cases these new materials can be appHed directly to a substrate, thus modifying the substrate in a novel way. 

It has long been established that all cell membranes in the body are composed of a fundamental structure called plasma membrane. This boundary surrounds single cells such as epithelial cells. More complex membranes such as intestinal epithelium and skin, are composed of multiples of this fundamental structure, which has been visualized as a bimolecular layer of lipid molecules with a monolayer of protein adsorbed into each surface. Cell membranes are further interspersed with small pores that can be protein line channels through the lipid layer or, simply, spaces between the lipid molecules. In membranes composed of many cells, the spaces between the cells constimte another kind of membrane pores (2). 

Integrated computational models comprising the physics of the plasma flow near boundaries, the atomic and molecular processes and the particle-surface... 

One innovative technique that has been developed to limit the mobility of a liquid plasma-facing surface is the capillary pore system [27]. This system has been successfully deployed in a tokamak environment [28]. Although this system addresses many of the mobility and erosion issues of a liquid plasmafacing surface, as will be discussed later in this chapter in order to achieve the full benefits from a low-recycling boundary a larger scaled-up version of this system will need to be developed. 

If this space scale 5 is smaller than the plasma sizes, then the external fields and currents are located only on the plasma surface layer with a penetration depth 5. This effect is known as the skin effect. The boundary layer, where the external fields penetrate and where plasma currents are located, is called the skin layer. The depth of the skin layer depends on the electromagnetic field frequency (/ = co/ln) and plasma conductivity. For calculation of the skin layer depth it is convenient to use the following numeric formula ... 

UHMWPE surface. This layer enhanced boundary lubrication, reduced the dynamic fiiction by 50%, and reduced static fiiction. Like most plasma surface modifications, this surface modification is relatively short-lived, but it lasts long enough to verify the concept that adsorption of protein fi om synovial fluid can be influenced by changing surface chemistry and can potentially improve boundary lubrication. 

In addn, for an ionized gas to be called a plasma, it must have an equal number of pos and neg charges for, by definition, a plasma has no net charge. Regions termed "sheaths , having large (net charges) do develop at the plasma boundaries. Such sheaths are to the plasma what the surface is to a solid or liquid, and their thickness is of the order of the "Debye length ... 

Conduction electrons in a metal are nearly free to move within the metal in response to an applied electric field. A surface plasma wave, also called a surface plasmon, is an electromagnetic wave that propagates along the boundary between a metal and a dielectric (an electrical insulator). The electromagnetic field decreases exponentially into both layers but is concentrated in the dielectric layer. 

One can impose additional spatial confinement on a Debye plasma such that the potential energy vanishes at the boundary of a given sphere of radius R. For the strongly coupled system, one can assume that no electron current passes through the boundary surface and the wavefunction must vanish at the Wigner-Seitz boundary R [154], Under such conditions, the radial one-particle wavefunction ir(r) satisfies... 

---