This chapter summarizes our current understanding of the various ionospheres in the solar system. The order of presentation of the planetary ionospheres follows their position with respect to the Sun, that is, it starts with Mercury and ends with Pluto. The amount of information currently available varies widely, from a reasonably good description for Venus to just a basic guess for Pluto. In the last section of this chapter, the ionospheres of the various moons and that of Comet Halley are described. Here again the existing data are limited, with the exception of Titan, which is currently undergoing extensive exploration by the Cassini Orbiter.

Mercury

Mercury does not have a conventional gravitationally bound atmosphere, as indicated in Section 2.4. The plasma population caused by photo and impact ionization of the neutral constituents, which is present in the neutral exosphere, is an ion exosphere, not a true ionosphere. No quantitative calculations of the plasma densities have been carried out to date. The global Na+ production rate was estimated to be a few times 1023 ions s−1, but no other studies have been published and there are no observations concerning the thermal plasma densities. The Messenger spacecraft is currently on its way and will be placed in orbit around Mercury in 2011. Our understanding of Mercury's environment will increase significantly with data from a successful Messenger mission.

Venus

Of all the nonterrestrial thermospheres and ionospheres in the solar system, those of Venus have been the most studied and best understood, mainly because of the Pioneer Venus Orbiter (PVO) spacecraft, which made measurements over the 14-year period from 1978 to 1992.

Plasma biology and medicine are rapidly growing new areas of non-thermal plasma science and engineering. Not so long ago, biomedical applications of non-thermal plasma were mostly focused on surface sterilization as well as treatment of different surfaces to control their compatibility with biomaterials. Now, plasma is also applied in solving novel sophisticated problems of tissue engineering, sterilization of reusable heat-sensitive medical instruments, and of large-volume air and water streams. Recently, non-thermal plasmas have also been directly applied in medicine, in treatment of living tissues, including sterilization and healing of wounds, blood coagulation, and treatment of skin deceases. Such applications are to be discussed, starting with the biological and finishing with the medical.

Non-Thermal Plasma Sterilization of Different Surfaces: Mechanisms of Plasma Sterilization

Non-thermal plasma is an effective source of active species and factors, such as radicals, ions, excited atoms and molecules, UV radiation and so on, which are able to deactivate, kill, or even completely disintegrate bacteria, viruses, and other micro-organisms without any significant temperature effects. It attracts interest to plasma applications in sterilization and disinfection. Disinfection usually implies a couple-order-of-magnitude reduction of populations of micro-organisms, while sterilization usually requires at least a 104–105-fold reduction in the number of micro-organisms. This section focuses on plasma sterilization and disinfection of different non-living surfaces (such as, for example, medical instruments and equipment).The following sections focus on sterilization of air and water streams, as well as on sterilization of living tissues (such as, for example, human skin and organs in vivo).

Plasma chemistry is an area of research that has consumed and inspired more than 35 years of the author's professional activities. During this period, plasma chemistry has become a rapidly growing area of scientific endeavor that holds great promise for practical applications for industrial and medical fields. Plasma has become a ubiquitous element that pervades many aspects of our lives. For example, the public is well aware of plasma TV, fluorescent lamps, and plasma thrusters, as well as popular-culture concepts such as plasma guns and plasma shields from Star Trek. Not many are aware, however, that computers, cell phones, and other modern electronic devices are manufactured using plasma-enabled chemical processing equipment; that most of the synthetic fibers used in clothing, photomaterials, and advanced packaging materials are plasma treated; that a significant amount of potable water in the world is purified using ozone-plasma technology; and that many different tools and special surfaces are plasma coated to protect and provide them with new extraordinary properties. The developments in plasma chemistry are enabling tremendous growth in a variety of applications for manufacturing, environmental remediation, and therapeutic and preventive medicine.

The motivation for this book is to provide engineers and scientists with a foundational understanding of the physical and chemical phenomena associated with both thermal and non-thermal discharge plasmas. Students pursuing degrees in electrical, chemical, mechanical, environmental, and materials engineering will find that the applications in plasma and plasma chemistry will have many important bearings in their own disciplinary areas.

Plasma-Chemical Conversion of Methane, Ethane, Propane, and Natural Gas into Syngas (CO–H2) and other Hydrogen-Rich Mixtures

General Features of Plasma-Assisted Production of Hydrogen from Hydrocarbons: Plasma Catalysis

Industrial demand for H2 and H2-rich gases is growing, resulting in the development of the hydrogen energy concept in energy production and transportation (Hoffmann & Harkin, 2002; Ewing, 2004). Hydrogen production plays a key role in the development of fuel cell technology and should be especially noted (Busby, 2005). Also the conversion of natural gas into syngas (CO–H2 mixture) plays an important role in natural gas liquefaction and numerous processes of organic synthesis (Smith, 2001). Conventional thermo-catalytic technology for H2 production is limited by relatively low specific productivity, high metal capacity, and the large equipment size, which becomes especially important in the case of small- and medium-scale H2-generation systems. The application of plasma in the production of hydrogen and syngas creates an attractive alternative to conventional thermo-catalytic technologies (Deminsky et al., 2002). Plasma stimulation of hydrogen-rich gas production results in a significant increase of specific productivity and reduces the capital and operational costs of the process. The use of traditional catalysis is often limited by a time delay based on heating the catalyst to the required high temperature, which is especially critical in transportation applications such as hydrogen production from hydrocarbons on board a vehicle. The plasma method of hydrogen production has almost no inertia, which makes the plasma approach very attractive to the automotive industry.

Plasma as the Fourth State of Matter

Although the term chemistry in the title of the book does not require a special introduction, the term plasma probably does. Plasma is an ionized gas, a distinct fourth state of matter. “Ionized” means that at least one electron is not bound to an atom or molecule, converting the atoms or molecules into positively charged ions. As temperature increases, molecules become more energetic and transform matter in the sequence: solid, liquid, gas, and finally plasma, which justifies the title “fourth state of matter.”

The free electric charges – electrons and ions – make plasma electrically conductive (sometimes more than gold and copper), internally interactive, and strongly responsive to electromagnetic fields. Ionized gas is usually called plasma when it is electrically neutral (i.e., electron density is balanced by that of positive ions) and contains a significant number of the electrically charged particles, sufficient to affect its electrical properties and behavior. In addition to being important in many aspects of our daily lives, plasmas are estimated to constitute more than 99% of the visible universe.

The term plasma was first introduced by Irving Langmuir (1928) because the multi-component, strongly interacting ionized gas reminded him of blood plasma. Langmuir wrote: “Except near the electrodes, where there are sheaths containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small.

Thermal Plasma Spraying

Plasma Spraying as a Thermal Spray Technology

Plasma spraying is a thermal spray technology in which finely ground metallic and non-metallic materials are deposited on a substrate in a molten or semimolten state (Kudinov et al., 1990; Zhukov & Solonenko, 1990; Pawlowski, 1995; Vurzel & Nazarov, 2000; Fauchais, Vardelle, & Dussoubs, 2001; Fauchais, 2004; Knight, 1991, 1998, 2005; Yoshida, 2005). Two other technologies are combustion spray and wire-arc spray. The thermal plasma heat source is usually based on direct-current (DC)-arc or radiofrequency inductively coupled plasma (RF-ICP) discharge. It provides very high temperatures, over 8000 K at atmospheric pressure, which allows melting of any material. This is actually a major distinctive feature of plasma spraying. Although thermal plasma enables extremely high temperatures, the operational melting temperature in plasma spraying is usually kept at least 300 K lower than the vaporization or decomposition temperature to avoid losses in energy efficiency. Ground materials are either injected into plasma (in the case of RF discharges) or in the plasma jet (in the case of DC arcs), where they are heated, melted or softened, accelerated, and directed toward the surface or substrate being coated. Particles or droplets making impact with the substrate rapidly cool, solidify, cool further, contract, and build up incrementally to form a deposit. The basic coating building blocks, known as “splats,” typically undergo cooling rates in excess of in the case of metals (Jackson et al., 1981).

Plasma Ignition and Stabilization of Flames

General Features of Plasma-Assisted Ignition and Combustion

Spark ignition is one of the oldest applications of plasma, known and successfully applied for thousands of years. Even in the automotive industry, spark ignition has been applied for more than a hundred years. Nevertheless, other plasma discharges, especially, non-thermal discharges, have been attracting more and more attention for use in ignition and stabilization of flames. An example, in this regard, is the non-thermal plasma ignition of fuel–air mixtures at moderate pressures and high velocities, including ignition in supersonic flows, plasma enhancement of combustion at atmospheric pressure, and stimulation of combustion of lean mixtures (Anikin et al., 2005; Starikovskaia, 2006). Numerous investigations have been focused on plasma ignition and stabilization of flames. The effectiveness of spark ignition relies on the essential non-uniformity of the thermal plasma of spark discharges and, therefore, restrictions of the system geometry (see, for example, Thiele, Warnatz, & Maas, 2000). Relevant application of thermal arc discharges, related in particular to hypersonic flows, has been analyzed, for example, by Takita (2002) and Matveev et al. (2005). Initiation of flame by a short-pulse thermal discharge and a conventional arc has been investigated in CH4–air mixtures using the time-resolved interferometry by Maly and Vogel (1979). The ignition effect of gliding arc discharges, which generates non-thermal plasma but also can result in some controlled heating, has been analyzed by Ombrello et al. (2006a, b).

The yield of a total plasma-chemical process is due to synergistic contributions of numerous different elementary reactions taking place simultaneously in a discharge system. The sequence of transformations of initial chemical substances and electric energy into products and thermal energy is usually referred to as the mechanism of the plasma-chemical process. Elementary reaction rates are determined by the micro-kinetic characteristics of individual reactive collisions (like, for example, reaction cross sections or elementary reaction probabilities) as well as by relevant kinetic distribution functions (like the electron energy distribution function [EEDF], or population function of excited molecular states). The elementary reaction rate is actually a result of integration of the reaction cross section or probability over the relevant distribution function and characterizes the energy or excitation state of reactants. We will focus in this chapter mostly on the micro-kinetics of the elementary reactions – on their cross sections and probabilities – assuming, if necessary, conventional Maxwellian or Boltzmann distribution functions. More sophisticated non-Maxwellian and non-Boltzmann kinetic distribution functions typical for strongly non-equilibrium discharge conditions, like the Druyvesteyn EEDF for electrons or Treanor distribution for vibrationally excited molecules, are to be considered in the next chapter.

Ionization Processes

Plasma is an ionized gas. The key process in plasma is ionization, which means conversion of neutral atoms or molecules into electrons and positive ions. Thus, ionization is the first elementary plasma-chemical processes to be considered.

Although the public understanding of plasmas may be limited to plasma TVs, low-temperature plasma processes are beginning to enter into a higher level of consciousness due to the importance of plasma in many aspects of technological developments. The use of plasma for industrial purposes began more than 100 years ago with plasma sources used to produce light. Since then, plasma processes have emerged in transforming wide-ranging technologies, including microelectronics, gas lasers, polymers and novel materials, protective coatings, and water purification, and finally found their ubiquitous place in our homes. Plasma systems or plasma-treated materials are now commonly used and can be found in air-cleaning systems; food containers; fruit, meat, and vegetable treatment; fabrics; and medical devices.

In recent years, new application areas of plasma chemistry and plasma processing have been established, such as plasma nanotechnology with the continuous growth of the “dusty plasmas” domain, plasma production and modification of nanotubes, plasma aerodynamics, and plasma ignition and stabilization of flames. With the recent emphasis on alternative energy and environmental concerns, plasma chemistry has revolutionized hydrogen production, biomass conversion, and fuel-cell technology. In the same manner, the use of non-thermal plasmas in biology and medicine will likely “explode” in the coming years for various applications. Plasma is expected to soon be widely used in surgery, decontamination and sterilization of surfaces and devices, and air and water streams, as well as in tissue engineering and direct treatment of skin diseases.