Non-equilibrium plasma sources

Cold and hot plasmas In Chapter 2, non-equilibrium gas plasmas are shown to be well suited to medical applications because of their capability of providing energetic electrons and ions to facilitate highly active reaction chemistry at a low gas temperature. This description is clear in that it characterizes the large difference of the electron energy (typically at 1 eV or above) from the gas temperature (usually below a couple of hundreds degree Celsius), as well as its potential for applications in healthcare. It is, however, ambiguous in what is meant by low gas temperature; this is possibly inevitable given the very wide range of medical applications addressable by non-equilibrium plasmas. Some applications require the gas temperature to be close to room temperature and remain relatively unchanged. This is the case for plasma disinfection of dental cavities, for which an increase in intrapulpal temperature of less than 2.2°C from the body temperature presents acceptable thermal stress. However, an increase of 5.5°C and 11°C would cause pulpal necrosis in 15% and 60% of cases, respectively (Zack & Cohen, 1965). For other applications, a modestly elevated gas temperature can be either acceptable or even beneficial. An example is plasma surgery in which plasma-generated heat is used as the main mechanism to achieve controlled and localized ablation of unwanted tissue. In the case of decontamination and sterilization of metallic surgical instruments, the autoclaving procedure is typically carried out at 121°C. It is, therefore, clear that low gas temperature has different meanings for different applications. This variation in the definition and indeed perception of what constitutes low gas temperature has led to a common use of largely ill-defined and ambiguous terminology, such cold, hot, and indeed non-thermal gas plasmas. These may be appropriate within a specific group of applications, but not to other applications, thus causing considerable confusion. Furthermore, these terms are not conducive to capturing clearly the relevant plasma properties.