Positively Charged Macroparticles in Low-Temperature Plasma
The evolution of a positively charged metallic macroparticle placed into the low-temperature plasma is considered. The influence of the value of the initial macroparticle charge on the dynamics of the electrical potential and temperature of the macroparticle, as well as the possibility of evaporation of a macroparticle due to its interaction with plasma particles are studied. The system of equations of the energy balance and the current balance based on the OML theory, that takes into account the changing of macroparticle potential and its temperature over the time is solved numerically. The solution of the system of equations shows the evolution of the macroparticle potential and temperature within the time interval from the moment when the macroparticle is placed in the plasma until the moment the macroparticle has charged to the floating potential. The positive charge of the macroparticle excludes the thermionic emission and secondary electron emission from the macroparticle surface, as well as the mechanisms of cooling of the macroparticle associated with these emission processes. Analytical expressions that determine the macroparticle potential, the electron current on the macroparticle, as well as the power transferred by plasma electrons in the case when the energy of attraction of electrons to the macroparticle strongly exceeds the energy of thermionic electrons, the energy of secondary electrons and the energy of plasma ions are obtained. A simplified system of equations of the energy balance and the current balance for a positively charged macroparticle is solved; the solution of the simplified equations matches with the solution of the general equations in the region of positive values of the macroparticle potential. Calculations show that during the charging of the macroparticle, its temperature increases up to the boiling point of the macroparticle substance. An equation that determines the conditions under which evaporation of macroparticles is possible has been obtained and solved numerically. The possibility of evaporation of macroparticles of a given size (critical value of the radius) due to initial charging to high positive values of potential is shown. The dependencies of the critical value of the radius on the initial value of the potential for tungsten and copper macroparticles that can be evaporated in a low-temperature plasma are obtained. These solutions bound the region of the parameters where evaporation of a macroparticle is possible and where it is not. The critical values of the potential for copper and tungsten particles with sizes of 0.1 and 1 μm are calculated. The dependence of the radius of a macroparticle on time during the process of vaporization is obtained.
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