Finite-element analysis of the effect of sheath-gas composition in an inductively-coupled plasma

Abstract

The sheath gas plays an important role in inductively-coupled plasmas (ICPs) in preventing thermal damage to the side wall of the torch. The sheath gas (hydrogen in our case) is more difficult to ionise than argon (the main plasma gas) due to its lower electrical conductivity at the working temperature, preventing plasma formation and concomitant high temperatures in the immediate vicinity of the torch inside wall. The sheath gas also has a higher flow rate, reducing the time for heat transfer from the plasma to the inside wall. Numerous models to simulate an ICP reactor have been published in the open literature, none of which take into account the effect of characteristics of the sheath gas on the performance of an ICP on the heat transfer from the plasma to the sidewall of the reactor. The H2:Ar gas ratio can have severely detrimental effects on the efficiency of the plasma because of the higher ionisation potential of H2. Excess hydrogen in the sheath gas may also constitute material wastefulness. Both of these factors have an implication on the economics of the plasma process. This research aims at finding the optimum sheath-gas flow for the Necsa torch, used for spheroidisation. The work was done on the commercial finite-element programme COMSOL Multiphysics.