Non-equilibrium processes in the plasma torches of inductively coupled plasma facilities

Characterizing the thermodynamic and chemical state of the plasma that interacts with heat shield materials during testing in inductively coupled plasma facilities is essential for comparing test data to computational fluid dynamics simulations. The formation of plasma sustained in argon(Ar) inside the plasma torch of inductively coupled plasma facilities is analyzed by solving electromagnetic, mass, momentum, energy, and charge conservation equations in a time-splicing manner. The electron energy distribution functions required to evaluate transport and thermodynamic properties, and the chemical rates of electron-related processes are obtained from the solution of the Boltzmann equation for electron energy distributions. Non-equilibrium processes for heavy species are modeled by explicitly simulating the electronically excited states of Ar as individual species and the inclusion of Ar dimers (Ar2* and Ar2⁺ ). Under the operating conditions of 30 kW and 250 Torr, two regions are observed within the plasma torch. A localized plasma forms near the coils of the torch with a high density of charged (ions and electrons) species with a significant amplitude of the electric field. The plasma quantities (electric field and density of charged species) decay rapidly in the radial direction resulting in a relatively unionized region close to the symmetry axis. The rapid decrease occurs because of the symmetry of the plasma – the azimuthal electric field on the symmetric axis is zero. With the power deposition scaling as the square of the electric field and plasma conductivity, the majority of the power deposition is localized in a small region near the coils of the torch where the electric field and conductivity are highest. Inclusion of Ar dimers in the reaction affects the concentration of excited states and the source terms that result in the production of electrons. Analysis indicates that the formation of Ar2* is preferred at high pressures because it is formed through a three-body reaction with neutral Ar.