Numerical simulations of a RF-RF hybrid plasma torch with argon at atmospheric pressure

TL;DR

The paper addresses sustaining a plasma torch at atmospheric pressure using a RF-RF hybrid approach with two coils at $f_{ m HF}=13.56~\mathrm{MHz}$ and $f_{ m MF}=200~\mathrm{kHz}$. It employs a 2D axisymmetric COMSOL Multiphysics model to quantify how the minimum sustaining current ($MSI$) and total power depend on the coil distance ($D_{ m coil}$) and HF power ($P_{ m HF}$), including impedance, temperature, velocity profiles, and heat transfer. The main contributions are the systematic mapping of $MSI$ versus $D_{ m coil}$ and $P_{ m HF}$, demonstration that dual-frequency operation reduces MF power requirements, and validation of grid-independence with robust impedance behavior. The findings have practical implications for designing cost-effective, stable ICP torches, while acknowledging limitations such as neglect of radiation and the absence of 3D effects, which guide future experimental and modeling work.

Abstract

We report numerical results regarding the minimum sustaining coil excitation current for a RF-RF hybrid torch operating at two different frequencies. The first coil is excited at a high-frequency, while the second coil is set at a medium frequency. The filling gas is argon, at atmospheric pressure. We use the modeling software COMSOL Multiphysics to describe the evolution of key parameters when: (i) the distance between the two coils changes, (ii) the power of the high frequency coil changes. We discuss the radial temperature profiles, the axial velocities and the heat convected at the end of the medium-frequency coil. The latter is compared with the total heat conduction to the plasma confinement tube wall.

Figures (15)

• Figure 1: 2D axisymmetric diagram of the plasma torch.
• Figure 2: Default mesh skewness: (a) global view, (b) zoom on coil grid and (c) zoom on boundary layers at the wall.
• Figure 3: Typical current curve with plateau, and its corresponding power, to find an accurate value of the MSI. The MSI is defined as the last current value giving a constant coil power. In this case, only the MF coil is on.
• Figure 4: MSI of the MF coil for different coil distances, with $P_{\rm HF} = 3~kW$. The corresponding MF coil power is shown on the right axis.
• Figure 5: 2D temperature plot when MSI is reached. (a)$D_{\rm coil} = 40~mm$ and (b)$D_{\rm coil} = 180~mm$.