Uncovering low-pressure non-equilibrium phenomena in a conical ICP torch for next-generation plasma applications: A two-temperature computational–experimental study

Conical radio-frequency inductively coupled plasma (RF-ICP) torches offer higher power density and improved energy efficiency than conventional cylindrical designs. However, their behavior under reduced pressure remains largely unexplored. In this study, a two-temperature non-local thermodynamic equilibrium (NLTE) model is developed to investigate plasma characteristics in a conical RF-ICP torch operating over a pressure range of 100–10 kPa. Agreement with temperature measurements obtained using optical emission spectrometry (OES) confirms the accuracy of the simulated thermal field. The results reveal strong pressure-dependent non-equilibrium behavior: decreasing pressure increases electron temperature while reducing heavy-particle temperature, creating a large electron–heavy particle temperature difference and shifting Joule heating toward the torch wall. This displacement moves the hot plasma core away from the central axis, increasing wall heat losses from approximately 8% to 42%, and highlighting the need for thermal management under low-pressure operation. Comparison with LTE predictions at 10 kPa demonstrates major discrepancies, including a maximum temperature difference of approximately 4500 K and significant changes in plasma spatial structure. Deviations from ionization equilibrium further confirm the breakdown of equilibrium assumptions at sub-atmospheric pressures. These findings clarify pressure-driven NLTE behavior in conical ICP torches and provide insight for plasma-assisted coating and vacuum processing applications.