Electron Density Measurements in an Inductively Coupled Plasma Torch Using the Stark Broadening of Hydrogen Lines

The detailed characterization of high-power inductively coupled plasma torches is challenging due to significant thermal and chemical non-equilibrium. One key quantity of interest that determines operating characteristics and controls plasma properties is the electron density. In this study, we infer electron number densities in an ICP torch using hydrogen seeding. The torch runs on either argon or nitrogen and we measure the Stark broadened emission lineshapes of the Balmer series Hβ and Hα transitions. This allows us to perform line-of-sight integrated measurements in the plasma generating region of the device, where optical access is extremely limited and thermal and chemical non-equilibrium states are possible. We discuss the methodology and challenges, and report axial profiles of electron number density in the plasma generating region of the ICP torch for both argon and nitrogen plasmas. The experimental measurements are compared to predictive simulations of the argon plasma case, directly and through the generation of synthetic hydrogen transition lines. The findings shed light on differences in ICP operation with argon or nitrogen feed gas, clarify limitations of the methodology, and elucidate how such measurements can be used in the comparison to and validation of numerical models.