An experimental study of the state of equilibrium in an inductively-coupled plasma torch

Abstract

An experimental study of the state of equilibrium existing in an atmospheric pressure argon inductively-coupled plasma torch (ICPT) has been undertaken using several diagnostic methods. In particular, a relaxation technique was used which involved pulsing off the RF field rapidly (< 3 μs) and monitoring the subsequent intensity changes in the optically-thin excited argon I spectral lines. The removal of the RF field is accompanied by a sharp intensity increase persisting for about 18μs before falling monotonically. This effect is explained in terms of the theory of collision-radiation decay of a plasma. The increase in spectral line intensity is used to ascertain whether the optically-thin excited Ar I states conform to a Boltzmann distribution for various operating conditions of the ICPT. For those cases where the excited states are close to a Boltzmann distribution, this method is used to determine the difference in temperature between the electrons (Tₑ) and the heavy particles (Tg) as well as the electron temperature. In addition, the same technique applied to a near saturated (optically thick) Ar I spectral line (811.5nm) is used to observe changes in radiative losses from the plasma. The results obtained show that a single flow (coolant only) ICPT plasma is in a state of pLTE in the central regions (r < 7mm). Not only do the optically-thin excited Ar I states conform to a Boltzmann distribution but centrally Tₑ = Tg to closer than 3% and Tₑ ≈ 10,000 K. The introduction of a second (aerosol) flow axially through the plasma is shown to cause a deviation from excitational equilibrium across the entire plasma radius. Increasing this flow from 1 1/min to 2 1/min appeared to return the plasma to a state of excitational equilibrium, but the temperatures obtained became inconsistent with the validity of Saha’s equation, i.e. indicating a non-Boltzmann distribution of the Ar I states. The radiative losses are increased significantly. The introduction of certain aerosols viz, 4%H₂ 4%Air, H₂O and KC1, into the plasma are shown to have negligible effect on the state of equilibrium. Four different methods used to obtain the electron density, (Stark broadening, interferometry, absolute intensity of the continuum, and Saha’s equation calculations) were compared for various cases. With a single flow plasma the results were consistent, and are typically in the range 10²¹ - 10²² m⁻³. With the introduction of the second flow, the method based on Saha’s equation yields results inconsistent with the other methods, confirming the significant deviation from equilibrium for this system. Calculations show that the deviation from excitational equilibrium is consistent with convective losses resulting from the forced flow of argon through the torch.