Experimental study on the dynamic characteristics of an analytical inductively coupled plasma and its tail flame

Abstract

Many numerical simulation models for analytical and industrial ICP sources have been developed; hence, experimental verification is essential. Compared with plasma temperature, the flow velocity profile is a direct and reliable criterion for model verification. In this paper, an experimental study on the dynamic properties of a home-made analytical ICP source and its tail flame is conducted using a high-speed colour camera and a high-speed fibre-optic spectrometer, and the spatially resolved pulsation frequency and flow velocity are presented. The pulsation frequencies of the plasma area and emission intensity were experimentally determined, respectively. The spatially resolved pulsation frequency indicates that pulsation of the normal analytical zone (NAZ) is very stable and synchronous, and the tail flame fluctuates due to ambient air entrainment. The flow velocity in the coolant gas was characterised by tracking the trajectories of injected alumina powder particles. After correcting for the velocity difference between the powder particle with high inertia and the surrounding flow, a plausible range of axial (V_z) and radial (V_r) velocity at the outer edge of the coolant gas is proposed. The flow velocity on the axis downstream of the NAZ was experimentally determined by tracking and interpolating the velocity of discrete erbium ion clouds originating from individual erbia suspension particles. By comparing the simulated profile of axial velocity with the experimental profile, the power coupling efficiency of the present ICP facility is estimated to be around 80%. A linear expression is presented to describe the variation of V_z with the axial position (z) in the range of 0 ≤ z ≤ 50 mm. Because erbium ion clouds were not distinguishable from the very bright emission background within the NAZ, a novel method is proposed to determine the flow velocity in the NAZ by combining the dependence of the audio frequency of plasma pulsation on the flow velocity profile, the simulated profile of axial velocity, and the experimental value of pulsation frequency. The determined value of axial velocity at the torch outlet axis operating at an r.f. power of 1200 W is in good agreement with the fitted value. This work presents complete experimental data on flow velocity in a single ICP facility and experimentally verifies the previously developed 2D numerical model.