An instrumental system based on a two-dimensional detector to measure electron number densities in plasmas is described. High-resolution H spectra are detected simultaneously at up to 200 spatial locations. Radially resolved electron number densities are determined, following Abel inversion and comparison of H line profiles to theoretical Stark-broadened spectra. The particular interpolation procedure used to calculate H spectra from discrete literature data can affect the calculated electron number density values. Radially and vertically resolved electron number maps for inductively coupled plasmas supported in Fassel-type and low-flow torches are presented, Electron number densities in the center of the 1.0-kW ICPs are less than 2.5 × 10<sup>14</sup> cm<sup>−3</sup>. Short-term precision is better than 1% relative standard deviation except low in the center of the plasma, where the H intensity is low on-axis and larger off-axis.
John W. Olesik, Su-Jane Den, and Kathryn R. Bradley, "An Instrumental System for Simultaneous Measurement of Spatially Resolved Electron Number Densities in Plasmas," Appl. Spectrosc. 43, 924-932 (1989)