LANGMUIR PROBE MEASUREMENTS OF THE ION EXTRACTION PROCESS IN INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY .1. SPATIALLY-RESOLVED DETERMINATION OF ELECTRON-DENSITY AND ELECTRON-TEMPERATURE

A single Langmuir probe is inserted through the skimmer into the supersonic jet extracted from an inductively coupled plasma (ICP). Electron temperature (T-e) and electron density (n(e)) are measured with axial spatial resolution from 2 mm behind the sampler to 40 mm behind the skimmer. In the upstream part of the supersonic jet n, drops as axial position increases, as expected for a quasineutral beam. However, the measurements indicate the presence of an unexpected disturbance in front of the skimmer. Electron density behind the skimmer is much lower than would be expected if the skimmer behaved ideally. Plasma conditions that yield a high plasma potential and secondary discharge lead to further reductions in n(e) behind the skimmer. The electrons stay hot during the extraction process, as seen previously, although T-e does drop by similar to 4000 K behind the skimmer. The measurements generally support the concept that the extracted plasma remains quasineutral as it passes through the sampler and skimmer.