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 super sonic 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 s kimmer. In the upstream part of the supersonic jet n, drops as axial p osition increases, as expected for a quasineutral beam. However, the m easurements indicate the presence of an unexpected disturbance in fron t of the skimmer. Electron density behind the skimmer is much lower th an would be expected if the skimmer behaved ideally. Plasma conditions that yield a high plasma potential and secondary discharge lead to fu rther reductions in n(e) behind the skimmer. The electrons stay hot du ring the extraction process, as seen previously, although T-e does dro p 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.