No abstract

Mass spectrometric measurements of positive ions emerging from Ar-plasmas at 0.1-0.6 Torr are described. The adaption of the mass spectrometer requires besides the extraction probe an ion optical system, which causes the focussing and bunching of the ion current on the entrance of the time-of-flight mass spectrometer. Systematic measurements of the velocity distributions of the different ion species reveal:1. The Bohm-criterion is confirmed. In front of the wall the ions are heated in the quasineutral zone and are accelerated in the space charge zone, which here is in the order of magnitude of the ion mean free path.2. Heating and accelerating increase with growing ion current to the extraction probe.3. A further increase of the probe current causes collisions in the space charge zone which gives rise to a drop of the transmission of the optical system.4. The measurement of the current of one ion species as a function of the probe current is a simple experimental proof for collisions of the ions in the space charge zone.