Low-frequency instability in systems with electron collection from a plasma that penetrates into a vacuum and possible diagnostic applications

It has been found that a low-frequency instability develops in open plasma systems when electrons are collected from a plasma that penetrates in the vacuum. This instability is related to the appearance of an opposed flux of ions that develops when the ion component of the flux of penetrating plasma is reflected from the electron extractor. The condition of reflection is that the potential Vex of the extractor should exceed an amount Vex equal to the potential Up of the plasma in the discharge from which the plasma flow into the vacuum. The process is periodic and the frequencies of the oscillations are determined by the time of flight of the ions through the potential well (a potential barrier for the electrons) that develops when electrons are extracted from the penetrating plasma. The depth of modulation of the extracted electron current is substantial and may reach 50%. Here the authors discuss some results from a study of the dynamics of the change in the steady-state parameters of the plasma in the systems and of the spatial and temporal characteristics of the wave processes that develop during this instability. They also demonstrate the possibility of using the conditions for the development of this instability as a remote plasma diagnostic