THE ARGON-HYDROGEN EXPANDING PLASMA - MODEL AND EXPERIMENTS

An argon expanding cascaded arc plasma, with small amounts (0-10 vol.% ) of hydrogen added to the flow, is investigated by means of Thomson-R ayleigh scattering and optical emission spectroscopy. The results, esp ecially the electron density behaviour as a function of the distance f rom the onset of the expansion, are interpreted by comparison with res ults of a quasi one-dimensional model. The associative charge exchange reaction between Ar+ ions and H2 molecules plays a dominant role in t he model. Assuming that H2 molecules from the wall enter the plasma in the shock region, the large ionization loss can be explained. Good ag reement between model and experiments is found for the electron and ne utral density and the electron temperature behavior. This makes plausi ble the existence of a recirculation flow inside the vacuum vessel, wh ich transports wall-associated hydrogen molecules towards the plasma.