Light and heavy impurity production mechanisms during radiofrequency plasma heating in the Uragan-3 torsatron

The authors study the mechanisms by which light (oxygen, carbon) and heavy (metal atoms) impurities are released from the walls in the Uragan-3 torsatron, in which radiofrequency (RF) heating is the only way of heating (and producing) the plasma. Having investigated the relationship between the quantity of impurities in the peripheral plasma, as well as the mean density, the confining magnetic field and the power input, they show that metal impurities appear as a result of sputtering from metal surfaces. Carbon, too, like the metal atoms, enters the plasma as a result of physical sputtering of carbon bearing materials. However, oxygen production occurs in a different way — through chemical interaction between hydrogen and metal oxides on the surface. By comparing impurity concentrations in the edge plasma and the fluxes of charge-exchange atoms as functions of e, power input and magnetic field, the authors show that these fluxes do not play a decisive role, either in surface sputtering of the antenna and the magnet casing or in the chemical interaction responsible for the release of oxygen from the surfaces. Sputtering from the surface of the magnet casing that faces the confinement volume is shown to be governed mainly by the high frequency current associated with RF potential oscillations of the peripheral plasma. Sputtering from the surface of the RF antenna is due to its interaction with the quasi-constant ion current, with an energy of ≥ 100 eV produced during RF heating. The formation of water molecules on the surface is due to interaction of surface oxides with hydrogen atoms formed during the dissociation of H2 molecules.