A simple technique for reducing the re-cycling rate and impurity concentration in a tokamak plasma is described. An active metal coating, in this case titanium, evaporated onto the surface of the vacuum vessel, provides a trap for neutral hydrogen and impurity atoms which would otherwise freely penetrate the plasma. With this treatment, the plasma density decays with time after the ionization of the initial filling gas is completed, in contrast to typical standard discharges which have a rising density throughout the entire period of the discharge, indicating a large gas influx. These discharges are observed to have resistances close to that of a pure hydrogen plasma, Zeff ≃ 1.0. There is a corresponding reduction in the intensity of highly ionized spectral lines of oxygen and iron as evidence of reduced impurity concentrations. The value of the effective ion charge, Zeff, can be varied, by pulsing controlled amounts of impurity gases into the hydrogen plasma.
Impurity transport measurements of biased H mode in hydrogen and deuterium plasmas on the Phaedrus-T tokamak