JET impurity results in helium plasmas

During JET operation in 3He and 4He, a higher density limit is observed than in deuterium, density control is easier and radiation losses are lower during ICRH. Also, there are lasting beneficial effects on subsequent D discharges. Impurity behaviour, particle confinement and residual deuterium content of these plasmas have been studied by visible and VUV spectroscopy. It has been found that He and D influxes are comparable in similar helium and deuterium discharges, respectively, i.e. τHe ≅ 0.5τD. Release of deuterium, but not helium, from carbon surfaces leads to higher deuterium fractions in ICRH helium plasmas and is responsible for a higher density and radiation increase in deuterium. Impurity diffusion coefficients and confinement times are the same in helium and deuterium plasmas. Nickel concentrations are higher in helium as expected from the higher sputtering yield. Oxygen is almost absent in pure helium discharges and is well correlated with the residual deuterium flux, suggesting a chemical production mechanism. Subsequent to longer periods of helium operation, a lasting reduction of oxygen has also been observed in deuterium plasmas. Carbon concentrations in both helium and deuterium show the signature of physical sputtering, i.e. they increase with Ip and decreases with ne. While carbon production and densities in helium can be explained by the theoretical sputtering yields, the situation in deuterium is more complicated owing to the presence of oxygen. The low oxygen levels and the trend of falling carbon concentrations with increasing ne are the reason for the high density limits of helium plasmas. (orig.)