Zeff Profile Research Articles

Impurity behaviour in PBX L- and H-mode plasmas

Intrinsic impurity behaviour and transport properties in neutral beam heated L- and H-mode PBX tokamak plasmas were studied with a variety of impurity diagnostics. Central impurity accumulation was most often observed in H-mode discharges; sometimes it resulted in a thermal collapse due to high central metallic radiation (∼1.5 W • cm−3). The impurity accumulation was evident from peaked Zeff and radiated power profiles and was further substantiated by specific vacuum ultraviolet and X-ray spectroscopy measurements. It is shown that impurity accumulation was neither unique nor inevitable in H-mode discharges and it could be suppressed by sufficient gas puffing. Central impurity accumulation was also seen in L-mode plasmas even with co-injected neutral beams. This usually occurred at high beam power and relatively low density. While there was no significant difference in the degree of accumulation between L-mode and H-mode discharges, the Zeff profile itself was more peaked in the H-mode because the electron density profiles are flatter in H-mode plasmas than in L-mode plasmas. The degree of accumulation increased as Zeff(0) itself increased, which suggests that neoclassical convection and diffusion driven by both impurity-impurity and impurity-plasma ion collisions contribute to the central plasma particle transport.

Measurements and implications of Zeff profiles on the DIII-D tokamak

Here Zeff profiles have been obtained across the entire minor radius of the DIII-D [Fusion Technol. 8, 441 (1985); Plasma Physics and Controlled Nuclear Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. 1, p. 159] tokamak from measurements of visible bremsstrahlung continuum radiation. The Zeff profiles are presented for Ohmic divertor and limiter plasmas as well as for neutral beam heated H-mode and L-mode plasmas. The measured Zeff profiles are found to have small gradients over the entire plasma cross section. This implies that resistivity gradient driven electrostatic modes are unlikely to influence the confinement properties in the edge region of these discharges. The H-mode plasma has a centrally peaked Zeff profile and a relatively flat electron density profile that suggests the existence of a negative value for the parameter ηi.

Some effects of MHD activity on impurity transport in the PBX Tokamak

The effects of MHD activity on intrinsic impurity transport are studied in ohmic discharges of the Princeton Beta Experiment (PBX) by measuring the Zeff profile from visible bremsstrahlung radiation and spectral line intensities from ultraviolet spectroscopy. A diffusive/convective transport model, including an internal disruption model, is used to simulate the data. The Zeff profile with no MHD activity is fitted with a strong inward convection, characterized by a peaking parameter cv(=-a2 nu /2rD)=11 (-3.5, +4.5). At the onset of MHD activity (a large m=1 n=1 oscillation is followed by sawteeth), this strongly peaked profile is flattened and subsequently reaches a new quasi-equilibrium shape. This new profile is characterized by reduced convection (cv=3.6 (-1.1, +1.6), D=1.4 (-0.7, +5.6)*104 cm2 s-1) in addition to the particle redistribution which accompanies the sawtooth internal disruptions, indicating that the transport processes themselves change with the onset of sawtooth activity.