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 Z_eff 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 Z_eff profile with no MHD activity is fitted with a strong inward convection, characterized by a peaking parameter c_v(=-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 (c_v=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.