Demixing of impurities and hydrogen as deduced from Zeff profiles in the boronized ASDEX

Substantial progress towards fusion has been made in the confinement, stability and heating of tokamak plasmas. The transport behaviour of magnetically confined plasmas, however, is still an unsolved problem. The transport mechanisms of hydrogen and of impurities are known to be different, leading to phenomena such as impurity accumulation on axis, especially in good confinement regimes, and more generally to demixing of the various species. Besides energy losses from impurity radiation, one has to be concerned about dilution of the fuel-ion density, and about effects that impurities may have on the main ion and electron transport. To understand the transport behaviour of the different plasma species, one needs their spatial density profiles. The evolution of the electron density profiles is well known, the impurity profiles may be inferred from spectroscopic measurements, but usually the proton density profiles are less well documented. From reliable Zeff measurements it is possible to infer the proton density profile using a typical species mix derived from spectroscopic data and assuming quasi-neutrality. It is convenient to represent the various impurities by a characteristic impurity ion with a density nz and a fictive charge Z(Z=ΣniZ2i/ΣniZi;i≥2). A typical value for Z is 7, indicating that light impurities are dominating. Comparing the different profiles, we find characteristic differences in the electron, proton and impurity transport behaviour. (author) 2 refs., 6 figs., 1 tab