Determination of metal impurity density,Delta Z(eff) and dilution on JET by VUV emission spectroscopy

The provision of measurements of metallic impurity densities, Z(eff) and dilution for a large number of discharges within a campaign facilitates the analysis of impurity trends. Such trends are of increasing importance as additional heating power and pulse length increase. This is particularly important for RF heating and therefore it is in particular relevant to the assessment of the ITER-like ICRF antenna (ILA) on JET. To this end, a method is presented for determining the metal impurity density, Delta Z(eff) and dilution in steady-state JET plasmas using passive VUV emission. The method is based on the combination of absolutely calibrated VUV transition intensity measurements with Universal Transport Code (UTC) simulations. In the analysis the line-integrated measurements of transitions in Li-like Ni, Fe and Cu have been used for test discharges characterized by widely varied plasma profiles. The simulations use a wide class of transport coefficients for diffusion D(r) and convection V (r). For a given pair of D(r) and V (r), the simulated line intensity has been matched to the line intensity measured in the experiment. An approximately linear dependence of the derived metal densities, Delta Z(eff) and dilution normalized to a Li-like line intensity on electron temperature has been obtained which is valid in a localized, mid-radius plasma region. These linear dependences are exploited to derive local metal densities for JET discharges.