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In the vast majority of plasmas produced in the JET tokamak the X-ray emission at energies above about 1 keV is dominated by bremsstrahlung radiation from the background deuterium ions, and both bremsstrahlung and recombination radiation from one or two light impurity species such as beryllium and carbon. Under these circumstances, local values of impurity densities and concentrations, or alternatively of the effective charge Z(eff) of the ion mixture, are derived from tomographic measurements of the local X-ray emissivity at energies above 1-3 keV, in conjunction with measurements of electron density and temperature profiles. These calculations are performed using the intrinsic emissivities due to the impurity ions. The emissivities into the relevant energy bands, determined by the detector and filter characteristics, are calculated assuming coronal ionisation equilibrium, and tabulated as a function of electron temperature. This method offers a good spatial and temporal resolution, and is successfully applied to most plasma conditions where the relative proportions of impurities are known. A detailed sensitivity study assesses the effects of errors in the input data. Tomography is compared with established methods based on visible bremsstrahlung measurements, and charge exchange spectroscopy.

Additional information

Authors: PASINI D, JET Joint Undertaking, Abingdon, Oxon. (GB);EDWARDS A W, JET Joint Undertaking, Abingdon, Oxon. (GB);WEISEN H, Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne, Lausanne (CH);WELLER A, Max-Planck-Institut für Plasmaphysik, Garching bei München (DE)
Bibliographic Reference: Report: JET-P(90)36 EN (1990) 23 pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon OX14 3EA (GB)
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