Temperature and impurity transport studies of heated tokamak plasmas by means of a collisional-radiative model of x-ray emission from Mo30+ to Mo39+

This work presents and interprets, by means of detailed atomic calculations , observations of L-shell (n = 3 --> n = 2) transitions in highly ionized m olybdenum, the main intrinsic heavy impurity in the Frascati tokamak upgrad e plasmas. These hot plasmas were obtained by additional electron cyclotron resonance heating (ECRH), at the frequency of 140 Ghz, during the current ramp-up phase of the discharge. Injecting 400 kW on axis and 800 kW slightl y off axis, the peak central electron temperature reached 8.0 and 7.0 keV, respectively, for a time much longer than the ionization equilibrium time o f the molybdenum ions. X-ray emissions from rarely observed high charge sta tes, Mo30+ to Mo39+ have been studied with moderate spectral resolution (la mbda/Delta lambda similar to 150) and a time resolution of 5 ms. A sophisti cated collisional-radiative model for the study of molybdenum ions in plasm as with electron temperature in the range 4-20 keV is presented. The sensit ivity of the x-ray emission to the temperature and to impurity transport pr ocesses is discussed. This model has been then used to investigate two diff erent plasma scenarios. In the first regime the ECRH heating occurs on axis during the current ramp up phase, when the magnetic shear is evolving from negative to zero up to the half radius. The spectrum is well reproduced wi th the molybdenum ions in coronal equilibrium and with a central impurity p eaking. In the second regime, at the beginning of the current flat top when magnetic sheer is monotonic and sawtoothing activity is appearing, the low est charge states (Mo33+ to Mo30+), populated off axis, are affected by ano malous transport and the total molybdenum profile is found to be flat up to the half radius. We conclude with the presentation of "synthetic spectra" computed for even higher temperature plasmas that are expected in future ex periments with higher ECRH power input.