Charge dependence of neoclassical and turbulent transport of light impurities on MAST

S S Henderson, L Garzotti, F J Casson, D Dickinson, M O'Mullane, A Patel, C M Roach, H P Summers, H Tanabe, M Valovic

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Carbon and nitrogen impurity transport coefficients are determined from gas puff experiments carried out during repeat L-mode discharges on the Mega-Amp Spherical Tokamak (MAST) and compared against a previous analysis of helium impurity transport on MAST. The impurity density profiles are measured on the low-field side of the plasma, therefore this paper focuses on light impurities where the impact of poloidal asymmetries on impurity transportis predicted to be negligible. A weak screening of carbon and nitrogen is found in the plasma core, whereas the helium density profile is peaked over the entire plasma radius. Both carbon and nitrogen experience a diffusivity of the order of 10 m2s-1 and a strong inward convective velocity of ~40 m s-1 near the plasma edge, and a region of outward convective velocity at mid-radius. The measured impurity transport coefficients are consistent with neoclassical Banana-Plateau predictions within ρ≤0.4. Quasi-linear gyrokinetic predictions of the carbon and helium particle flux at two flux surfaces, ρ= 0.6 and ρ= 0.7, suggest that trapped electron modes are responsible for the anomalous impurity transport observed in the outer regions of the plasma. The model, combining neoclassical transport with quasi-linear turbulence, is shown to provide reasonable estimates of the impurity transport coefficients and the impurity charge dependence.
Original languageEnglish
Article number095001
Number of pages10
JournalPlasma Physics and Controlled Fusion
Issue number9
Early online date30 Jul 2015
Publication statusPublished - Sep 2015


  • MAST
  • impurity transport
  • charge dependence


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