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

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

Charge dependence of neoclassical and turbulent transport of light impurities on MAST 150 150 Mathew
CCFE-PR(15)48

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

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 lowfield side of the plasma, therefore this paper focuses on light impurities where the impact of poloidal asymmetries on impurity transport is 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 m 2 /s and a strong inward convective velocity of » 40 m/s near the plasma edge, and a region of outward convective velocity at midradius. The measured impurity transport coefficients are consistent with neoclassical Banana-Plateau predictions within ½ · 0 . 4. Quasi-linear gyrokinetic predictions of the carbon and helium particle flux at two flux surfaces, ½ = 0 . 6 and ½ = 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.

Collection:
Journals
Journal:
Plasma Physics and Controlled Fusion
Publisher:
IOP
Published date:
07/01/2015