[en] The impact of resonant magnetic perturbations (RMPs) on the power required to access H-mode is examined experimentally on MAST. Applying RMP in n = 2, 3, 4 and 6 configurations delays the L-H transition at low applied fields and prevents the transition at high fields. The experiment was primarily performed at RMP fields sufficient to cause moderate increases in ELM frequency, f _m_i_t_i_g_a_t_e_d/ f _n_a_t_u_r_a_l ∼ 3. To obtain H-mode with RMPs at this field, an increase of injected beam power is required of at least 50% for n = 3 and n = 4 RMP and 100% for n = 6 RMP. In terms of power threshold, this corresponds to increases of at least 20% for n = 3 and n = 4 RMPs and 60% for n = 6 RMPs. This ‘RMP affected’ power threshold is found to increase with RMP magnitude above a certain minimum perturbed field, below which there is no impact on the power threshold. Extrapolations from these results indicate large increases in the L-H power threshold may be required for discharges requiring large mitigated ELM frequency. (paper)

[en] 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 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 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 $\mathrm{\rho <!--\; ??\; -->}⩽<!--\; ???\; -->0.4$. Quasi-linear gyrokinetic predictions of the carbon and helium particle flux at two flux surfaces, $\mathrm{\rho <!--\; ??\; -->}=0.6$ and $\mathrm{\rho <!--\; ??\; -->}=0.7$, 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. (paper)

[en] Total radiated line power coefficients for ions of medium to heavy weight elements, called $\mathcal{P}\mathcal{L}\mathcal{T}$ coefficients in the atomic data and analysis structure, have been improved by algorithmically optimising the selection of configuration sets that underpin the calculation to include the most important radiating transitions driven by both the ground and metastable configurations and to establish and limit the error of truncation. The optimised calculations typically differ from Pütterich by 20%–30% with truncation error $\lesssim <!--\; ???\; -->5\mathrm{\%<!--\; \%\; -->}$. Further appraisal of error due to atomic level bundling, atomic structure and collision strength calculation methods has been carried out. It is shown that bundling to configurations is accurate to $\lesssim <!--\; ???\; -->10\mathrm{\%<!--\; \%\; -->}$ for all ions except those with closed-shell ground configurations which give errors up to a factor 2–3. For near neutral, closed-shell ions, plane-wave Born collision strength calculations, which omit spin-change, give substantial error in comparison with distorted-wave calculations of $\mathcal{P}\mathcal{L}\mathcal{T}$. For highly charged ions, spin-system breakdown reduces the error in the $\mathcal{P}\mathcal{L}\mathcal{T}$ markedly, typically $\lesssim <!--\; ???\; -->10\mathrm{\%<!--\; \%\; -->}$. The error introduced by the atomic structure codes used here, autostructure and the Cowan code, is probably limited to $\lesssim <!--\; ???\; -->30\mathrm{\%<!--\; \%\; -->}$. (paper)

[en] The impurity concentration in the tokamak divertor plasma is a necessary input for predictive scaling of divertor detachment, however direct measurements from existing tokamaks in different divertor plasma conditions are limited. To address this, we have applied a recently developed spectroscopic N II line ratio technique for measuring the N concentration in the divertor to a range of H-mode and L-mode plasma from the ASDEX Upgrade and JET tokamaks, respectively. The results from both devices show that as the power crossing the separatrix, P_sep, is increased under otherwise similar core conditions (e.g. density), a higher N concentration is required to achieve the same detachment state. For example, the N concentrations at the start of detachment increase from approximate to 2% to approximate to 9% as P_sep, is increased from ≅2.5 MW to ≅7 MW. These results tentatively agree with scaling law predictions (e.g. Goldston et al.) motivating a further study examining the parameters which affect the N concentration required to reach detachment. Finally, the N concentrations from spectroscopy and the ratio of D and N gas valve fluxes agree within experimental uncertainty only when the vessel surfaces are fully-loaded with N. (authors)