JET carbon screening experiments were performed using methane gas injection. L-Mode experiments scanned parameters influencing the JET scrape-off-layer (SOL) and/or intrinsic impurity level. Scaling relations are derived to describe methane injected into L-Mode plasmas from the JET horizontal mid-plane. L-Mode screening was 3–20 times better for plasmas connected to the divertor than for similar limited plasmas. The screening was worse for methane injection from the mid-plane and best for injection from the divertor. The screening was 1.5–2 times worse for H-Mode than L-Mode. Both ELM-averaged and inter-ELM H-Mode screening was documented. The screening results were used to understand the intrinsic impurity levels. Zeff reduced at higher densities partly due to better carbon screening at the higher density, and partly due to decreased carbon influxes. Diverted L-Mode intrinsic carbon levels arose from both main chamber and divertor sources, while H-mode carbon primarily originated from the divertor. DIVIMP and EDGE2D were used to model the observed screening. The modelling indicated that carbon removal to the divertor required lower temperatures for Coulomb collisions to couple the impurity ions to the SOL deuterium flows. The carbon removal occurred primarily in the outer SOL regions.

Recent experiments at ASDEX Upgrade have achieved advanced scenarios with high β N (>3) and confinement enhancement over ITER98(y, 2) scaling, H H98y2 = 1.1-1.5, in steady state. These discharges have been obtained in a modified divertor configuration for ASDEX Upgrade, allowing operation at higher triangularity, and with a changed neutral beam injection (NBI) system, for a more tangential, off-axis beam deposition. The figure of merit, β N H ITER89-P , reaches up to 7.5 for several seconds in plasmas approaching stationary conditions. These advanced tokamak discharges have low magnetic shear in the centre, with q on-axis near 1, and edge safety factor, q 95 in the range 3.3-4.5. This q-profile is sustained by the bootstrap current, NBI-driven current and fishbone activity in the core. The off-axis heating leads to a strong peaking of the density profile and impurity accumulation in the core. This can be avoided by adding some central heating from ion cyclotron resonance heating or electron cyclotron resonance heating, since the temperature profiles are stiff in this advanced scenario (no internal transport barrier). Using a combination of NBI and gas fuelling line, average densities up to 80-90% of the Greenwald density are achieved, maintaining good confinement. The best integrated results in terms of confinement, stability and ability to operate at high density are obtained in highly shaped configurations, near double null, with δ = 0.43. At the highest densities, a strong reduction of the edge localized mode activity similar to type II activity is observed, providing a steady power load on the divertor, in the range of 6 MW m −2 , despite the high input power used (>10 MW).

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Edge profiles of electron temperature and density are measured in ASDEX Upgrade with a high spatial resolution of 2–3 mm with Thomson scattering. In the region of the edge transport barrier in ELMy H-mode, the gradient lengths of Te and ne are found closely coupled, with the temperature decay length two times shorter than the density decay length corresponding to ηe ≈ 2. The ηe constraint allows us to calculate the electron temperature and density profiles from the pressure profile if the density and temperature values are known at one spatial position. The edge density in the region of the barrier foot is closely coupled to the main chamber recycling, with no strong dependence on other parameters. In contrast, the density rise from the outer barrier foot to the pedestal exhibits pronounced dependence on plasma current and shaping, indicating quite different mechanisms determining the absolute density and its gradient.