Deeply nanostructured tungsten with an arborescent shape was found for the first time to be formed on tungsten-coated graphite by a high-flux helium plasma irradiation at surface temperatures of 1250 and 1600 K, an incident ion energy of 12 eV (well below the physical sputtering threshold) and a helium ion fluence of 3.5 × 10 27 m −2 .

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A comprehensive investigation has been performed of the static and dynamic behaviour of detached recombining plasmas in the linear divertor plasma simulator NAGDIS-II. For stationary plasma detachment, the transition from electron-ion recombination (EIR) to molecular activated recombination (MAR) has been observed by injecting hydrogen gas into high density helium plasmas. The particle loss rate due to MAR is found to be comparable to that of EIR. Experiments have also been performed by the injection of a plasma heat pulse produced by RF heating into the detached helium plasma to demonstrate the dynamic behaviour of volumetric plasma recombination. Negative spikes in the Balmer series line emission were observed and found to be similar to the so called negative ELM observed in tokamak divertors. Observed Balmer spectra were analysed in detail using the collisional-radiative model. A rapid increase of the ion flux to the target plate was observed associated with the re-ionization of the highly excited atoms generated by EIR.

The effects of a transient heat load on tungsten damaged by helium plasma irradiation have been investigated using a ruby laser with long pulse duration in the divertor simulator NAGDIS-II (Takamura et al 2002 Plasma Sources Sci. Technol. 11 A42). The pulse width of the ruby laser was ∼0.6 ms, which is close to that of the expected heat load accompanied by type-I edge localized modes (ELMs) in ITER operation. Helium holes/bubbles, which were formed in the surface region of powder metallurgy tungsten due to the exposure to the helium plasma, disappeared after the laser pulse irradiation to the tungsten surface with sufficient pulse energy. The results indicated that the transient heat loads similar to those expected by ELMs will mitigate damages such as bubbles and holes produced by helium irradiation. When a vacuum plasma sprayed tungsten coating on graphite was exposed to the helium plasma, the surface was covered with arborescent nanostructured tungsten containing many helium bubbles inside the structure. Melting traces were found on the surface after the laser pulses irradiated the surface even though the pulse energy was lower than that for melting bulk tungsten. A numerical temperature calculation of the sample suggested that the effective thermal conductivity near the surface dramatically decreased by several orders of magnitude due to the formation of nanostructured tungsten.

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A power-balance model, with radiation losses from impurities and neutrals, gives a unified description of the density limit (DL) of the stellarator, the L-mode tokamak, and the reversed field pinch (RFP). The model predicts a Sudo-like scaling for the stellarator, a Greenwald-like scaling, , for the RFP and the ohmic tokamak, a mixed scaling, , for the additionally heated L-mode tokamak. In a previous paper (Zanca et al 2017 Nucl. Fusion 57 056010) the model was compared with ohmic tokamak, RFP and stellarator experiments. Here, we address the issue of the DL dependence on heating power in the L-mode tokamak. Experimental data from high-density disrupted L-mode discharges performed at JET, as well as in other machines, are taken as a term of comparison. The model fits the observed maximum densities better than the pure Greenwald limit.

Characteristics of toroidal plasma rotation have been experimentally investigated using charge exchange recombination spectroscopy in the ASDEX Upgrade tokamak. Ion cyclotron resonance frequency (ICRF) heating is found to cause a reduction of the toroidal rotation velocity, V φ , driven by neutral beam injection (NBI) in the co-and counter-current directions. The reduction of plasma rotation is attributed to an increasing momentum diffusivity connected with the confinement degradation by the additional ICRF power flux, and not to an ICRF induced toroidal force related to radial non-ambipolar transport of resonant particles. Toroidal momentum transport is found to be anomalous in various plasma regimes including standard and improved H-modes and ioninternal transport barrier (ITB) plasmas. In the inner half of those plasmas, except for high density H-modes with on-axis NBI only, the momentum diffusivity, χ φ , is found to be similar to the ion and electron heat diffusivities, χ i and χ e . In the outer half region, χ φ becomes smaller than χ i , while χ φ is still comparable with χ e except for ITB plasmas. It is found that the normalized gradient length of the toroidal rotation velocity, R/L V φ is smaller than that of the ion temperature, R/L T i , in H-modes and ITB plasmas. The magnitude of R/L V φ in an ITB region exceeds that in H-modes, as seen for the T i profile. In H-modes, the T i profile is stiff (R/L T i ∼ 5), while the V φ profile is not stiff, with R/L V φ ranging from 0 to 7. The V φ profile tends to become flat at high densities with on-axis NBI only. Additional ICRF heating can lead to a small decrease in both R/L T i and R/L V φ , while it sometimes causes a flattening of the V φ profile in the inner region. It is shown that the neoclassical correction of V φ does not affect strongly the results obtained with the measured V φ .

The electron heat transport in low density H-mode plasmas heated by neutral beam injection (NBI) is investigated in ASDEX Upgrade using electron cyclotron heating (ECH) combining both steady-state and transient response analysis by modulating the ECH power. Under these conditions, more than 60% of the NBI power (>3 MW) is delivered to the ions, while approximately 20% (∼1 MW) is delivered to the electrons. In the confinement region, the electron-to-ion temperature ratio, T e /T i , varies between 0.5 and 0.7 in the NBIonly phase and between 0.8 and 1.0 when the ECH is also applied. Due to the low collisional coupling, the power in the electron channel is locally more than doubled by applying up to the available 2 MW of ECH, while the power in the ion channel is locally increased by less than 30%. A dependence on the density of the reaction of the plasma parameters to the ECH is observed. For plasmas with average density ne < 4.0 × 10 19 m −3 (defined as 'hot-ion' H-modes), when the ECH is applied, T e increases, the central T i drops and the density flattens. These effects disappear with increasing density and are not observed for ne > 4.5 × 10 19 m −3 (defined as 'regular' H-modes). Power balance analysis of both the hot-ion and regular H-modes points to a strong resilient behaviour of the T e profiles. In the hot-ion cases, the ECH heating induces a strong increase in transport in the ion channel. Power balance and transient response analysis of the regular H-modes are consistent with an inverse scale length transport model with a threshold in R/L T e = R • |∇T e |/T e , above which the electron heat transport is increased. Comparison with recent studies in pure EC heated L-modes points to a stronger resilience of T e in the NBI heated H-modes.