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Optical diagnostic systems of ITER are foreseen to include metallic, plasma-facing, electromagnetic radiation reflecting components called first mirrors (FMs). Molybdenum coatings are important candidates for these components. Depending on the local plasma parameters of the reactor, the mirrors may be under net erosion or deposition conditions. In this work, we exposed molybdenum coatings to a high-flux deuterium plasma in order to test their roughening limits under erosion conditions. The high energy of deuterium ions (500 eV on average) results in more vigorous roughening of the surface compared with lower energy ions (200 eV). Longer exposure (3 × 1020 ions cm−2) of the 200 eV ions results in only a slightly increased roughness compared with shorter exposure (6.8 × 1019 ions cm−2). Both phenomena match to the theory regarding roughening dynamics of physical sputtering. A comparison of results in this work with previous studies gives support to the hypothesis that roughening is flux and temperature dependent. Partial delamination of the coatings is observed upon exposure at room temperature, but not at an elevated temperature (200 °C). In summary, Mo coatings will remain functional in the ITER environment under the expected conditions. However, changes in the expected conditions such as 500 eV mean energy of impinging charge exchange neutrals or

Metallic first mirrors (FMs) are foreseen to play a crucial role for all optical diagnostics in ITER. It is highly important for the FMs to maintain a good reflectivity both in erosion and deposition zones in the harsh ITER environment. Molybdenum mirrors, which are important candidates for the FMs, exhibit a reflectivity spectrum different from that of bulk molybdenum after exposure to low temperature (4–5 eV) deuterium plasma. This difference is mainly due to the presence of deuterium and deuterium-induced defects in the metal. The results presented show that these reflectivity changes are similar for single and nanocrystalline molybdenum mirrors. Moreover, exposure of magnetron sputtered nanocrystalline molybdenum films to deuterium plasma revealed that after a certain deviation of the spectrum has been reached, the reflectivity remains constant upon further exposure. Exposures were carried out in a range of fluences corresponding to up to 18 ITER discharges in equatorial ports and 38 discharges in the upper ports in the first wall positions. Constant conditions of −200 V bias and 150 °C temperature were maintained on the samples. Further exposures performed in a tokamak result in reflectivity changes that are comparable to those obtained with deuterium plasma exposure. No mechanical damage, such as blistering and increase in roughness, is observed on the coated molybdenum films upon any of the mentioned exposures. The complex permittivity of the exposed molybdenum is determined from ellipsometry measurements and corroborated with core and valence level photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy and surface resistivity measurements.

Several improvements to the MAST plant and diagnostics have facilitated new studies advancing the physics basis for ITER and DEMO, as well as for future spherical tokamaks (STs). Using the increased heating capabilities P NBI ⩽ 3.8 MW H-mode at I p = 1.2 MA was accessed showing that the energy confinement on MAST scales more weakly with I p and more strongly with B t than in the ITER IPB98(y, 2) scaling. Measurements of the fuel retention of shallow pellets extrapolate to an ITER particle throughput of 70% of its original designed total throughput capacity. The anomalous momentum diffusion, χϕ, is linked to the ion diffusion, χi, with a Prandtl number close to P ϕ ≈ χϕ/χi ≈ 1, although χi approaches neoclassical values. New high spatial resolution measurements of the edge radial electric field, E r , show that the position of steepest gradients in electron pressure and E r (i.e. shearing rate) are coincident, but their magnitudes are not linked. The T e pedestal width on MAST scales with rather than ρpol. The edge localized mode (ELM) frequency for type-IV ELMs, new in MAST, was almost doubled using n = 2 resonant magnetic perturbations from a set of four external coils (n = 1, 2). A new internal 12 coil set (n ⩽ 3) has been commissioned. The filaments in the inter-ELM and L-mode phase are different from ELM filaments, and the characteristics in L-mode agree well with turbulence calculations. A variety of fast particle driven instabilities were studied from 10 kHz saturated fishbone like activity up to 3.8 MHz compressional Alfvén eigenmodes. Fast particle instabilities also affect the off-axis NBI current drive, leading to fast ion diffusion of the order of 0.5 m2 s−1 and a reduction in the driven current fraction from 40% to 30%. EBW current drive start-up is demonstrated for the first time in a ST generating plasma currents up to 55 kA. Many of these studies contributed to the physics basis of a planned upgrade to MAST.