• Energy Research

Pre-plasma mobilization of magnetic dust can be an important issue for future fusion reactors where plasma breakdown is critical. A combined on-line and off-line study of magnetic dust in ASDEX Upgrade is reported. Post-mortem collection revealed similar composition and morphology compared to other tokamaks, but the overall amount was much smaller. Optical and IR camera diagnostics excluded dust flybys prior to plasma start-up. The negative detection is discussed in light of the magnetic dust properties, the strength of mobilizing forces and the temporal evolution of the magnetic field.

Arcing is a mechanism of dust production and erosion in fusion devices. The masses and velocities of tungsten (W) particles produced by arcing is of special interest given that W is used as wall material in current experimental devices and is planned for use in new machines such as ITER. This work presents a new technique for evaluation of the parameters of particles produced by vacuum arcs in laboratory conditions based on direct observation with a high-speed video camera. Tracking of particle trajectory provides a measurement of velocity and angle of emission. Additionally, the emitted thermal radiation of the particles is measured and its evolution over time is compared with a model of its cooling in order to obtain a measurement of size and temperature at the moment of emission for each individual particle. Surprisingly, temperature measurements reveal the production of initially solid particles. The newly established video technique allows automated measurement of a high number of particles in order to obtain distributions of the particle parameters.

Analytical expressions for the divertor radiation around the onset of detachment based on the divertor neutral pressure and the upstream separatrix density are tested against measurements on strongly nitrogen seeded high confinement mode (H-mode) discharges in ASDEX Upgrade. The divertor pressure is used as a key experimental parameter as far as possible, since it can be regarded as engineering parameter based on the balance of puffing and pumping. For a tentative extrapolation to future tokamak power plants, limits set by the maximum core impurity concentration via the burn conditions are related to the divertor impurity content via the divertor enrichment factor. Combined with the H-mode density limit, which is most likely a limit for the separatrix density nesep/nGreenwald ≤ 0.5, estimates for the maximum separatrix power Psep compliant with the achievement of detachment are presented for a density related 2-point model and a flux related radiation model. The detachment, which is assumed necessary for the achievement of sufficiently long target lifetimes, requires in H-mode the dissipation of a major fraction of the power flowing to the divertor by impurity radiation. Favorable conditions for power exhaust feature a high Greenwald density / high divertor neutral pressure. The latter also facilitates a faster helium exhaust, which increases the margin for the seed impurity concentration in the core. Major uncertainties in the extrapolation of seeded scenarios regard the divertor impurity enrichment and the power width λ in the divertor for detached conditions. Keywords: 52.55.Fa Tokamaks 52.40.Hf plasma-materials interactions, Boundary layer effects

Arcs, a source of dust particles and a localized erosion mechanism of the plasma-facing components, are found in all major fusion plasma devices. Measurements of arcs require diagnostics with high temporal and local resolution, which are not available at arc dominated locations in ASDEX Upgrade (AUG). To understand the erosion by arcing and to allow extrapolation for future fusion devices different materials are used to scan the material properties. In AUG, inserts were installed at the inner baffle region to measure the erosion by arcing. The use of polished inserts allows an accurate determination of the arc traces by depth maps obtained by laser profilometery. It turned out that the melting temperature of the materials is the main parameter for erosion. For tungsten mounted at the inner baffle, a region which is deposition dominated, an erosion rate by arcing of 1.2·1013 at cm−2 s−1 is measured. For Beryllium, 9.5·1013 at cm−2 s−1 is extrapolated from its thermal properties. As martensitic–ferritic low-activation steel is under discussion for the use in DEMO, magnetic steels were also investigated. Comparing stainless steel with magnetic steel, much deeper and wider craters are found in the latter one: they reach a depth of −80 μm. The erosion of magnetic steel by arcs is 40 times higher compared to stainless steel, which has almost the same physical properties.

AbstractAt the inner baffle of the AUG divertor massive polished inserts of tungsten and P92 steel were installed to measure the erosion by arcing. For tungsten most of the traces are less than 0.4µm deep and a similar amount of tungsten is deposited close to the traces. Few craters up to 4µm resulting in an average erosion rate of 2×1013 at cm−2s−1 are observed. The behaviour for P92 steel is quite different: most of the traces are 4µm deep, up to 80µm were observed. An average erosion rate of 400×1013 at cm−2s−1, i.e. more than a factor of hundred higher compared to tungsten, is found. Therefore, erosion by arcing has to be taken into account to determine the optimal material mix for future fusion devices.

In 2013 a redesigned solid tungsten divertor, Div-III, was installed in ASDEX Upgrade. In two experimental campaigns more than 2700 plasma discharges with up to 10s duration, 110MJ plasma heating, and reaching a Psep/R of 10MW/m were conducted. Div-III is an adiabatically loaded divertor with tungsten target tiles clamped onto a water cooled structure. The tungsten target tiles were inspected and characterized during the shut-down period in 2014. As a result of the target inspection characteristic modifications of the plasma exposed tiles were detected that were not found during the target qualification in a high heat load test facility. 126 out of 128 tiles reveal deep cracks through the target. A lot of tiles show shallow cracks in the high heat load region and finally protruding tiles with strong local damages were found. It should be noted that none of these target damages have caused an unscheduled opening of ASDEX Upgrade. Keywords: Solid tungsten, Divertor, ASDEX Upgrade, Plasma energy

Tungsten (W) is the prime candidate for the plasma facing material in present and future fusion devices. When exposed to a plasma containing He ions, W can exhibit creation of sub-surface nano-bubbles leading to formation of nano-tendrils called fuzz. Formation of W fuzz was confirmed by many laboratory experiments including He loading in linear plasma devices. There is, however, limited experience related to the parameter space for fuzz formation and re-erosion in a tokamak environment. For this reason, a dedicated experiment was carried out in ASDEX Upgrade (AUG) during its 2022 He campaign to study the evolution of fuzz-like W structures. Nuclear materials and energy 37, 101539 - (2023). doi:10.1016/j.nme.2023.101539 Published by Elsevier, Amsterdam [u.a.]