Experimental estimation of tungsten impurity sputtering due to Type I ELMs in JET-ITER-like wall using pedestal electron cyclotron emission and target Langmuir probe measurements

The ITER baseline scenario, with 500 MW of DT fusion power and Q = 10, will rely on a Type I ELMy H-mode and will be achieved with a tungsten (W) divertor. W atoms sputtered from divertor targets during mitigated ELMs are expected to be the dominant source in ITER. W impurity concentration in the plasma core can dramatically degrade its performance and lead to potentially damaging disruptions. Understanding the physics of the target W source due to sputtering during ELMs and inter-ELMs is important and can be helped by experimental measurements with improved precision. It has been established that the ELMy target ion impact energy has a simple linear dependence with the pedestal electron temperature measured by Electron Cyclotron Emission (ECE). It has also been shown that Langmuir Probes (LP) ion flux measurements are reliable during ELMs due to the surprisingly low electron temperature. Therefore, in this paper, LP and ECE measurements in JET-ITER-Like-Wall (ILW) unseeded Type I ELMy H-mode experiments have been used to estimate the W sputtering flux from divertor targets in ELM and inter-ELM conditions. Comparison with similar estimates using W I spectroscopy measurements shows a reasonable agreement for the ELM and inter-ELM W source. The main advantage of the method involving LP measurements is the very high time resolution of the diagnostic (∼10 μs) allowing very precise description of the W sputtering source during ELMs.EURATOM 633053MSMT INGO LG14002Fundação para a Ciência e Tecnologia UID/FIS/50010/201

Adhesion force of W dust on tokamak W plasma-facing surfaces: The importance of the impact velocity

The adhesion of tungsten (W) dust particles on W tokamak plasma-facing surfaces is investigated. When spherical dust hits the tokamak walls, its shape is altered as a contact area is created by the adhesion work and irreversible plastic deformation. The van der Waals adhesion force between such a deformed grain and a flat surface is estimated, and the link between the force and the impact velocity is determined. We show that adhesion can be increased by several orders of magnitude when the impact velocity approaches the adhesion velocity. Finally, the effects of surface roughness are incorporated

Electron collection and thermionic emission from a spherical dust grain in the space-charge limited regime

International audienc

Dust remobilization from rough planar surfaces in tokamak steady-state plasmas

The ability of tungsten (W) dust to be remobilized from rough tokamak plasma-facing surfaces is investigated in this study. Atomic Force Microscopy is used to evaluate the adhesion force distribution between W spheroids of ∼1−10 µm radius and a rough W substrate. Our results confirm that the Rabinovich model describes reasonably well the mean adhesion force in tokamak-relevant surface roughness regimes. The external (electric and ion drag) forces are estimated as well using a simplified sheath model. The results reveal that micron-size dust can be resuspended by attainable electric fields in tokamak conditions

Magnetized electron emission from a small spherical dust grain in fusion related plasmas

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Simulation of W dust transport in the KSTAR tokamak, comparison with fast camera data

In this paper, dust transport in tokamak plasmas is studied through both experimental and modeling aspects. Image processing routines allowing dust tracking on CCD camera videos are presented. The DUMPRO (DUst Movie PROcessing) code features a dust detection method and a trajectory reconstruction algorithm. In addition, a dust transport code named DUMBO (DUst Migration in a plasma BOundary) is briefly described. It has been developed at CEA in order to simulate dust grains transport in tokamaks and to evaluate the contribution of dust to the impurity inventory of the plasma. Like other dust transport codes, DUMBO integrates the Orbital Motion Limited (OML) approach for dust/plasma interactions modeling. OML gives direct expressions for plasma ions and electrons currents, forces and heat fluxes on a dust grain. The equation of motion is solved, giving access to the dust trajectory. An attempt of model validation is made through comparison of simulated and measured trajectories on the 2015 KSTAR dust injection experiment, where W dust grains were successfully injected in the plasma using a gun-type injector. The trajectories of the injected particles, estimated using the DUMPRO routines applied on videos from the fast CCD camera in KSTAR, show two distinct general dust behaviors, due to different dust sizes. Simulations were made with DUMBO to match the measurements. Plasma parameters were estimated using different diagnostics during the dust injection experiment plasma discharge. The experimental trajectories show longer lifetimes than the simulated ones. This can be due to the substitution of a boiling/sublimation point to the usual vaporization/sublimation cooling, OML limitations (eventual potential barriers in the vicinity of a dust grain are neglected) and/or to the lack of a vapor shielding model in DUMBO

Ion target impact energy during Type I edge localized modes in JET ITER-like Wall

Équipe 107 : Physique des plasmas chaudsInternational audienceThe ITER baseline scenario, with 500 MW of DT fusion power and Q = 10, will rely on a Type I ELMy H-mode, with Delta W = 0.7 MJ mitigated edge localized modes (ELMs). Tungsten (W) is the material now decided for the divertor plasma-facing components from the start of plasma operations. W atoms sputtered from divertor targets during ELMs are expected to be the dominant source under the partially detached divertor conditions required for safe ITER operation. W impurity concentration in the plasma core can dramatically degrade its performance and lead to potentially damaging disruptions. Understanding the physics of plasma-wall interaction during ELMs is important and a primary input for this is the energy of incoming ions during an ELM event. In this paper, coupled Infrared thermography and Langmuir Probe (LP) measurements in JET-ITER-Like-Wall unseeded H-mode experiments with ITER relevant ELM energy drop have been used to estimate the impact energy of deuterium ions (D+) on the divertor target. This analysis gives an ion energy of several keV during ELMs, which makes D+ responsible for most of the W sputtering in unseeded H-mode discharges. These LP measurements were possible because of the low electron temperature (T-e) during ELMs which allowed saturation of the ion current. Although at first sight surprising, the observation of low T-e at the divertor target during ELMs is consistent with the `Free-Streaming' kinetic model which predicts a near-complete transfer of parallel energy from electrons to ions in order to maintain quasi-neutrality of the ELM filaments while they are transported to the divertor targets