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Overview of the JET results in support to ITER
No full textThe 2014–2016 JET results are reviewed in the light of their significance for optimising the ITER research plan for the active and non-active operation. More than 60 h of plasma operation with ITER first wall materials successfully took place since its installation in 2011. New multi-machine scaling of the type I-ELM divertor energy flux density to ITER is supported by first principle modelling. ITER relevant disruption experiments and first principle modelling are reported with a set of three disruption mitigation valves mimicking the ITER setup. Insights of the L–H power threshold in Deuterium and Hydrogen are given, stressing the importance of the magnetic configurations and the recent measurements of fine-scale structures in the edge radial electric. Dimensionless scans of the core and pedestal confinement provide new information to elucidate the importance of the first wall material on the fusion performance. H-mode plasmas at ITER triangularity (H = 1 at β N ~ 1.8 and n/n GW ~ 0.6) have been sustained at 2 MA during 5 s. The ITER neutronics codes have been validated on high performance experiments. Prospects for the coming D–T campaign and 14 MeV neutron calibration strategy are reviewed
Impact of divertor geometry on H-mode confinement in the JET metallic wall
No full textRecent experiments with the ITER-like wall have demonstrated that changes in divertor strike point position are correlated with strong modification of the global energy confinement. The impact on energy confinement is observable both on the pedestal confinement and core normalised gradients. The corner configuration shows an increased core density gradient length and ion pressure indicating a better ion confinement. The study of neutral re-circulation indicates the neutral pressure in the main chamber varies inversely with the energy confinement and a correlation between the pedestal total pressure and the neutral pressure in the main chamber can be established. It does not appear that charge exchange losses nor momentum losses could explain this effect, but it may be that changes in edge electric potential are playing a role at the plasma edge. This study emphasizes the importance of the scrape-off layer (SOL) conditions on the pedestal and core confinement.</p
ELM-induced arcing on tungsten fuzz in the COMPASS divertor region
No full textMaterials exposed to plasma may undergo various forms of surface modifications. Among the important phenomena for tungsten - as the prime candidate plasma-facing material for fusion devices – is a formation of helium-induced fibreform nanostructure, so-called tungsten fuzz. In this paper, we report direct observations of the interaction of the pre-prepared fuzzy tungsten surfaces with ELMy H-mode plasmas in the COMPASS tokamak as well as consequent ex-situ morphological analyses, with a particular focus on arcing as a potential erosion mechanism. Arcing events are documented from high-speed camera imaging. The sample surfaces are examined by scanning electron microscopy. Arc traces were observed on all samples, while their number was dependent on the sample position and orientation. Inside the arc traces, localized melting and densification of the original fuzz was observed, resulting in thickness reduction. The modified structure still retained some porosity and did not extend into the bulk
The 2017 Plasma Roadmap: Low temperature plasma science and technology
No full textJournal of Physics D: Applied Physics published the first Plasma Roadmap in 2012 consisting of the individual perspectives of 16 leading experts in the various sub-fields of low temperature plasma science and technology. The 2017 Plasma Roadmap is the first update of a planned series of periodic updates of the Plasma Roadmap. The continuously growing interdisciplinary nature of the low temperature plasma field and its equally broad range of applications are making it increasingly difficult to identify major challenges that encompass all of the many sub-fields and applications. This intellectual diversity is ultimately a strength of the field. The current state of the art for the 19 sub-fields addressed in this roadmap demonstrates the enviable track record of the low temperature plasma field in the development of plasmas as an enabling technology for a vast range of technologies that underpin our modern society. At the same time, the many important scientific and technological challenges shared in this roadmap show that the path forward is not only scientifically rich but has the potential to make wide and far reaching contributions to many societal challenges.</p
Insight into CO2 Dissociation in Plasmas from Numerical Solution of a Vibrational Diffusion Equation
Get PDFThe dissociation of CO2 molecules in plasmas is a subject of enormous importance for fundamental studies and in view of the recent interest in carbon capture and carbon-neutral fuels. The vibrational excitation of the CO2 molecule plays an important role in the process. The complexity of the present state-to-state (STS) models makes it difficult to find out the key parameters. In this paper we propose as an alternative a numerical method based on the diffusion formalism developed in the past for analytical studies. The non-linear Fokker-Planck equation is solved by the time-dependent diffusion Monte Carlo method. Transport quantities are calculated from STS rate coefficients. The asymmetric stretching mode of CO2 is used as a test case. We show that the method reproduces the STS results or a Treanor distribution depending on the choice of the boundary conditions. A positive drift, whose energy onset is determined by the vibrational to translational temperature ratio, brings molecules from mid-energy range to dissociation. Vibrational-translational energy transfers have negligible effect at the gas temperature considered in this study. The possibility of describing the dissociation kinetics as a transport process provides insight towards the goal of achieving efficient CO2 conversion.</p
Secondary electron emission of tin and tin-lithium under low energy helium plasma exposure
No full textSecondary electron emission (SEE) yields of tin (Sn) and tin-lithium (SnLi) eutectic (20 at.% Li) samples were measured in He-plasma at a mean incoming electron energy up to 120 eV. SnLi shows a maximum yield of about 1.45 at 110 eV electron energy while the yield of the Sn surface was measured to be maximally 1.05 at 120 eV. X-ray photoelectron spectroscopy (XPS) analysis demonstrated the segregation effect of Li to the surface of the eutectic, both after melting in the argon atmosphere and in molten state with simultaneous He-plasma exposure. At least the top 10 nm of the SnLi samples were heavily enriched with Li, and Sn/Li ratios varied in the range 0.8–5% depending on eutectic treatment conditions. After the plasma exposure Sn3d is detected predominantly in the oxidized state while after extended atmospheric oxidation there was still a significant amount of Sn3d detected in the metallic state. The liquid surface of SnLi indicated a possible decrease of SEE yield. All measurements gave values of SEE yield close to or above unity. Such values can lead to significant plasma sheath disturbances and subsequent additional heat flux from electrons on such a plasma-facing material, thus, should be accounted for in designing fusion reactors using these components
The influence of the Ar/O2 ratio on the electron density and electron temperature in microwave discharges
No full textThe aim of this work is to study the properties of Ar-O-2 microwave driven surfatron plasmas as a function of the Ar/O-2 ratio in the gas mixture. The key parameters are the plasma electron density and electron temperature, which are estimated with Thomson scattering (TS) for O-2 contents up to 50% of the total gas flow. A sharp drop in the electron density from 10(20) m(-3) to approximately 10(18) m(-3) is estimated as the O-2 content in the gas mixture is increased up to 15%. For percentages of O-2 lower than 10%, the electron temperature is estimated to be about 2-3 times higher than in the case of a pure argon discharge in the same conditions (T-e approximate to 1 eV) and gradually decreases as the O-2 percentage is raised to 50%. However, for O-2 percentages above 30%, the scattering spectra become Raman dominated, resulting in large uncertainties in the estimated electron densities and temperatures. The influence of photo-detached electrons from negative ions caused by the typical TS laser fluences is also likely to contribute to the uncertainty in the measured electron densities for high O-2 percentages. Moreover, the detection limit of the system is reached for percentages of O-2 higher than 25%. Additionally, both the electron density and temperature of microwave discharges with large Ar/O-2 ratios are more sensitive to gas pressure variations
On the synergistic effect of inorganic/inorganic barrier layers: An ellipsometric porosimetry investigation
No full textIn this paper, plasma-enhanced chemical vapor deposited SiO2 layers capped by an ultra-thin plasma-assisted atomic layer deposited Al2O3 over-layer are analyzed by means of ellipsometric porosimetry (EP). In a very recent contribution, we have shown that the combination of the two layers provided excellent intrinsic moisture permeation barrier performance down to the 10−5–10−6 g · day−1 · m−2 regime. The present paper therefore addresses the microstructural changes which the SiO2 layers undergo upon Al2O3 deposition, as monitored by ellipsometric porosimetry (EP). It was found that the Al2O3 deposition primarily affects the relative content of open pores with d > 0.3 nm (water as probe) and d > 0.42 nm (ethanol as probe) from 5.35 to 2.81% and from 2.50 to 0.32%, respectively
Profile control simulations and experiments on TCV: a controller test environment and results using a model-based predictive controller
No full textThe successful performance of a model predictive profile controller is demonstrated in simulations and experiments on the TCV tokamak, employing a profile controller test environment. Stable high-performance tokamak operation in hybrid and advanced plasma scenarios requires control over the safety factor profile (q-profile) and kinetic plasma parameters such as the plasma beta. This demands to establish reliable profile control routines in presently operational tokamaks.
We present a model predictive profile controller that controls the q-profile and plasma beta using power requests to two clusters of gyrotrons and the plasma current request. The performance of the controller is analyzed in both simulation and TCV L-mode discharges where successful tracking of the estimated inverse q-profile as well as plasma beta is demonstrated under uncertain plasma conditions and the presence of disturbances. The controller exploits the knowledge of the time-varying actuator limits in the actuator input calculation itself such that fast transitions between targets are achieved without overshoot.
A software environment is employed to prepare and test this and three other profile controllers in parallel in simulations and experiments on TCV. This set of tools includes the rapid plasma transport simulator RAPTOR and various algorithms to reconstruct the plasma equilibrium and plasma profiles by merging the available measurements with model-based predictions. In this work the estimated q-profile is merely based on RAPTOR model predictions due to the absence of internal current density measurements in TCV. These results encourage to further exploit model predictive profile control in experiments on TCV and other (future) tokamaks
Experimental validation of the analytical model for tungsten dust - wall mechanical impacts incorporated in the MIGRAINe dust dynamics code
No full textMechanical dust-wall collisions are unavoidable in fusion devices and their accurate modeling is essential for the understanding of dust transport. The MIGRAINe dust dynamics code features analytical models addressing all facets of dust-surface impacts, some aspects of which have not been experimentally validated thus far. Dedicated dust injection experiments have been carried out in Pilot-PSI resulting to the visualization of in-plasma tungsten dust-surface impacts with an unprecedented resolution. They allowed for a calibration of key quantities of the MIGRAINe impact model