189 research outputs found
Investigating the thermal stability of highly radiative discharges on JET with a new tomographic method
No full textThe next generation of Tokamak devices is expected to work at very high radiated fractions, well above 90%, to preserve the integrity of the plasma facing components in general and the divertor in particular. In addition to maintaining high confinement, these configurations will also have to guarantee a low disruptivity. An accurate determination of the emitted radiation will therefore become increasingly important, not only for the global power balances but also for specific regions of the plasma cross section (for example to properly control detachment). In this perspective, a new tomographic inversion method, based on the maximum likelihood (ML) approach, capable of providing routinely confidence intervals in the estimates of the radiated power, has been applied to the investigation of high radiative discharges on JET with the ITER like wall (ILW). The emission has been increased with injection of extrinsic impurities. Taking into account all the major sources of uncertainties, a systematic analysis of the configurations has shown that it has not been possible to develop stable configurations with radiated fraction higher than 70% of the input power. At higher radiated fractions the discharges always disrupt. Therefore significant work remains to be done to extend JET operation in a reactor relevant regime of sufficient radiation in preparation for ITER and DEMO
Designing a high resolution microcontroller-based electrostatic probe system for plasma characterization
No full textInternational audienceIn this work, an automated system capable of biasing electrostatic probes in cold plasmas and acquiring the associated data is implemented. The step-by-step design, fabrication, and fine calibration of the entire system are presented. High resolution and accuracy, increased acquisition rate and high noise rejection, are the main claims for the system presented hereby. The device efficiency is eventually demonstrated through measurements in the negative ion H- source "Prometheus I"
Automated electrostatic probe device of high resolution and accuracy
No full textInternational audienceIn this work, an automated apparatus for driving single electrostatic probes and acquiring the plasmarelateddata has been designed and fabricated. The voltage range of the present system is ±110 Vwith an adjustable voltage step as low as 3 mV. Voltage and current measurements are carried out withhigh resolution and high accuracy circuits, both based on 16 bit analog-to-digital converters. The codeembedded in a micro-controller, schedules the operation of the device and transfers the experimentaldata to a personal computer. The modular design of the system makes possible its modification andthus increases its adaptability to different plasma setups. Finally, the reliable operation of the entiredevice is confirmed by tests in Electron Cyclotron Resonance plasma. © 2014 AIP Publishing LLC
The evolution of particulates across the sooting limit in turbulent premixed opposed jet flames
No full textSoot formation in combustors is a complex process comprising highly intermittent interactions between physical and chemical processes across a wide range of time-scales. The influence of turbulence on the molecular pathways initiating particulate formation remains unquantified. Controlling soot emissions to the atmosphere will require overcoming large gaps in the understanding of soot formation/oxidation especially in turbulent combustion. The complexities of soot formation in turbulent flames suggests that the use of a flexible compact burner configuration with well–defined boundary conditions and precise control of flow characteristics is of significant advantage. The novel back–to–burnt opposed jet configuration features fractal grid generated turbulence and provides accurate control of flow parameters. The study includes the analyses of the overall flame structure of turbulent premixed ethylene/air flames, the relative concentrations of PAHs associated with soot inception and particle size distributions. The experiments covered a series of sooting flame conditions with variations in the equivalence ratio (1.7 ≤ \phi_{UN} ≤ 2.2), the total rate of strain (255 ≤ a_{T} [s−1] ≤ 610) and burnt gas temperature (1400 ≤ T_{LN} [K] ≤ 1700). The conditions traverse the soot inception limit, e.g. the transition from lightly to heavily sooting flames, with non- intrusive ELS and PAH–PLIF combined probe sampling to quantify gaseous and PAH species using GC–TCD and GC–MS, respectively. The probe sampling features comprehensive sampling steps used to provide accurate concentrations of major gaseous, PAH species and particles with minimum losses. It is shown that the rate of strain exerts a substantial influence on both PAH concentrations and soot formation. Hence, it is likely that soot formation in turbulent flames becomes dominated by contributions from low strain regions. It is also found that the stoichiometry of the mixture controls the concentrations of PAHs associated with soot inception. The results obtained clearly show that benzo(a)pyrene is prevalent in flame structures and that relatively large amounts are condensed onto soot particles. A transition between bimodal and unimodal shapes of the particle size distributions shows strong competitions between oxidation, aggregation and surface growth processes in the turbulent flames.Open Acces
Validation of SOLPS-ITER simulations with kinetic, fluid, and hybrid neutral models for JET-ILW low-confinement mode plasmas
No full textsponsorship: This work has been carried out within the framework of the EU-ROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 - EUROfusion) . Views and opinions expressed are however those of the author (s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them. W. Van Uytven is funded by a PhD fellowship of the Research Foundation-Flanders. The computational resources and services used in this work were provided by the VSC (Flemish Supercomputer Center) , funded by the Research Foundation Flanders (FWO) and the Flemish Government-department EWI. (European Union|101052200 - EUROfusion, Research Foundation-Flanders, Research Foundation Flanders (FWO), Flemish Government-department EWI)status: Publishe
Modulation of Integral Length Scales of Turbulence in an Optical SI Engine by Direct Injection of Gasoline, iso-Octane, Ethanol and Butanol Fuels
No full textIn-cylinder air flow structures are known to play a major role in mixture preparation and engine operating limits for spark-ignition engines. In this paper PIV measurements were undertaken in an optical spark-ignition at 1500 RPM, part-load with 0.5 bar intake plenum pressure. One of the PIV planes was vertical, cutting through the centrally located spark plug (tumble plane). The other plane was horizontal 1 mm below the spark plug ground electrode (swirl plane). The effect of engine head temperature was also examined by using engine-head coolant temperatures of 20 °C and 80 °C. The flow field was examined late in the compression stroke at typical ignition timing. The study was conducted under air-only motoring engine conditions but also under fuelled conditions in the early intake stroke using direct injection of gasoline, iso-octane, ethanol and butanol fuels. The flow field under air-only motored conditions showed velocities between 3 and 5 m/s predominantly from intake to exhaust. Little differences were observed between hot and cold engine-head temperature; typically ∼10% larger mean velocity and turbulent kinetic energy was seen on the intake side. Integral length scales were on the tumble plane between 2 and 4.5 mm in the vertical and 4–7 mm in the horizontal direction. The swirl view showed scales between 4 and 10 mm, larger at cold than at hot engine-head conditions. The vertical length scales appeared to be limited by the clearance height, scaling typically by about 10–15%. The horizontal components scaled to the cylinder bore diameter by about 5–12%. The fuel injection process in the early intake stroke led to little differences in the general mean flow structure at ignition timing, except for a small increase in the maximum velocities, ∼10%. The turbulent kinetic on the tumble plane was highest towards the exhaust side of the engine under non-fuelled engine conditions but fuel injection resulted in highest values on the intake side. The integral length scales with fuel injection were of the same order of magnitude to those of air only measurements on the tumble plane, but showed distinctly larger areas with length scales up to 9 mm on the swirl plane. Differences between fuels for their fuel specific injection duration were small, with average length scales between 4 and 6 mm. Ethanol exhibited typically largest scales and butanol smallest
Innovative dud detection based on JET DT experience
No full textDT operations at JET gave a unique and invaluable opportunity to design, develop and test real-time controllers that will be applied in future burning plasma devices, as ITER and SPARC. Among them, the dud detector [L. Piron et al. 2019 Fusion Eng. Design 146 1364] is of primary importance since it allows to optimize the limited neutron and tritium budget consumptions. Such kind of a detector has been finalized and routinely used during DT experiments performed in 2021. In this work, we are proposing an innovative dud detector based on the experience gained during DT operation, namely add a metrics which supervises the H mode behaviour and allows redundancy on the input signals to cope with possible diagnostic and/or signal failures
production
No full textInternational audienceOptimization of negative ion sources operated with deuterium may be limited by the lack ofdata on fundamental processes. Insight can be obtained from studies focusing on a directcomparison of H2 and D2 plasmas. Herein, the ECR volume negative ion source ‘Prometheus I’operates with both H2 and D2 gases and the properties of the generated plasmas are probed bymeans of electrostatic probe and laser induced photo-detachment. A parametric study, involvingpressure and microwave (2.45 GHz) power variation, reveals similar qualitative trends for mostof the plasma properties in both gases. However, quantitively, differences do exist for the plasmapotential, the electron density and temperature, and the negative ion to electron density ratio.Electron energy distribution functions are thus isotope dependent. Overall, nearly the samemaximal H− and D− negative ion densities are achieved (i.e. 4×109 cm−3). The results areeventually elucidated with respect to the ECR heating mechanism, and the production anddestruction paths of the negative ions
Beryllium erosion in pure D and nitrogen seeded limiter plasmas in JET
No full textBeryllium erosion in pure D and nitrogen seeded limiter plasmas in JETT. Dittmar, J. Romazanov, S. Aleiferisc, D. Borodin, S. Brezinsek, A. Drenika, A. Huber, E. Pawelecb, S. Silburnc, V. S. Neverovd, and the JET contributors*Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung – Plasmaphysik, Partner of the Trilateral Euregio Cluster (TEC), 52425 Jülich, Germanya Max-Planck-Institut für Plasmaphysik, 85748 Garching b. München, Germayb University of Opole, Institute of Physics, Oleska 48, Opole, Polandc UKAEA-EURATOM Association, Culham Science Centre, Abingdon, OX14 3DB, UKdNational Research Centre “Kurchatov Institute”, Moscow 123182, Russian [email protected] (Be) is the material used for the inner and outer limiters of the JET tokamak and will be the plasma-facing material of the first wall in the ITER fusion experiment. The erosion of Be is a main driver for fuel retention due to the build-up of hydrogen rich Be-codeposits and is responsible for a large fraction of the tungsten sputtering in the divertor by material migration of Be from the main chamber into the W divertor. Besides physical sputtering there is evidence that chemically assisted physical sputtering (CAPS) play an important role in Be erosion [1]. Contrary to physical sputtering CAPS relies additionally on chemical processes for the Be erosion, and therefore erosion yields are very sensitive regarding surface temperature and chemical composition. A series of similar JET limiter discharges (Bt = 2.4 T, Ip = 1.9 MA, ne,core ~ 7–8·1019 m-³, Te ~ 1.9 keV) in deuterium, comparable to studies in unseeded discharges in earlier experiment [2] was performed. Fast, continuous repetition of identical discharges ratcheted up the limiter base temperature and allowed to study the influence of the limiter surface temperatures (450...640 K) on the interaction of the plasma with the beryllium components. Compared with previous studies, the edge plasma temperature during the limiter heat-up phase was decreased from ~15 eV to less than 5 eV, thus, the impact energy of the projectiles dropped accordingly. Additionally, plasma density variations were performed at elevated limiter temperature and reduction of CAPS. Besides VIS and UV spectroscopic observation at the midplane contact point, additional narrowband filtered camera images of Be, BeD and Be II allow a direct comparison of emission patterns with ERO 2.0 modeling [3]. Similar discharges were also carried-out with nitrogen and neon seeding in order to study suppression of CAPS by nitrogen species. In this contribution, we will give an overview of the available experimental data on CAPS and discuss the influence of surface temperature and ion energy. Moreover, we show that the nitrogen gives raise to an cumulative, surface temperature independent reduction of BeI, BeII and molecular BeD measured intensities and discuss that this reduction is likely due to a suppression of chemical erosion of the beryllium. [1] C. Björkas, et al. Plasma Phys. Control. Fusion 55 (2013) 074004[1] S. Brezinsek, et al., Nuc. Fus. 54 (2014) 103001[3] D. Borodin, et al., this conference* See the author list of E. Joffrin et al. published in Nuclear Fusion Special issue 2019, https://doi.org/10.1088/1741-4326/ab227
Particle Size Distributions in Turbulent Premixed Ethylene Flames Crossing the Soot Inception Limit
No full textThe current study presents novel experimental data on soot particle size distributions (PSDs) with mobility diameters in the size range 4 ≤ Dm [nm] ≤ 230 obtained from four premixed turbulent ethyleneair flames crossing the soot inception limit. The flames are stabilised against hot combustion products
from nitrogen diluted hydrogen flames in a back-to-burnt (or fresh-to-burnt) opposed jet configuration.
The burner features fractal grid generated turbulence to provide accurate control of the turbulence. A
scanning mobility particle sizer (SMPS) equipped with nano- and long–DMA columns coupled with a
dual dilution port quartz probe is used and a comprehensive analysis of optimal sampling conditions
to minimise particle losses is presented. Spatially resolved data along the stagnation point streamline
is obtained to show the evolution of PSDs through the turbulent flame brush. It is shown that turbulent transport distributes soot particles across the mixing layer between the two jets with the maximum
median and mean mobility diameters found close to the stagnation point. The impact of the estimated
total mean rate of strain (420 ≤ aT [s−1] ≤ 610) and equivalence ratio (1.8 ≤ φUN ≤ 2.2) on PSDs is also
quantified
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