1,204 research outputs found
Bifurcation of the radial electric field in the presence of edge polarization in Tokamaks
No full textIn guiding-center simulations of a tokamak plasma edge, bifurcations of both the radial electric field and the electrode current are observed for a threshold value of the electrode bias voltage, in qualitative agreement with experiments. The simulations are neoclassical and Monte Carlo based, also containing the perpendicular viscosity effect. Above the voltage threshold, an inward moving, solitary radial electric field structure is generated with a concomitant drop in the electrode current. The position, width, and height of the soliton depend on the magnitude and radial variation of the conductivity (and, thus, on local plasma parameters), finite orbit effects, and also on the perpendicular viscosity and neutral density. The obtained Er structures are narrower and higher than the ones measured in the edge polarization experiments in the Jülich tokamak TEXTOR-94 [S. Jachmich et al., Plasma Phys. Controlled Fusion 40, 1105 (1998)]
Plasma edge density and lower hybrid current drive in JET (Joint European Torus)
No full textExternally launched lower hybrid (LH) waves do not propagate into the plasma core during operations of JET with radial profiles with relatively high density even at the periphery, approaching the condition necessary for ITER. Modelling results indicate that this problem would be caused by parametric instability (PI)induced LH spectral broadening, which is expected to occur in the plasma edge and prevents the coupled LH power penetrating the plasma core. However, operation with relatively high electron temperature at the edge is expected to diminish the PI effect and extend the LH current drive effectiveness to reactor-grade high density plasmas, consistent with results obtained in other experiments
The isotope effect on the detachment density in JET-ILW Ohmically confined plasmas
No full textThe isotope effect on the detachment density in JET-ILW Ohmically confinedplasmasV Solokha1, M Groth1, S Brezinsek2, M Brix3, G Corrigan3, C Guillemaut4, D Harting3,S Jachmich5, U Kruezi6, S Marsen7, S Wiesen2 and JET contributors∗1 Aalto University, P.O. Box 14100, FI-00076, Aalto, Espoo, Finland2 Forschungszentrum Julich GmbH, Institut fur Energie- und Klimaforschung, IEK-4 - Plasmaphysik, 52425 Julich, Germany3 EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK4 Instituto de Plasmas e Fusao Nuclear, Instituto Superior Tecnico, Lisbon, Portugal5 Ecole Royale Militaire School, Av de la Renaissance 30, Brussels, Belgium6 ITER Organization Route de Vinon sur Verdon, Saint Paul-lez-Durance, France7 Max-Planck-Institute for Plasma Physics, Greifswald, Germany∗ See the author list of E. Joffrin et al. accepted for publication in Nuclear Fusion Special Issue 2019,https://doi.org/10.1088/1741-4326/ab2276Studies in JET with the ITER-like wall (JET-ILW) Ohmic plasmas show that the density range, in which thepartial detachment at both divertor targets is accessible, is affected by the hydrogen isotope species as well asby the divertor geometry. These studies are consistent with previous research showing that the detachmentonset density for different hydrogen species in JET with the carbon wall (JET-C) scales inversely with the ionmass for vertical target (VT) configuration L-mode discharges [1].The isotope effect on the detachment density was reassessed for hydrogen and deuterium plasmas in thevertical and horizontal target (HT) configurations. The density of the detachment onset in the HT configurationfor the deuterium discharges is 10% lower than for the hydrogen at the outer target (OT), while in the VTconfiguration the isotope effect is stronger (30%) and affects detachment onset density at both targets.Simulations with the EDGE2D-EIRENE code [2,3] showed that the deuterium cases have a 50% higher neutraldensity than the hydrogen cases due to the subdivertor entrance conductance dependence on the thermalvelocity of the molecules. The increased neutral densities in the deuterium cases cause two times higherradiation losses in the simulations, while in the experiments bolometry measurements showed only 30% higherradiation losses. On the other hand, the Balmer-alpha emission profiles across the outer divertor plate aremeasured similar in shape and magnitude. Due to the pumping plenum, which reflects 80% of the incidentparticles, and subdivertor geometry of JET, the probability of the recycled particles from the inner target (IT) toreach the cryopump via the IT subdivertor entrance is close to zero. The poloidal ExB drift direction affects themagnitude of the isotope effect by moving particles from the OT to the IT in the normal BT direction (weakerisotope effect) or towards the OT and the cryopump in the reversed BT configuration (stronger isotope effect). Inthe HT configuration, the molecules at the IT can not be pumped, whereas in the VT configuration moleculesfrom both targets could be pumped due to connection to the OT subdivertor entrance by the private flux region.The higher detachment onset densities for heavier isotope species caused by the higher molecular densities.The dependence of molecular transport on the strike-point (SP) position due to the curvature of the target platesis predicted to cause the higher detachment onset densities in cases with higher strike-point positions. Theusage of the low SP position decreases the detachment onset density and increases pumping efficiency, whichleads to better particle control as well as to a bigger influence of the subdivertor entrance conductance andisotope species on the divertor plasma.[1] C.F. Maggi et al, Nucl. Fusion 39 979 (1999).[2] R. Simonini et al, Contrib. Plasma Physics, 34 (1994), 368-373.[3] S. Wiesen, JET ITC-Report, (2006), http://www.eirene.de/e2deir report 30jun06.pdfCorresponding author: tel.: +358(40)257-44-98, e-mail: [email protected] (V. Solokah
First mirror test in JET for ITER: Complete overview after three ILW campaigns
No full textThe First Mirror Test for ITER has been carried out in JET with mirrors exposed during: (i) the third ILW campaign (ILW-3, 2015-2016, 23.33 h plasma) and (ii) all three campaigns, i.e. ILW-1 to ILW-3: 2011-2016, 63,52 h in total. All mirrors from main chamber wall show no significant changes of the total reflectivity from the initial value and the diffuse reflectivity does not exceed 3% in the spectral range above 500 nm. The modified layer on surface has very small amount of impurities such as D, Be, C, N, O and Ni. All mirrors from the divertor (inner, outer, base under the bulk W tile) lost reflectivity by 20-80% due to the beryllium-rich deposition also containing D, C, N, O, Ni and W. In the inner divertor N reaches 5 x 10(17) cm(-2), W is up to 4.3 x 10(17) cm(-2), while the content of Ni is the greatest in the outer divertor: 3.8 x 10(17) cm(-2). Oxygen-18 used as the tracer in experiments at the end of ILW-3 has been detected at the level of 1.1 x 10(16) cm(-2). The thickness of deposited layer is in the range of 90 nm to 900 nm. The layer growth rate in the base (2.7 pm s(-1)) and inner divertor is proportional to the exposure time when a single campaign and all three are compared. In a few cases, on mirrors located at the cassette mouth, flaking of deposits and erosion occurred
Plasma impact on diagnostic mirrors in JET
No full textMetallic mirrors will be essential components of all optical systems for plasma diagnosis in ITER. This contribution provides a comprehensive account on plasma impact on diagnostic mirrors in JET with the ITER-Like Wall. Specimens from the First Mirror Test and the lithium-beam diagnostic have been studied by spectrophotometry, ion beam analysis and electron microscopy. Test mirrors made of molybdenum were retrieved from the main chamber and the divertor after exposure to the 2013–2014 experimental campaign. In the main chamber, only mirrors located at the entrance of the carrier lost reflectivity (Be deposition), while those located deeper in the carrier were only slightly affected. The performance of mirrors in the JET divertor was strongly degraded by deposition of beryllium, tungsten and other species. Mirrors from the lithium-beam diagnostic have been studied for the first time. Gold coatings were severely damaged by intense arcing. As a consequence, material mixing of the gold layer with the stainless steel substrate occurred. Total reflectivity dropped from over 90% to less than 60%, i.e. to the level typical for stainless steel. Keywords: JET, First mirror test, Diagnostic mirrors, Erosion-depositio
Single Langmuir probe characteristic in a magnetized plasma at the text tokamak
No full textA single Langmuir probe tip was used at TEXT-Upgrade to obtain I-V characteristics in a magnetized plasma. Noisy data were reduced by a boxcar-averaging routine. Unexpected effects, namely nonsaturation of ion current, hysterises in the characteristics and I(V)-data were observed, which are in disagreement to the common single probe model. A double probe model allows parameterization of the I(V) curves and to determine the plasma properties in the scrape-off layer. It is shown in this model that a Langmuir probe does perturb the local space potential in the plasma. Comparisons were made with the triple probe technique of measuring temperatures. The nonsaturation of ion current leads to an error in the triple probe technique of order 20%
Impact of global plasma parameters on transport suppression due to ExB shear
No full textPositive radial electric fields have been applied by means of electrode biasing to the plasma edge of TEXTOR-94 in order to study the transport and confinement changes with the generated ExB plasma flow. Discharges performed at various plasma parameters have shown a change in the transport reduction and improvement of confinement, which polarisation experiments undergo. Theory predicts that in a low B-t-discharge less ExB shear has to be applied to reach the same level of transport suppression than in a high B-t-discharge, whereas a decrease of the isotopic mass should impede the formation of transport barriers. Our data have confirmed such expectations. In this paper we will try to disclose the possible influences of global plasma parameters on the underlying mechanism of transport reduction in improved confinement
ELMs of types I and II observed by triple probes at the targets of the gasbox divertor of JET
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