Papers by Isabel Alessandra Miranda Nunes
Physics of Plasmas, 2013
To consolidate ITER design choices and prepare for its operation, JET has implemented ITER's plas... more To consolidate ITER design choices and prepare for its operation, JET has implemented ITER's plasma facing materials, namely Be at the main wall and W in the divertor. In addition, protection systems, diagnostics and the vertical stability control were upgraded and the heating capability of the neutral beams was increased to over 30 MW. First results confirm the expected benefits and the limitations of all metal plasma facing components (PFCs), but also yield understanding of operational issues directly relating to ITER. H-retention is lower by at least a factor of 10 in all operational scenarios compared to that with C PFCs. The lower C content (Section 1factor 10) have led to much lower radiation during the plasma burn-through phase eliminating breakdown failures. Similarly, the intrinsic radiation observed during disruptions is very low, leading to high power loads and to a slow current quench. Massive gas injection using a D 2 /Ar mixture restores levels of radiation and vessel forces similar to those of mitigated disruptions with the C wall. Dedicated L-H transition experiments indicate a reduced power threshold by 30%, a distinct minimum density and pronounced shape dependence. The L-mode density limit was found up to 30% higher than for C allowing stable detached divertor operation over a larger density range. Stable H-modes as well as the hybrid scenario could be only re-established when using gas puff levels of a few 10 21 es −1. On average the confinement is lower with the new PFCs, but nevertheless, H factors up to 1 (H-Mode) and 1.3 (at b N ≈ 3, hybrids) have been achieved withWconcentrationswell below themaximum acceptable level.
Nuclear Fusion, 2012
This paper compares the confinement in JET low and high triangularity hybrid and baseline ELMy H-... more This paper compares the confinement in JET low and high triangularity hybrid and baseline ELMy H-mode regimes in a large database of 112 plasmas and identifies the location and source for the enhanced confinement factors in the hybrid plasmas. JET hybrid plasmas feature enhanced performance, and typically have b N~2 .5-3, H 98 < 1.5 compared to baseline H-mode plasmas, which typically have b N~1 .5-2, H 98 ~1. The higher global pressure achieved in hybrid plasmas is obtained through a tight coupling of the core and pedestal pressures. The confinement in hybrid plasmas is therefore strongly linked to the obtainable pedestal confinement. A comparison of the EPED1 predictive pedestal model with the experimental pedestal pressures shows good agreement of the model predictions with the measured data, offering a possible explanation for the tight link between global and pedestal pressure through the stabilising effect of high b on the pedestal MHD stability. The electron kinetic profiles do not show a net profile peaking of the electron pressure, although the electron density and temperature show respectively decreasing and increasing trends with collisionality. Hence the confinement benefit from the electron profile peaking as reported in previous studies [most recently in M. Maslov et al., Nucl. Fusion 49] is not confirmed by the current study. Both low and high triangularity hybrid plasmas feature T i /T e > 1 across the entire profile leading to a confinement benefit through the ions. Additional performance is obtained in the low triangularity hybrid plasmas through increased ion temperature profile peaking at low collisionality.
Nuclear Fusion, 2013
The recent installation of a full metal, ITER-like, first wall provided the opportunity to study ... more The recent installation of a full metal, ITER-like, first wall provided the opportunity to study the impact of the plasma-facing materials on plasma initiation or breakdown. This study for the first time presents a full experimental characterisation of tokamak breakdown at JET, using all discharges since 2008, covering both operations with a main chamber carbon and a beryllium ITER-like main chamber wall. It was found that the avalanche phase was unaffected by the change in wall material. However, changes in out-gassing by the wall and lower carbon levels resulted in better controlled density and significantly lower radiation during the burn-through phase with the ITER-like wall. Breakdown failures, that usually developed with a carbon wall during the burn-through phase (especially after disruptions) were absent with the ITER-like wall. These observations match with the results obtained from a new model of plasma burn-through that includes plasma-surface interactions. The simulations show that chemical sputtering of carbon is the determining factor for the impurity content, and hence also radiation, during the burn-through phase for operations with a carbon wall. As seen experimentally, with a beryllium main wall, the plasma surface effects predicted by the model do not raise the radiation levels much above those expected for pure deuterium plasmas. With the ITER-like wall, operation with higher prefill pressures, and thus higher breakdown densities, was possible, which helped maintaining the density after breakdown.
Fusion Engineering and Design, 2013
The aim of the JET ITER-like Wall Project was to provide JET with the plasma facing material comb... more The aim of the JET ITER-like Wall Project was to provide JET with the plasma facing material combination now selected for the DT phase of ITER (bulk beryllium main chamber limiters and a full tungsten divertor) and, in conjunction with the upgraded neutral beam heating system, to achieve ITER relevant conditions. The design of the bulk Be plasma facing components had to be compatible with increased heating power and pulse length, as well as to reuse the existing tile supports originally designed to cope with disruption loads from carbon based tiles and be installed by remote handling. Risk reduction measures (prototypes, jigs, etc) were implemented to maximize efficiency during the shutdown. However, a large number of clashes with existing components not fully captured by the configuration model occurred. Restarting the plasma on the ITER-like Wall proved much easier than for the carbon wall and no deconditioning by disruptions was observed. Disruptions have been more threatening than expected due to the reduced radiative losses compared to carbon, leaving most of the plasma magnetic energy to be conducted to the wall and requiring routine disruption mitigation. The main chamber power handling has achieved and possibly exceeded the design targets.
The Scientific World Journal, 2019
Objective. To quantifyEnterococcus faecalisdensity in root canal dentin after chemomechanical pre... more Objective. To quantifyEnterococcus faecalisdensity in root canal dentin after chemomechanical preparation (CMP) using alternated irrigating regimen.Methodology. Root canals (RC) were contaminated withE. faecalis(ATCC 19433) for 3 weeks and evident biofilms were obtained. After initial sampling (S1), the CMP was aided by irrigants: saline solution (control; n=12), a conventional regimen (CR) (group 1; n=12) using 5.25% NaOCl and a final rinse with 17% EDTA, and an alternating regimen (AR) of intercalated use of NaOCl and EDTA (group 2, n=12), followed by a second sampling (S2). After 2 weeks, S3 was obtained. Two roots were analyzed by scanning electron microscopy. Each root was divided into cervical, mild, and apical segments and sampling of the superficial (n=90) and deep (n=90) dentin layers was obtained using Gates-Glidden burs. TheE. faecalisdensity (CFU/mg) in log10was categorized as residual (0 > 0.2), moderate (0.2 ≥ 0.5), or elevated (> 0.5). The prevalence of positive...
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Papers by Isabel Alessandra Miranda Nunes