| 1. |
- Grumer, Jon, et al.
(författare)
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Unexpected transitions induced by spin-dependent, hyperfine and external magnetic-field interactions
- 2014
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Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 89:11
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Tidskriftsartikel (refereegranskat)abstract
- Unexpected transitions are induced by weaker interactions not included in the gross structure model of the ion under investigation. We discuss different examples of such decay channels, starting with relativistic spin-induced transitions. These represented an important field of study a few decades ago, and we illustrate how some challenging cases can be treated very accurately with today's computational techniques, while close degeneracy sometimes still prevents ab initio methods from obtaining accurate results. For hyperfine induced transitions we review some recent results and discuss remaining challenges for experiment and theory. Finally, we discuss the newly opened field of accurate calculations for transitions induced by an external magnetic field and point to some examples of where these are accessible for experimental tests.
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| 2. |
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| 3. |
- Broglia, Ricardo, et al.
(författare)
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Pygmy resonances: what's in a name?
- 2019
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Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 94:11
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Tidskriftsartikel (refereegranskat)abstract
- The centroid, width and percentage of energy weighted sum rule of dipole resonances can be strongly affected by dynamical fluctuations and static deformations of the nuclear surface, deformations and fluctuations which, in turn, depend on pairing, and thus on Cooper pairs. Because of angular momentum conservation, such insight is restricted, to lowest order, to fluctuations/deformations of quadrupole and monopole type. The latter being closely connected with the neutron (excess) skin and thus with soft dipole modes. From the values (N − Z)/A ≈ 0.18, 0.21, and 0.45 for the nuclei 122Sn, 208Pb, and 11Li, it is expected that the latter system, which is weakly bound by pairing effects (spatially extended single Cooper pair and odd proton acting as spectator), constitutes an attractive laboratory to study the properties of soft E1-modes and thus of isospin nuclear deformation. From the calculation of the full dipole response function in QRPA, discretizing the continuum in a spherical box of radius of 40 fm, one finds a GDR with centroid E x ≈ 24 MeV, width Γ ≈11 MeV and carrying 90% of the EWSR, and a low-lying collective resonance characterized by E X = 0.75 MeV, Γ = 0.5 MeV and 6.2% EWSR. The wave function of the latter resonance is built out of about fifteen components (both protons and neutrons), typical of a collective mode. The transition densities indicate this soft E1-mode to be generated by surface density oscillation of the neutron (halo) skin (Δr np ≈ 1.71 fm) relative to an approximately isospin-saturated core. Through a detailed study of the full dipole response of 11Li we will draw a comparison between the soft E1-mode of this halo nucleus and the PDR of heavy stable nuclei, pointing to the physical similarities and also to the basic differences.
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| 4. |
- Ragnarsson, I.
(författare)
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How deformed are the TSD bands in odd Lu isotopes?
- 2017
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Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 92:12
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Tidskriftsartikel (refereegranskat)abstract
- The experimental fingerprints for large deformation in the triaxial strongly deformed (TSD) bands of 163,165,167Lu are discussed. It is argued that these fingerprints are not very convincing. On the contrary, especially the fact that there exist strong interactions between the TSD bands and normal-deformed (ND) bands indicates that the deformation of the TSD bands cannot be very different from that of the ND bands. The need for detailed new experimental data is underlined.
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| 5. |
- Thomasson, Anna, et al.
(författare)
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Science facilitets and stakeholder management : How a pan-European research facility ended up in a small Swedish University town
- 2017
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Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 92:6
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Tidskriftsartikel (refereegranskat)abstract
- This is the story of how a large research facility of broad European and global interest, the European Spallation Source (ESS), ended up in the small university town of Lund in Sweden. This happened in spite of the fact that a number of influential European countries were at one time or another competitors to host the facility. It is also a story about politics which attempts to illustrate how closely intertwined politics and science are, and how the interplay between those interests affects scientific progress. ESS became an arena for individual ambitions and political manoeuvring. The different stakeholders, in their striving to ensure that their own interests were realised, in various ways and with different degrees of success over the years, have influenced the key decisions that, during the already 30 year history of ESS, have driven the course that this project has taken. What emerges is that the interests of the stakeholders and the interests of the project itself are frequently not in harmony. This imposes challenges on the management of large research facilities as they have to not only navigate in the scientific landscape, which they often are more familiar with, but also in the political landscape. This story is therefore an attempt to shed light on the role of managers of large research facilities and the often delicate balancing act they have to perform when trying to comply with the different and often conflicting stakeholder interests. What is especially worthwhile examining, as we do in this paper, is the role that individuals, and the interaction between individuals, have played in the process. This shows that the focus of stakeholder theory on organisations, rather than the people in the organisations, needs to be redirected on to the individuals representing those organisations and their inter-relationships. At the same time it is clear that the developing field of stakeholder management theory has not emerged into the consciousness of science facility managers or their governing bodies and is far down the list of priorities of researchers who use the facilities.
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| 6. |
- Korn, Andreas, 1972-
(författare)
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NLTE line formation
- 2008
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Ingår i: Physica Scripta. - Bristol : IOP. - 0031-8949 .- 1402-4896. ; T133, s. 014009-
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Tidskriftsartikel (refereegranskat)abstract
- I review some of the present achievements and future challenges of non-local thermodynamicequilibrium (NLTE) line-formation calculations for solar-type stars. It is concluded that thefull potential of NLTE still remains to be tapped, in particular in view of the current transitionfrom one-dimensional (1D) hydrostatic to 3D hydrodynamic model atmospheres.
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| 7. |
- Borodkina, I., et al.
(författare)
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Intra-ELM tungsten sputtering in JET ITER-like wall : analytical studies of Be impurity and ELM type influence
- 2017
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Ingår i: Physica Scripta. - : IOP PUBLISHING LTD. - 0031-8949 .- 1402-4896. ; T170
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Tidskriftsartikel (refereegranskat)abstract
- The W source strength in JET H-mode discharges depends on the W sputtering in the inter and the intra-ELM phase due to impinging hydrogenic ions (D or H) and impurities (mainly Be). The analytical approach for interpretation of the Langmuir probe measurements is applied to model the ELM ion parallel transport and the W sputtering flux in intra-ELM and inter-ELM conditions in JET-ITER-like wall (ILW) hydrogen and deuterium plasmas. The impact of the Be ion charge and the Be concentration in the impinging ion flux on the W sputtering was estimated. Be2+ concentrations of 0.5% and 1% in the impinging ion flux increases the W sputtering fluence per ELM by 20%-30% and 35%-55% correspondingly with respect to pure deuterium plasma; the charge state of Be ions has no substantial effect on W sputtering in the intra-ELM phase. The analysis of JET ELMy H-mode discharges in hydrogen with different types of ELMs is presented. The W sputtering source under inter-and intra-ELM conditions is estimated using the analytical approach and validated by optical emission spectroscopy in these discharges. The intra-ELM W sputtering flux increases 2-4 times in comparison to the inter-ELM flux.
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| 8. |
- Catarino, N., et al.
(författare)
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Time-resolved deposition in the remote region of the JET-ILW divertor : measurements and modelling
- 2017
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Ingår i: Physica Scripta. - : IOP PUBLISHING LTD. - 0031-8949 .- 1402-4896. ; T170
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Tidskriftsartikel (refereegranskat)abstract
- One crucial requirement for the development of fusion power is to know where, and how much, impurities collect in the machine, and how much of the fuelling isotope tritium will be trapped therein. The most relevant information on this issue comes from the operation of the Joint European Tokamak (JET), which is the world's largest operating tokamak and has the same interior plasma-facing materials as the next step machine, ITER. Much of the information gained so far has been from post-mortem analysis of samples collected after whole campaigns involving varied types of operation. This paper describes time-resolved measurements of the deposition rate using rotating collectors (RC) placed in remote areas of the JET divertor during the 2013-2014 campaign with the ITER-like Wall (ILW). These techniques allow the effects of different types of operation to be distinguished. Rotating collectors made of silicon discs housed behind an aperture are exposed to the plasma. Each time the magnetic field coils are ramped up for a discharge the disc rotates, providing a linear relationship between the exposed region and the discharge number. Post-mortem ion beam analyses provide information on the deposit composition as a function of the discharge number. The results show that the Be deposition average for the RC in the corners of the inner and outer divertor are 4.9 x 10(16) cm(-2) and 1.8 x 10(17) cm(-2), respectively, accumulated over an average of similar to 25 pulses. Data from the rotating collector below Tile 5 in the central region of divertor indicate a Be deposition rate of 9.3 x 10(15) cm(-2), per similar to 25 pulses.
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| 9. |
- Coenen, J. W., et al.
(författare)
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Transient induced tungsten melting at the Joint European Torus (JET)
- 2017
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Ingår i: Physica Scripta. - : IOP PUBLISHING LTD. - 0031-8949 .- 1402-4896. ; T170
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Tidskriftsartikel (refereegranskat)abstract
- Melting is one of the major risks associated with tungsten (W) plasma-facing components (PFCs) in tokamaks like JET or ITER. These components are designed such that leading edges and hence excessive plasma heat loads deposited at near normal incidence are avoided. Due to the high stored energies in ITER discharges, shallow surface melting can occur under insufficiently mitigated plasma disruption and so-called edge localised modes-power load transients. A dedicated program was carried out at the JET to study the physics and consequences of W transient melting. Following initial exposures in 2013 (ILW-1) of a W-lamella with leading edge, new experiments have been performed on a sloped surface (15 degrees slope) during the 2015/2016 (ILW-3) campaign. This new experiment allows significantly improved infrared thermography measurements and thus resolved important issue of power loading in the context of the previous leading edge exposures. The new lamella was monitored by local diagnostics: spectroscopy, thermography and high-resolution photography in between discharges. No impact on the main plasma was observed despite a strong increase of the local W source consistent with evaporation. In contrast to the earlier exposure, no droplet emission was observed from the sloped surface. Topological modifications resulting from the melting are clearly visible between discharges on the photographic images. Melt damage can be clearly linked to the infrared measurements: the emissivity drops in zones where melting occurs. In comparison with the previous leading edge experiment, no runaway melt motion is observed, consistent with the hypothesis that the escape of thermionic electrons emitted from the melt zone is largely suppressed in this geometry, where the magnetic field intersects the surface at lower angles than in the case of perpendicular impact on a leading edge. Utilising both exposures allows us to further test the model of the forces driving melt motion that successfully reproduced the findings from the original leading edge exposure. Since the ILW-1 experiments, the exposed misaligned lamella has now been retrieved from the JET machine and post mortem analysis has been performed. No obvious mass loss is observed. Profilometry of the ILW-1 lamella shows the structure of the melt damage which is in line with the modell predictions thus allowing further model validation. Nuclear reaction analysis shows a tenfold reduction in surface deuterium concentration in the molten surface in comparison to the non-molten part of the lamella.
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| 10. |
- Drenik, A., et al.
(författare)
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Evaluation of the plasma hydrogen isotope content by residual gas analysis at JET and AUG
- 2017
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Ingår i: Physica Scripta. - : IOP PUBLISHING LTD. - 0031-8949 .- 1402-4896. ; T170
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Tidskriftsartikel (refereegranskat)abstract
- The isotope content of the plasma reflects on the dynamics of isotope changeover experiments, efficiency of wall conditioning and the performance of a fusion device in the active phase of operation. The assessment of the isotope ratio of hydrogen and methane molecules is used as a novel method of assessing the plasma isotope ratios at JET and ASDEX-Upgrade (AUG). The isotope ratios of both molecules in general shows similar trends as the isotope ratio detected by other diagnostics. At JET, the absolute values of RGA signals are in relatively good agreement with each other and with spectroscopy data, while at AUG the deviation from neutral particle analyser data are larger, and the results show a consistent spatial distribution of the isotope ratio. It is further shown that the isotope ratio of the hydrogen molecule can be used to study the degree of dissociation of the injected gas during changeover experiments.
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