| 1. |
- Fenstermacher, M.E., et al.
(författare)
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DIII-D research advancing the physics basis for optimizing the tokamak approach to fusion energy
- 2022
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 62:4
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Tidskriftsartikel (refereegranskat)abstract
- DIII-D physics research addresses critical challenges for the operation of ITER and the next generation of fusion energy devices. This is done through a focus on innovations to provide solutions for high performance long pulse operation, coupled with fundamental plasma physics understanding and model validation, to drive scenario development by integrating high performance core and boundary plasmas. Substantial increases in off-axis current drive efficiency from an innovative top launch system for EC power, and in pressure broadening for Alfven eigenmode control from a co-/counter-I p steerable off-axis neutral beam, all improve the prospects for optimization of future long pulse/steady state high performance tokamak operation. Fundamental studies into the modes that drive the evolution of the pedestal pressure profile and electron vs ion heat flux validate predictive models of pedestal recovery after ELMs. Understanding the physics mechanisms of ELM control and density pumpout by 3D magnetic perturbation fields leads to confident predictions for ITER and future devices. Validated modeling of high-Z shattered pellet injection for disruption mitigation, runaway electron dissipation, and techniques for disruption prediction and avoidance including machine learning, give confidence in handling disruptivity for future devices. For the non-nuclear phase of ITER, two actuators are identified to lower the L-H threshold power in hydrogen plasmas. With this physics understanding and suite of capabilities, a high poloidal beta optimized-core scenario with an internal transport barrier that projects nearly to Q = 10 in ITER at ∼8 MA was coupled to a detached divertor, and a near super H-mode optimized-pedestal scenario with co-I p beam injection was coupled to a radiative divertor. The hybrid core scenario was achieved directly, without the need for anomalous current diffusion, using off-axis current drive actuators. Also, a controller to assess proximity to stability limits and regulate β N in the ITER baseline scenario, based on plasma response to probing 3D fields, was demonstrated. Finally, innovative tokamak operation using a negative triangularity shape showed many attractive features for future pilot plant operation.
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| 2. |
- Pfeffer, W. Tad, et al.
(författare)
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The Randolph Glacier Inventory : a globally complete inventory of glaciers
- 2014
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Ingår i: Journal of Glaciology. - 0022-1430 .- 1727-5652. ; 60:221, s. 537-552
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Tidskriftsartikel (refereegranskat)abstract
- The Randolph Glacier Inventory (RGI) is a globally complete collection of digital outlines of glaciers, excluding the ice sheets, developed to meet the needs of the Fifth Assessment of the Intergovernmental Panel on Climate Change for estimates of past and future mass balance. The RGI was created with limited resources in a short period. Priority was given to completeness of coverage, but a limited, uniform set of attributes is attached to each of the similar to 198 000 glaciers in its latest version, 3.2. Satellite imagery from 1999-2010 provided most of the outlines. Their total extent is estimated as 726 800 +/- 34 000 km(2). The uncertainty, about +/- 5%, is derived from careful single-glacier and basin-scale uncertainty estimates and comparisons with inventories that were not sources for the RGI. The main contributors to uncertainty are probably misinterpretation of seasonal snow cover and debris cover. These errors appear not to be normally distributed, and quantifying them reliably is an unsolved problem. Combined with digital elevation models, the RGI glacier outlines yield hypsometries that can be combined with atmospheric data or model outputs for analysis of the impacts of climatic change on glaciers. The RGI has already proved its value in the generation of significantly improved aggregate estimates of glacier mass changes and total volume, and thus actual and potential contributions to sea-level rise.
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| 3. |
- Glasser, N. F., et al.
(författare)
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Global sea-level contribution from the Patagonian Icefields since the Little Ice Age maximum
- 2011
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Ingår i: Nature Geoscience. - 1752-0894 .- 1752-0908. ; 4:5, s. 303-307
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Tidskriftsartikel (refereegranskat)abstract
- The melting of mountain glaciers and ice caps is expected to contribute significantly to sea-level rise in the twenty-first century(1-)3, although the magnitude of this contribution is not fully constrained. Glaciers in the Patagonian Icefields of South America are thought to have contributed about 10% of the total sea-level rise attributable to mountain glaciers in the past 50 years(3). However, it is unclear whether recent rates of glacier recession in Patagonia are unusual relative to the past few centuries. Here we reconstruct the recession of these glaciers using remote sensing and field determinations of trimline and terminal moraine location. We estimate that the North Patagonian Icefield has lost 103 +/- 20.7 km(3) of ice since its late Holocene peak extent in AD 1870 and that the South Patagonian Icefield has lost 503 +/- 101.1 km(3) since its peak in AD 1650. This equates to a sea-level contribution of 0.0018 +/- 0.0004 mm yr(-1) since 1870 from the north and 0.0034 +/- 0.0007 mm yr(-1) since 1650 from the south. The centennial rates of sea-level contribution we derive are one order of magnitude lower than estimates of melting over the past 50 years(3), even when we account for possible thinning above the trimline. We conclude that the melt rate and sea-level contribution of the Patagonian Icefields increased markedly in the twentieth century.
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| 4. |
- Andersen, J. L., et al.
(författare)
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Ice surface changes during recent glacial cycles along the Jutulstraumen and Penck Trough ice streams in western Dronning Maud Land, East Antarctica
- 2020
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 249
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Tidskriftsartikel (refereegranskat)abstract
- Reconstructing past ice-sheet surface changes is key to testing and improving ice-sheet models. Data constraining the past behaviour of the East Antarctic Ice Sheet are sparse, limiting our understanding of its response to past, present and future climate change. Here, we report the first cosmogenic multinuclide (Be-10, Al-26, Cl-36) data from bedrock and erratics on nunataks along the Jutulstraumen and Penck Trough ice streams in western Dronning Maud Land, East Antarctica. Spanning elevations between 741 and 2394 m above sea level, the samples have apparent exposure ages between 2 ka and 5 Ma. The highest-elevation bedrock sample indicates (near-) continuous minimum exposure since the Pliocene, with a low apparent erosion rate of 0.15 +/- 0.03 m Ma(-1), which is similar to results from eastern Dronning Maud Land. In contrast to studies in eastern Dronning Maud Land, however, our data show clear indications of a thicker-than-present ice sheet within the last glacial cycle, with a thinning of similar to 35-120 m during the Holocene (similar to 2-11 ka). Difficulties in separating suitable amounts of quartz from the often quartz-poor rock-types in the area, and cosmogenic nuclides inherited from exposure prior to the last deglaciation, prevented robust thinning estimates from elevational profiles. Nevertheless, the results clearly demonstrate ice-surface fluctuations of several hundred meters between the current grounding line and the edge of the polar plateau for the last glacial cycle, a constraint that should be considered in future ice-sheet model simulations.
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| 5. |
- Glasser, Neil F., et al.
(författare)
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Cosmogenic nuclide exposure ages for moraines in the Lago San Martin Valley, Argentina
- 2011
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Ingår i: Quaternary Research. - : Cambridge University Press (CUP). - 0033-5894 .- 1096-0287. ; 75:3, s. 636-646
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Tidskriftsartikel (refereegranskat)abstract
- At several times during the Quaternary, a major eastward-flowing outlet glacier of the former Patagonian Ice Sheet occupied the Lago San Martin Valley in Argentina (49 degrees S, 72 degrees W). We present a glacial chronology for the valley based on geomorphological mapping and cosmogenic nuclide ((10)Be) exposure ages (n = 10) of boulders on moraines and lake shorelines. There are five prominent moraine belts in the Lago San Martin Valley, associated with extensive sandar (glaciofluvial outwash plains) and former lake shorelines. Cosmogenic nuclide exposure ages for boulders on these moraines indicate that they formed at 14.3 +/- 1.7 ka, 22.4 +/- 2.3 ka, 34.4 +/- 3.4 ka to 37.6 +/- 3.4 ka (and possibly 60 +/- 3.5 ka), and 99 +/- 11 ka (1 sigma). These dated glacier advances differ from published chronologies from the Lago San Martin Valley based on (14)C age determinations from organic sediments and molluscs in meltwater channels directly in front of moraines or in kettleholes within end moraine ridges. The moraine boulder ages also point to possible pre-LGM glacial advances during the last glacial cycle and a key observation from our data is that the LGM glaciers were probably less extensive in the Lago San Martin Valley than previously thought.
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| 6. |
- Glasser, Neil F., et al.
(författare)
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Glacial lake drainage in Patagonia (13-8 kyr) and response of the adjacent Pacific Ocean
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- Large freshwater lakes formed in North America and Europe during deglaciation following the Last Glacial Maximum. Rapid drainage of these lakes into the Oceans resulted in abrupt perturbations in climate, including the Younger Dryas and 8.2 kyr cooling events. In the mid-latitudes of the Southern Hemisphere major glacial lakes also formed and drained during deglaciation but little is known about the magnitude, organization and timing of these drainage events and their effect on regional climate. We use 16 new single-grain optically stimulated luminescence (OSL) dates to define three stages of rapid glacial lake drainage in the Lago General Carrera/Lago Buenos Aires and Lago Cohrane/Pueyrredon basins of Patagonia and provide the first assessment of the effects of lake drainage on the Pacific Ocean. Lake drainage occurred between 13 and 8 kyr ago and was initially gradual eastward into the Atlantic, then subsequently reorganized westward into the Pacific as new drainage routes opened up during Patagonian Ice Sheet deglaciation. Coupled ocean-atmosphere model experiments using HadCM3 with an imposed freshwater surface hosing to simulate glacial lake drainage suggest that a negative salinity anomaly was advected south around Cape Horn, resulting in brief but significant impacts on coastal ocean vertical mixing and regional climate.
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| 7. |
- Glasser, Neil F., et al.
(författare)
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Younger Dryas and early Holocene age glacier advances in Patagonia
- 2012
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 58, s. 7-17
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Tidskriftsartikel (refereegranskat)abstract
- Reliable dating of Southern Hemisphere glacier fluctuations since the Last Glacial Maximum (LGM) is crucial to resolving debates about millennial-scale climate change. Here we present Be-10 dates for lateral, valley-mouth and cross-valley moraines formed between the contemporary South American North Patagonian Icefield (NPI) and its LGM position in four separate valleys around 47 degrees S. This is an area near the core of the precipitation-bearing southern westerly winds, where it is known that rapid shifts in climate occurred during Lateglacial times. The dates indicate that outlet glaciers advanced, or at least stabilised, to form large moraines east of an expanded NPI at 11.0 +/- 0.5/11.2 +/- 0.6, 11.5 +/- 0.6, 11.7 +/- 0.6 and 12.8 +/- 0.7 ka (Putnam southern-hemisphere production rates and Dunai scaling scheme, assumed boulder erosion rate of 2 mm/ka). Four of these ages are statistically indistinguishable and probably represent a single, regional ice advance. The dates indicate that glaciers in Patagonia were larger during these times than at any point since the LGM and provide evidence in Patagonia for glacier advances around the time of the European Younger Dryas (12.9-11.7 ka) and into the very early Holocene. Although palaeoclimatic records from this area are often contradictory, these glacier advances were probably associated with a period of cooling or regionally increased precipitation related to the changes in the position of the southern westerly winds.
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| 8. |
- Harrison, Stephan, et al.
(författare)
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Early and mid-Holocene age for the Tempanos moraines, Laguna San Rafael, Patagonian Chile
- 2012
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 31, s. 82-92
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Tidskriftsartikel (refereegranskat)abstract
- Data about the nature and timing of Holocene events from the Southern Hemisphere, especially in southern South America, are required to provide insight into the extent and nature of past climate change in a region where land-based records are restricted. Here we present the first use of single grain Optically Stimulated Luminescence (OSL) dating of a moraine sequence recording glacial advance along the western side of the Patagonian Icefields. Dates from the Tempanos moraines at Laguna San Rafael (LSR) show that the San Rafael Glacier (SRG) advanced to maximum Holocene positions during the period 9.3 to 9.7 ka and at 5.7 ka. Outwash lying beneath the moraine in its northern portion, dated to 7.7 ka, indicates that the glacier front was also advanced at this time. Since these advances span both the regional early Holocene warm-dry phase (11.5 ka to 7.8 ka) and the subsequent cooling and rise in precipitation in the mid-late Holocene (since 6.6 ka) we infer that the advances of the SRG are not simply climate-driven, but that the glacier has also probably responded strongly to non-climatic stimuli such as internal ice dynamics and the transition between calving and non-calving. Many westwards-flowing glaciers in Patagonia were probably calving during much of the Late Pleistocene and Holocene, so we conclude that establishing robust glacial chronologies where climatic and non-climatic factors cannot be distinguished is likely to remain a challenge.
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| 9. |
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| 10. |
- Jansson, Krister, et al.
(författare)
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Using a GIS filtering approach to replicate patterns of glacial erosion
- 2011
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Ingår i: Earth Surface Processes and Landforms. - : Wiley. - 0197-9337 .- 1096-9837. ; 36:3, s. 408-418
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Tidskriftsartikel (refereegranskat)abstract
- In order to extend our knowledge of glacial relief production in mountainous areas new methods are required for landscape reconstructions on a temporal resolution of a glacial cycle and a spatial resolution that includes the most important terrain components. A generic data set and a 50 m resolution digital elevation model over a study area in northern Sweden and Norway (the present day landscape data set) were employed to portray spatial patterns of erosion by reconstructing the landscape over successive cycles of glacial erosion. A maximum-value geographic information system (GIS) filtering technique using variable neighbourhoods was applied such that existing highpoints in the landscape were used as erosional anchor points for the reconstruction of past landscape topography. An inherent assumption, therefore, is that the highest surfaces have experienced insignificant down-wearing over the Quaternary. Over multiple reconstruction cycles, proceeding backwards in time, the highest summits increase in area, valleys become shallower, and the valley pattern becomes increasingly simplified as large valleys become in-filled from the sides. The sum of these changes reduces relief. The pattern of glacial erosion, which is to 60% correlated to slope angle and to 70% correlated to relative relief, is characterized by (i) an abrupt erosional boundary below preserved summit areas, (ii) enhanced erosion in narrow valleys, (iii) restricted erosion of smooth areas, independently of elevation, (iv) eradication of small-scale irregularities, (v) restricted erosion on isolated hills in low-relief terrain, and (vi) a valley widening independent of valley directions. The method outlined in this paper shows how basic GIS filtering techniques can mimic some of the observed patterns of glacial erosion and thereby help deduce the key controls on the processes that govern large-scale landscape evolution beneath ice sheets.
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