| 1. |
- Lutman, A. A., et al.
(author)
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Polarization control in an X-ray free-electron laser
- 2016
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In: Nature Photonics. - : Springer Science and Business Media LLC. - 1749-4885 .- 1749-4893. ; 10:7, s. 468-472
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Journal article (peer-reviewed)abstract
- X-ray free-electron lasers are unique sources of high-brightness coherent radiation. However, existing devices supply only linearly polarized light, precluding studies of chiral dynamics. A device called the Delta undulator has been installed at the Linac Coherent Light Source (LCLS) to provide tunable polarization. With a reverse tapered planar undulator line to pre-microbunch the beam and the novel technique of beam diverting, hundreds of microjoules of circularly polarized X-ray pulses are produced at 500-1,200 eV. These X-ray pulses are tens of femtoseconds long, have a degree of circular polarization of 0.98(+0.02)(-0.04) at 707 eV and may be scanned in energy. We also present a new two-colour X-ray pump-X-ray probe operating mode for the LCLS. Energy differences of Delta E/E = 2.4% are supported, and the second pulse can be adjusted to any elliptical polarization. In this mode, the pointing, timing, intensity and wavelength of the two pulses can be modified.
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| 2. |
- Dammann, M., et al.
(author)
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Reliability and degradation mechanism of AlGaN/GaN HEMTs for next generation mobile communication systems
- 2009
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In: Microelectronics and Reliability. - : Elsevier BV. - 0026-2714. ; 49:5, s. 474-477
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Journal article (peer-reviewed)abstract
- Excellent reliability performance of AlGaN/GaN HEMTs on SiC substrates for next generation mobile communication systems has been demonstrated using DC and RF stress tests on 8 x 60 mu m wide and 0.5 mu m long AlGaN/GaN HEMTs at a drain voltage of V-d = 50 V. Drain current recovery measurements after stress indicate that the degradation is partly caused by slow traps generated in the SiN passivation or in the HEMT epitaxial layers. The traps in the SiN passivation layer were characterized using high and low frequency capacitance-voltage (CV) measurements of MIS test structures on thick lightly doped GaN layers. (C) 2009 Elsevier Ltd. All rights reserved.
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| 3. |
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| 4. |
- Dammann, Dyre Oliver, 1985, et al.
(author)
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Applications of radar interferometry for measuring ice motion
- 2021
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In: Proceedings of the International Conference on Port and Ocean Engineering under Arctic Conditions, POAC. - 0376-6756 .- 2077-7841. - 9781713841005 ; 2021-June
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Conference paper (peer-reviewed)abstract
- Ongoing sea ice decline has major implications for human activities near sea ice due to shorter seasons for on-ice operations and thinner ice with reduced load-bearing capacity and stability in many regions. This may in turn lead to increased sea ice mobility and impacts of ice movement on structures. We investigate space-borne radar interferometry (InSAR) as a technique to measure mm-scale sea ice motion of stationary ice over weeks to months. We find that InSAR enables mapping of bottomfast and stabilized landfast ice as regions with near-zero or reduced deformation. We further use this data to derive landfast ice strain and stress enabling estimation of the fracturing potential along the Northstar Island ice road in the Beaufort Sea, Alaska. We further examine ground-based radar interferometry (GRI) as a tool to collect continuous near real-time measurements on the km-scale not possible with InSAR. Based on GRI measurements conducted in Utqiaġvik, Alaska, we demonstrate the ability to evaluate ice strain in stationary ice and track vertical ice displacement due to tides. We also collected GRI measurements at 500 Hz during a drifting ice camp in the Beaufort Sea. The high sampling rate enabled tracking of continuous strain and detection of subtle variations in behavior between ice regimes. Through this work, interferometry shows promise as a tool to observe ice motion at the relevant scales needed for assessing sea ice stability, identify precursors to failure events, and better understand the relationship between different ice properties and loads on structures.
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| 5. |
- Dammann, Dyre Oliver, 1985, et al.
(author)
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Assessing sea ice trafficability in a changing arctic
- 2018
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In: Arctic. - : The Arctic Institute of North America. - 1923-1245 .- 0004-0843. ; 71:1, s. 59-75
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Journal article (peer-reviewed)abstract
- Arctic sea ice has undergone rapid changes during the last few decades, with negative implications for over-ice travel and on-ice operations, which benefit from services provided by the sea ice. A Parameter-based Trafficability Hierarchy (PATH) is presented here as a framework for developing quantitative assessment strategies that can guide planning and execution of operations on or near sea ice and quantify the impacts of recent changes on ice use. A PATH assessment has been completed for three case studies in Arctic Alaska. These cases, which correspond to a range of different icescapes and ice uses, identify and quantify different parameters linked to trafficability and safe operations. For ice road applications, PATH was used to determine an ice thickness compensation factor, a factor increasing the minimal thickness threshold for operations, to help translate sporadic auger ice thickness measurements along the Kotzebue-Kiana community ice road into an envelope for safe operations. A compensation factor as high as 1.5 was found to be necessary to ensure safety because of the high local thickness variability that is currently a concern for ice road operators. A PATH assessment of ice roughness for ice trail routing at Utqiaġvik draws on satellite remote sensing and is relevant for over-ice travel in general, including escape, evacuation, and rescue. We compared the routing of local snowmobile trails with Synthetic Aperture Radar (SAR) data products to identify specific ranges of ice conditions, roughness, and topography favored for ice trail construction. The same combination of data sources was used to identify potentially beneficial trail routes. Finally, an ice stability and safety assessment was completed for ice road construction and maintenance by industry near the Northstar Island oil production facility. We evaluated small-scale ice displacement data obtained from SAR interferometry to infer internal ice strain and stress and used these data in assessing the potential for fractures to reduce load-bearing capacity.
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| 6. |
- Dammann, Dyre Oliver, 1985, et al.
(author)
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Evaluating landfast sea ice stress and fracture in support of operations on sea ice using SAR interferometry
- 2018
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In: Cold Regions Science and Technology. - : Elsevier BV. - 0165-232X. ; 149, s. 51-64
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Journal article (peer-reviewed)abstract
- Recent Arctic warming has led to reduced sea-ice thickness and a more dynamic landfast ice cover with potential widespread consequences for ice users. Here, we develop an approach to assess the small-scale deformation of landfast ice critical to on-ice operations using synthetic aperture radar interferometry (InSAR). InSAR has previously proven successful in determining long-term qualitative climatology of ice deformation around on-ice operations, but is now used to explore its potential for providing quantitative guidance for ice road planning, construction, and maintenance. A validation effort using X-band SAR and high-precision GPS data over Elson Lagoon, Alaska, confirms the ability of InSAR to accurately estimate 3-dimensional sea ice strain values accumulated between SAR image acquisitions, using an inverse model. The inverse model was further applied to L-band InSAR data over the Northstar Island ice road near Prudhoe Bay, Alaska. Assuming an elasto-brittle rheology, the derived strain values yielded a spatial distribution of internal stress consistent with preexisting ice defects and morphology. In several localized regions of the study area, stress values exceeded expected yield stress. Resulting relative fracture intensity potential was shown to conform with local knowledge based on road inspections by engineers, and may be used to guide ice road planning, construction and maintenance efforts. The results presented here demonstrate that InSAR is an accurate tool for estimating landfast ice deformation and stability in support of ice use. The findings may also provide substantial new insights into the mechanics of landfast ice.
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| 7. |
- Dammann, Dyre Oliver, 1985, et al.
(author)
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Iceberg topography and volume classification using TanDEM-X interferometry
- 2019
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In: Cryosphere. - : Copernicus GmbH. - 1994-0424 .- 1994-0416. ; 13:7, s. 1861-1875
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Journal article (peer-reviewed)abstract
- Icebergs in polar regions affect water salinity, alter marine habitats, and impose serious hazards on maritime operations and navigation. These impacts mainly depend on the iceberg volume, which remains an elusive parameter to measure. We investigate the capability of TanDEM-X bistatic single-pass synthetic aperture radar interferometry (InSAR) to derive iceberg subaerial morphology and infer total volume. We cross-verify InSAR results with Operation IceBridge (OIB) data acquired near Wordie Bay, Antarctica, as part of the OIB/TanDEM-X Antarctic Science Campaign (OTASC). While icebergs are typically classified according to size based on length or maximum height, we develop a new volumetric classification approach for applications where iceberg volume is relevant. For icebergs with heights exceeding 5 m, we find iceberg volumes derived from TanDEM-X and OIB data match within 7 %. We also derive a range of possible iceberg keel depths relevant to grounding and potential impacts on subsea installations. These results suggest that TanDEM-X could pave the way for future single-pass interferometric systems for scientific and operational iceberg mapping and classification based on iceberg volume and keel depth.
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| 8. |
- Dammann, Dyre Oliver, 1985, et al.
(author)
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Instantaneous sea ice drift speed from TanDEM-X interferometry
- 2019
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In: Cryosphere. - : Copernicus GmbH. - 1994-0424 .- 1994-0416. ; 13:4, s. 1395-1408
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Journal article (peer-reviewed)abstract
- The drift of sea ice is an important geophysical process with widespread implications for the ocean energy budget and ecosystems. Drifting sea ice can also threaten marine operations and present a hazard for ocean vessels and installations. Here, we evaluate single-pass along-track synthetic aperture radar (SAR) interferometry (S-ATI) as a tool to assess ice drift while discussing possible applications and inherent limitations. Initial validation shows that TanDEM-X phase-derived drift speed corresponds well with drift products from a ground-based radar at Utqiagvik, Alaska. Joint analysis of TanDEM-X and Sentinel-1 data covering the Fram Strait demonstrates that S-ATI can help quantify the opening/closing rate of leads with possible applications for navigation. S-ATI enables an instantaneous assessment of ice drift and dynamic processes that are otherwise difficult to observe. For instance, by evaluating sea ice drift through the Vilkitsky Strait, Russia, we identified short-lived transient convergence patterns. We conclude that S-ATI enables the identification and analysis of potentially important dynamic processes (e.g., drift, rafting, and ridging). However, current limitations of S-ATI are significant (e.g., data availability and they presently only provide the cross-track vector component of the ice drift field) but may be significantly reduced with future SAR systems.
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| 9. |
- Dammann, Dyre Oliver, 1985, et al.
(author)
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Mapping Arctic Bottomfast Sea Ice Using SAR Interferometry
- 2018
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In: Remote Sensing. - : MDPI AG. - 2072-4292. ; 10:5
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Journal article (peer-reviewed)abstract
- Bottomfast sea ice is an integral part of many near-coastal Arctic ecosystems with implications for subsea permafrost, coastal stability and morphology. Bottomfast sea ice is also of great relevance to over-ice travel by coastal communities, industrial ice roads, and marine habitats. There are currently large uncertainties around where and how much bottomfast ice is present in the Arctic due to the lack of effective approaches for detecting bottomfast sea ice on large spatial scales. Here, we suggest a robust method capable of detecting bottomfast sea ice using spaceborne synthetic aperture radar interferometry. This approach is used to discriminate between slowly deforming floating ice and completely stationary bottomfast ice based on the interferometric phase. We validate the approach over freshwater ice in the Mackenzie Delta, Canada, and over sea ice in the Colville Delta and Elson Lagoon, Alaska. For these areas, bottomfast ice, as interpreted from the interferometric phase, shows high correlation with local bathymetry and in-situ ice auger and ground penetrating radar measurements. The technique is further used to track the seasonal evolution of bottomfast ice in the Kasegaluk Lagoon, Alaska, by identifying freeze-up progression and areas of liquid water throughout winter.
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| 10. |
- Dammann, Dyre Oliver, 1985, et al.
(author)
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Mapping pan-Arctic landfast sea ice stability using Sentinel-1 interferometry
- 2019
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In: Cryosphere. - : Copernicus GmbH. - 1994-0424 .- 1994-0416. ; 13:2, s. 557-577
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Journal article (peer-reviewed)abstract
- Arctic landfast sea ice has undergone substantial changes in recent decades, affecting ice stability and including potential impacts on ice travel by coastal populations and on industry ice roads. We present a novel approach for evaluating landfast sea ice stability on a pan-Arctic scale using Synthetic Aperture Radar Interferometry (InSAR). Using Sentinel-1 images from spring 2017, we discriminate between bottomfast, stabilized, and nonstabilized landfast ice over the main marginal seas of the Arctic Ocean (Beaufort, Chukchi, East Siberian, Laptev, and Kara seas). This approach draws on the evaluation of relative changes in interferometric fringe patterns. This first comprehensive assessment of Arctic bottomfast sea ice extent has revealed that most of the bottomfast sea ice is situated around river mouths and coastal shallows. The Laptev and East Siberian seas dominate the aerial extent, covering roughly 4100 and 5100 km(2), respectively. These seas also contain the largest extent of stabilized and nonstabilized landfast ice, but are subject to the largest uncertainties surrounding the mapping scheme. Even so, we demonstrate the potential for using InSAR for assessing the stability of landfast ice in several key regions around the Arctic, providing a new understanding of how stability may vary between regions. InSAR-derived stability may serve for strategic planning and tactical decision support for different uses of coastal ice. In a case study of the Nares Strait, we demonstrate that interferograms may reveal early-warning signals for the breakup of stationary sea ice.
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