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  • 1
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    Climatic controls on mountain glacier basal thermal regimes dictate spatial patterns of glacial erosion (2021)
    In:  Earth Surface Dynamics
    Details
    Publication Date: 2021-10-14
    Description: Climate has been viewed as a primary control on the rates and patterns of glacial erosion, yet our understanding of the mechanisms by which climate influences glacial erosion is limited. We hypothesize that climate controls the patterns of glacial erosion by altering the basal thermal regime of glaciers. The basal thermal regime is a first-order control on the spatial patterns of glacial erosion. Polythermal glaciers contain both cold-based portions that protect bedrock from erosion and warm-based portions that actively erode bedrock. In this study, we model the impact of various climatic conditions on glacier basal thermal regimes and patterns of glacial erosion in mountainous regions. We couple a sliding-dependent glacial erosion model with the Parallel Ice Sheet Model (PISM) to simulate the evolution of the glacier basal thermal regime and glacial erosion in a synthetic landscape. We find that both basal thermal regimes and glacial erosion patterns are sensitive to climatic conditions, and glacial erosion patterns follow the patterns of the basal thermal regime. Cold temperature leads to limited glacial erosion at high elevations due to cold-based conditions. Increasing precipitation can overcome the impact of cold temperature on the basal thermal regime by accumulating thick ice and lowering the melting point of ice at the base of glaciers. High precipitation rates, therefore, tend to cause warm-based conditions at high elevations, resulting in intensive erosion near the peak of the mountain range. Previous studies often assessed the impact of climate on the spatial patterns of glacial erosion by integrating climatic conditions into the equilibrium line altitudes (ELAs) of glaciers, and glacial erosion is suggested to be maximal around the ELA. However, our results show that different climatic conditions produce glaciers with similar ELAs but different patterns of basal thermal regime and glacial erosion, suggesting that there might not be any direct correlation between ELAs and glacial erosion patterns.
    Language: English
    Type: info:eu-repo/semantics/article
    Format: application/pdf
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  • 2
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    Numerical modeling of groundwater‐driven stream network evolution in low‐relief post‐glacial landscapes (2022)
    In:  Earth Surface Processes and Landforms
    Details
    Publication Date: 2022-02-15
    Description: In the low-relief post-glacial landscapes of the Central Lowlands of the United States, fluvial networks formed and expanded following deglaciation despite the low slopes and large fraction of the land surface occupied by closed depressions. Low relief topography allows for subtle surface water divides and increases the likelihood that groundwater divides do not coincide with surface water divides. We investigate how groundwater transfer across subtle surface water divides facilitates channel network expansion using a numerical model built on the Landlab platform. Our model simulates surface and subsurface water routing and fluvial erosion. We consider two end-member scenarios for surface water routing, one in which surface water in closed depressions is forced to connect to basin outlets (routing) and one in which surface water in closed depressions is lost to evapotranspiration (no routing). Groundwater is modeled as fully saturated flow within a confined aquifer. Groundwater emerges as surface water where the landscape has eroded to a specified depth. We held the total water flux constant and varied the fraction of water introduced as groundwater versus precipitation. Channel growth is significantly faster in routing cases than no-routing cases given identical groundwater fractions. In both routing and no-routing cases, channel expansion is fastest when ~30% of the total water enters the system as groundwater. Groundwater contributions also produce distinctive morphology including steepened channel profiles below groundwater seeps. Groundwater head gradients evolve with topography and groundwater-fed channels can grow more quickly than channels with larger surface water catchments. We conclude that rates of channel network growth in low-relief post-glacial areas are sensitive to groundwater contributions. More broadly, our findings suggest that landscape evolution models may benefit from more detailed representation of hydrologic processes.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 3
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    Development of Foreland Intracratonic Plateaus (Ozark Plateau and Appalachian Plateaus): A Consequence of Topographic Inversion Due To Erosion of Adjacent Fold‐Thrust Belts (2022)
    In:  Tectonics
    Details
    Publication Date: 2022-05-18
    Description: Unlike well-known plateaus associated with Cenozoic orogens, the Appalachian and Ozark Plateaus of the eastern United States fringe the foreland side of a long inactive and deeply eroded orogen. These foreland intracratonic plateaus (FIPs), which are underlain by sub-horizontal cratonic-platform strata and, in places, foreland-basin strata, now lie 0.5–1.2 km above sea level, notably higher than adjacent fold-thrust belts. An escarpment lies at or near the boundary between the FIPs and the fold-thrust belts. Why did the topographic inversion leading to the development of the FIPs take place? To address this question, we built a numerical model, using Landlab, to simulate how topography evolves as foreland lithosphere flexes upward when post-tectonic erosion causes unloading. In this model, flat-lying cap-rock strata (sandstone and limestone) of the foreland have greater resistance to erosion than do the deformed, tilted, cleaved, and fractured strata of the fold-thrust belt, especially where the fold-thrust belt contains argillaceous facies. We tested the model by characterizing the development of the Ozark Plateau in the foreland of the Ouachita fold-thrust belt. Results demonstrate that regional isostatic uplift due to erosion, given reasonable differences in resistance to erosion between the fold-thrust belt and the foreland, can generate the observed topographic inversion and a distinct escarpment, yielding a plateau. This model may help explain the post-Paleozoic evolution of the Catskill Mountains, the Deep Valleys Province, and the Cumberland Plateau, highlands which border the Appalachian fold-thrust belt.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 4
    Electronic Resource
    Electronic Resource
    Vacuum arc cathode spot parameters from high-resolution luminosity measurements (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 4763-4770 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Cathode spots on arc-cleaned copper and molybdenum electrodes in vacuum were studied by fast image converter framing and streak camera photography with high temporal and spatial resolution. The frame exposure time was 20 ns and the interframe period was 200 ns; the streak sweep time was between 200 ns and 1 μs. Spatial structures with a resolution of 5 μm could be determined by observing the spot movement with a small slit at the streak camera and a high sweep speed. Strong fluctuations of the light emission of the spot were found with characteristic times of 50–100 ns. When the spot moved out of the slit field of view a stepwise decrease occurred in the measured light, indicating an internal substructure of the spot with distances between fragments less than 10 μm and even smaller fragment diameters. The current per fragment was estimated to be 20–40 A. The frames confirm the short time constants of the spot. From frame to frame a spot motion was observed in most cases, yielding spot residence times 〈200 ns.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.350668
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  • 5
    Electronic Resource
    Electronic Resource
    Pulsed vacuum-arc ion source operated with a "triggerless" arc initiation method : The 8th international conference on ion sources (2000)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 71 (2000), S. 827-829 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Vacuum arcs can be initiated by simply applying a sufficiently high voltage (∼ 1 kV) between the anode and cathode, provided that there is a conducting path between these electrodes. Typically, the conducting path is obtained by coating the ceramic insulator. Plasma is formed explosively at the coating-cathode interface. Since neither a trigger supply nor a trigger electrode are required, the method has been dubbed "triggerless" arc initiation. Triggerless operation of a vacuum arc ion source was demonstrated for a number of cathode materials. It was found that triggerless operation is very reliable as long as the balance of deposition and erosion of the conducting material leads to a steady-state path resistance in the range from 1 Ω to 100 kΩ. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1150305
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  • 6
    Electronic Resource
    Electronic Resource
    Surface modification of magnetic recording heads by plasma immersion ion implantation and deposition (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 1656-1664 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Plasma immersion ion implantation and deposition is a novel process for surface modification. By combining plasma deposition and ion implantation and using filtered vacuum arc plasmas, thin film formation, direct and recoil ion implantation, and ion-beam-assisted intermixing of the film and substrate can be accomplished simultaneously. The implications of this technique in tribology of magnetic recording media have been investigated experimentally. Surfaces of Al2O3-TiC heads were modified with silver, carbon, and titanium ions at doses of 3×1016 ions/cm2 and mean implantation energies between 2.0 and 4.2 keV. Simulation results indicated that the modified regions exhibited high concentrations of implanted species in the top 2–3 nm, atomically mixed interfaces, and thicknesses between 10 and 25 nm. Surface imaging with an atomic force microscope and nanoindentation testing revealed that the modified heads possessed smoother topographies and increased hardnesses. Contact start-stop and continuous sliding experiments with modified heads and carbon-coated magnetic rigid disks and microscopy observations demonstrated that significant enhancement of the friction and wear characteristics can be achieved with the present surface modification technique. The possible reasons for the improved tribological behavior and the predominant mechanisms during contact start-stop and continuous sliding are interpreted in light of the obtained experimental results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358516
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  • 7
    Electronic Resource
    Electronic Resource
    Effect of pretreatment process parameters on diamond nucleation on unscratched silicon substrates coated with amorphous carbon films (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 485-492 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Diamond nucleation on unscratched silicon substrates was investigated using a conventional microwave plasma-enhanced chemical vapor deposition system. Silicon substrates were coated with thin films of amorphous carbon using a vacuum arc technique. The carbon-coated silicon substrates were pretreated with a methane-rich plasma at relatively low temperatures and were subsequently exposed to the diamond nucleation conditions. The significance of the pretreatment on the diamond nucleation density was examined by varying the methane concentration, chamber pressure, and exposure time. Scanning electron microscopy demonstrated that densely packed spherical nanoparticles on the pretreated surfaces played the role of diamond nucleation seeds. Raman spectroscopy analysis showed that the nucleation seeds consisted of nonhydrogenated carbon and that their structure was influenced by the pretreatment conditions. Transmission electron microscopy revealed that the nucleation seeds comprised disordered graphitic carbon and ultrafine diamond crystallites. Submicrometer films of good quality diamond possessing significantly higher nucleation densities (∼5×1010 cm−2) were grown from nanoparticles produced under optimum pretreatment conditions. The enhancement of the diamond nucleation density is mainly attributed to the formation of a large number of nanoparticles, which provided sufficient high-surface free-energy sites for diamond nucleation, in conjunction with their high etching resistance to atomic hydrogen stemming from the significant percentage of sp3 atomic carbon configurations, as evidenced by the presence of nanocrystalline diamond in the nanoparticle structure. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360855
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  • 8
    Electronic Resource
    Electronic Resource
    Erratum: "Surface modification of magnetic recording heads by plasma immersion ion implantation and deposition'' [J. Appl. Phys. 76, 1656 (1994)] (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 3223-3223 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358541
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  • 9
    Electronic Resource
    Electronic Resource
    Parametric study of a room-temperature metal vapor laser (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 64 (1988), S. 4285-4292 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have demonstrated laser operation at room temperature on the Cui (511,578 nm) and Aui (312,628 nm) transitions and obtained specific pulse energies for the Cui 511 nm and Aui 628 nm lines of about 5 and 2 μJ cm−3, respectively, from a single-section tube. A simple theoretical model has been used to show that the reduced outputs for the Cui 578 nm and Aui 312 nm lines are due to the significantly smaller fraction of the working volume in saturation in each case. The metal vapor column of length 20 cm and local ground-state concentration in the 1014–1015 cm−3 range is produced on demand in a single-section tube in less than 10 ms. The highest output power obtained is for a 90% He in an Ar/He mixture at a total flowrate of 1.8 l min−1, the tube pressure being 20 mbar. A specific sputtering discharge energy of approximately 600 mJ cm−3 is needed to obtain a peak power of 1 kW on either the Cui 511 nm or Aui 628 nm lines.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.341302
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  • 10
    Electronic Resource
    Electronic Resource
    Streaming metal plasma generation by vacuum arc plasma guns : Proceedings of the 7th international conference on ion sources (1998)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 69 (1998), S. 801-803 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We have developed several different embodiments of repetitively pulsed vacuum arc metal plasma gun, including miniature versions, multicathode versions that can produce up to 18 different metal plasma species between which one can switch, and a compact high-duty cycle well-cooled version, as well as a larger dc gun. Plasma guns of this kind can be incorporated into a vacuum arc ion source for the production of high-energy metal ion beams, or used as a plasma source for thin film formation and for metal plasma immersion ion implantation and deposition. The source can also be viewed as a low-energy metal ion source with ion drift velocity in the range 20–200 eV depending on the metal species used. Here we describe the plasma sources that we have developed, the properties of the plasma generated, and summarize their performance and limitations. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1148718
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