bluuebunny/crossref_metadata_2025 huggingface dataset:
Scaling of dominant runoff generation processes: Nested catchments approach using multiple tracers
Authors: Didszun Jens , Uhlenbrook Stefan
Published: February 2008

Low‐flow stream chemistry and rainfall event reactions of nested catchments in the mesoscale Dreisam catchment in the Black Forest Mountains, southwest Germany, were analyzed to investigate how the dominant runoff generation processes change with scale. The catchment sizes range from a 0.015 km2 headwater catchment to the 258 km2 Dreisam catchment. Synoptic sampling during low flows was used to address the spatial heterogeneity of the investigated tracers. Six events were sampled using three different experimental designs with the environmental tracers dissolved silica, oxygen‐18, deuterium, and potassium to investigate event processes at different scales. Results are elucidated with respect to the widely noticed Representative Elementary Area (REA) concept. Most of the observed differences between the catchments could be related to changes in the topography and other catchment properties (i.e., soils, geology, land use). The test site catchments less than 1–2 km2 were found to be nonrepresentative of the runoff generation in larger catchments due to the topographic structure and a reduced number of hydrological response units (HRU) and, consequently, generated runoff components. In catchments larger than 40 km2 an additional runoff component, the surface runoff from urban areas, became increasingly important. However, surprisingly small differences in the tracer responses at catchments between 1 and 40 km2 were observed. Although the lower threshold (1–2 km2) was similar for both methods (low‐flow and event investigations), results suggest that the thresholds depend on the investigated scale and hydrological parameters as well as the catchment properties. Applying microscale tracer methods at the mesoscale provided detailed insights into the scaling of the dominant runoff processes. The results show that this approach is an important component when addressing the scaling behavior besides the numerous microscale studies and modeling approaches. However, quantitative interpretations are limited owing to inherent heterogeneity at this scale.
Modeling of solute transport in snow using conservative tracers and artificial rain‐on‐snow experiments
Authors: Lee Jeonghoon , Feng Xiahong , Posmentier Eric S. , Faiia Anthony M. , Osterhuber Randall , Kirchner James W.
Published: February 2008

We report a study of solute transport in snow, using artificial rain‐on‐snow experiments with conservative anions (F−, Br−, and SO42−). The tracers were mixed into tap water and sprayed onto the snow surface from two water supply tanks. The water flux out of the base of the snowpack was recorded, and discharge samples were collected and analyzed for the three tracers. The chemical concentration of tracers in the discharge was negatively associated with the water flux. The objectives of the experiment were to test whether the mobile‐immobile model (MIM) with variable mobile‐immobile water exchange coefficient can simulate both positive and negative concentration‐discharge relationships in this and previous tracer experiments. By simulating our experimental data, we confirm that it is necessary for the exchange coefficient to increase with water velocity. In addition, we use the model to show that with a diurnal variation of clean water flux, a negative concentration‐discharge relationship occurs when solutes are evenly distributed in the mobile and immobile fluids, while a positive relationship occurs when the solutes were present only in the immobile fluid near the surface. This result may help in explaining the complicated concentration‐discharge relationships observed in catchments.
Estimating plume degradation rates in aquifers: Effect of propagating measurement and methodological errors
Authors: Jarsjö Jerker , Bayer‐Raich Martí
Published: February 2008

Field estimates of plume degradation rates λ [T−1] in aquifers provide a basis for assessing the possible impact of (toxic) organic pollutants on downstream environments; however, difficulties with measurement and methodology mean that estimated site‐specific rates potentially involve considerable uncertainty. Here, we specifically show that if mass flow or average concentration measurements are associated with errors of ∼20% (or even less), the errors may in many cases propagate, magnify, and cause order‐of‐magnitude errors in estimates of λ. We also investigate uncertainties in the integral pumping test method, in which average concentrations are determined based on concentrations measured from pumping wells. In this method, the small‐scale variability that may bias the results of point measurements can be favorably averaged out when pumping; however, the novelty of the approach means that questions remain regarding its application. For example, the magnitude of methodological errors, such as those associated with the assumption of constant concentration along the flow direction within the extent of the well capture zone, remain poorly understood. This assumption can be violated by the biodegradation of contaminants, thereby leading to a bias in subsequent interpretations. We provide an analytical expression from which the prediction error due to attenuating concentrations can be evaluated and show its dependence on the degradation rate, the degradation function, and the extent of the well capture zone. Even for considerable first‐order degradation, the mass flow error generally remains small and is not magnified in estimates of λ.
Day to night variation in meteor trail measurements: Evidence for a new theory of plasma trail evolution
Authors: Oppenheim Meers M. , Sugar Glenn , Bass Elizabeth , Dimant Yakov S. , Chau Jorge
Published: February 2008

A recent theory of meteor trail plasma diffusion made the prediction that meteors will generate more and longer lasting non‐specular echoes at night than during the day. This letter presents the first evidence of a dramatic day to night difference in non‐specular meteor trail occurrence rates and their duration. These observations were made by the 50MHz radar at the Jicamarca Radio Observatory (JRO) in Peru. In one 20 minute period starting 95 minutes before sunrise, this radar detected 1288 head echoes and 341 trails while a similar time after dawn, it measured 1240 head echoes but only 50 trails. Also, the duration of the nighttime trails greatly exceeded the daytime ones. This pattern was confirmed by a second experiment in July 2007. This data provides strong evidence that it is necessary to account for the effect of the ionospheric plasma density to explain meteor diffusion.
Energetic ion precipitation at Titan
Authors: Cravens T. E. , Robertson I. P. , Ledvina S. A. , Mitchell D. , Krimigis S. M. , Waite J. H.
Published: February 2008

Energetic protons and oxygen ions have been observed in Saturn's outer magnetosphere and can precipitate into Titan's atmosphere where they deposit energy, ionize, and drive ionospheric chemistry. Ion production rates caused by this precipitation are calculated using fluxes of incident 27 keV to 4 MeV protons measured by the Cassini MIMI instrument. We find that significant ion production rates exist in the 500 km to 1000 km altitude range and estimate associated electron densities of about 200–2000 cm−3 in reasonable agreement with measured densities. We demonstrate that energetic oxygen ions do not penetrate below about 650 km, but they can also generate significant ionization. We suggest that a few percent of the oxygen flux is converted to negative O ions as a consequence of charge exchange collisions, which might help explain the negative ions observed near 960 km by the Cassini CAPS instrument.
Oceanic precondition and evolution of the 2006 Indian Ocean dipole
Authors: Horii Takanori , Hase Hideaki , Ueki Iwao , Masumoto Yukio
Published: February 2008

Details of subsurface ocean conditions associated with the Indian Ocean Dipole event (IOD) were observed for the first time by mooring buoys in the eastern equatorial Indian Ocean. Large‐scale sea surface signals in the tropical Indian Ocean associated with the positive IOD started in August 2006, and the anomalous conditions continued until December 2006. Data from the mooring buoys, however, captured the first appearance of the negative temperature anomaly at the thermocline depth with strong westward current anomalies in May 2006, about three months earlier than the development of the surface signatures. These subsurface evolutions within the ocean would be a key factor for better understanding of IOD mechanisms and its predictability, and are providing a fundamental dataset for validation of modeling outputs.
Seismic signals from tsunamis in the Pacific Ocean
Authors: Shields Gordon , Bowman J. Roger
Published: February 2008

We examine seismic signals from tsunamis for 14 earthquakes of Mw 7.1 to 8.4 in the circum‐Pacific region that generated tsunamis measured previously from tide gauges or coastal runup. Low‐frequency signals of 0.5 mHz up to 2–8 mHz are observed at the expected tsunami arrival time on the horizontal components of low elevation seismic stations near coastlines for eight of the 14 earthquakes. Lower amplitude signals are observed for the other six. For two earthquakes (Mw 8.2–8.4) we observe a dispersed tsunami signal between 1 and 8 mHz, with lower frequencies arriving earlier. This dispersion is consistent with predictions from the standard model for gravity waves for the bathymetry along these source‐station paths. These observations, along with earlier observations from the Indian Ocean, suggest that seismic data could be used to complement tide gauges and ocean bottom pressure recorders to indicate the arrival of a tsunami.
Contrasting spectral changes limit albedo impact on land‐atmosphere coupling during the 2003 European heat wave
Authors: Teuling A. J. , Seneviratne S. I.
Published: February 2008

Soil moisture related changes in the Earth's reflectance have been suggested to impact warm season climate variability through their direct impact on surface shortwave radiative forcing. Here we investigate the potential contribution of albedo to land‐atmosphere coupling during the 2003 European heat wave. MODIS land surface albedo data for the years 2000–2004 are utilized to identify visible, near‐infrared, and total shortwave anomalies for 2003. Changes in vegetation structure result in significant but contrasting albedo anomalies in the visible and near‐infrared broadbands for most ecosystem types. Although locally the associated total shortwave radiative forcing can be significant (−10 W m−2) especially over cropland areas, the effects almost cancel out at the sub‐continental scale (−1 W m−2). The net effect is too small to support the hypothesis that albedo impacts large‐scale land‐atmosphere coupling.
Extended SuperDARN and IMAGE observations for northward IMF: Evidence for dual lobe reconnection
Authors: Marcucci M. F. , Coco I. , Ambrosino D. , Amata E. , Milan S. E. , Bavassano Cattaneo M. B. , Retinò A.
Published: February 2008

We present observations of ionospheric convection in the Northern Hemisphere made by the SuperDARN radar network during a 3 h period on 3 December 2001. The interplanetary magnetic field (IMF) during the time of observations is predominately northward with the By component changing from positive to slightly negative. During this period Cluster is skimming the southern high latitude dusk magnetopause and reveals that reconnection is going on quasi‐continuously with the reconnection site being most of the time tailward of the southern cusp and always near the satellite location (Retinò, et al., 2005). Detailed analysis of the three dimensional distribution function indicates that Cluster samples magnetosheath lines connected with geomagnetic field lines tailward of the cusps in both hemispheres (Bavassano Cattaneo et al., 2006). The evolution of the ionospheric convection measured by SuperDARN, together with IMAGE FUV observations of aurorae and DMSP particle precipitation data, confirms Cluster observations and shows that simultaneous reconnection poleward of both the northern and southern cusps occurs at a variable rate on the dusk part of the magnetosphere when the IMF clock angle is small.
Two chloride sources in soils of the McMurdo Dry Valleys, Antarctica
Authors: Bao Huiming , Barnes Jaime D. , Sharp Zachary D. , Marchant David R.
Published: February 2008

Sulfate, nitrate, and chloride are major water‐soluble anions in soils of the McMurdo Dry Valleys (MDV), Antarctica. We measured the chloride stable isotope composition of 40 soil samples from 16 different soil profiles. The soil profiles come from three different microclimate zones in the MDV: a coastal thaw zone, which fosters a seasonally saturated active layer; a stable upland zone, in which air and soil temperatures are too cold to foster active layers and/or significant snowmelt; and an intermediate, inland mixed zone. Results show that the total water‐soluble chloride content is low in both the coastal and the upland zones, but highly variable in the inland mixed zone (average 770 to 11520 ppm). Individual soil δ37Cl values range from −2.10‰ to +0.56‰, with profile average δ37Cl values broadly decreasing from the coast toward upland valleys. Vertically, the δ37Cl increases with depth in soil profiles developed on basal tills but decreases on sublimation tills. We propose that the spatial [Cl−] and δ37Cl patterns are the result of continuous influx and leaching of two major chloride sources, an airborne sea‐salt chloride (SSC) with a δ37Cl value ≥ ∼0.0‰ and a secondary atmospheric chloride (SAC) with a δ37Cl value ≤ −2.10‰. Our model suggests that sublimation tills have their Cl− input from two boundaries: a till‐atmosphere interface at the top and a buried‐ice‐till interface at the bottom, differing distinctively from basal tills where there is no salt contribution from the bottom. Our model also suggests a higher influx of SSC relative to SAC in the MDV in the past. Overall, characterizing the sources, budgets, and spatial/vertical patterns of chloride constitutes a first step toward unraveling climate, tectonic, and landscape changes archived in MDV soil profiles.
Antarctic climate change over the twenty first century
Authors: Bracegirdle Thomas J. , Connolley William M. , Turner John
Published: February 2008

Here we present a new assessment of Antarctic climate change over the 21st century based on data from the models that were developed as part of the Intergovernmental Panel on Climate Change (IPCC) 4th Assessment Report (AR4). To provide more reliable estimates of future change, a weighting scheme was applied to the model output, which depends on a measure of their ability to reproduce the mean climate of the late 20th century. The results show a seasonal variation of increases in circumpolar westerlies around Antarctica, which show the largest increases of 27% in autumn. This seasonal cycle was found to be consistent with projected changes in the semi‐annual oscillation (SAO). In summer and autumn the increases of the westerly wind component migrate sufficiently far south to be manifested as a reduction of the coastal easterlies. The surface warming averaged over the continent is projected to be 0.34°C dec−1 with an inter‐model standard deviation of 0.10°C dec−1. More rapid warming occurs during the winter over regions of sea ice retreat, e.g., 0.51 ± 0.26°C dec−1 around East Antarctica. Projections of total sea‐ice area show a decrease of 2.6 ± 0.73 × 106 km2 (33%). There is a projected increase of net precipitation averaged over the continent of 2.9 ± 1.2 mm a−1 dec−1. The weighting gives a larger increase of the autumn SAO peak, up to 30% larger for April. This is consistent with larger weighted autumn increases of circumpolar westerlies, more sea ice reduction and resulting larger skin temperature increases.
Multiple flooding events in Martian outflow channels
Authors: Harrison Keith P. , Grimm Robert E.
Published: February 2008

The large outflow channels of Chryse Planitia, Mars, are thought to have been carved by floodwaters discharged from an aquifer beneath a confining cryosphere. However, conventional models of groundwater flow require optimistically high permeabilities to produce, in a single flooding event, the discharge rates and volumes inferred from channel morphology. Additionally, discharge likely ceased upon refreezing of chaotic terrain fractures carrying it to the surface, further limiting the volume of water produced in a single flood. It is thus probable that multiple surface discharge events were required, and we quantify this hypothesis with regional groundwater simulations. Each discharge event is triggered by cryosphere disruption due to superlithostatic hydraulic head at the channel source region and is terminated by cryosphere fracture refreezing. Before the next event, head is allowed to recover with the aid of distal aquifer recharge constrained by previous global groundwater models. Our baseline model, which emulates the source region of Kasei Valles, yields a minimum of 2900 flooding events even for high depth‐averaged aquifer permeabilities corresponding to near‐surface values of 10−9m2. Although fewer events are required for other circum‐Chryse channels, formation by a single flood remains improbable. We suggest that unrealistically high numbers of floods may be circumvented by an alternative model involving the local ponding of surface discharge to form standing bodies of water. Episodic failure of these bodies produces flood outbursts which erode the channels.
A dynamic model of oceanic sulfur (DMOS) applied to the Sargasso Sea: Simulating the dimethylsulfide (DMS) summer paradox
Authors: Vallina S. M. , Simó R. , Anderson T. R. , Gabric A. , Cropp R. , Pacheco J. M.
Published: February 2008

A new one‐dimensional model of DMSP/DMS dynamics (DMOS) is developed and applied to the Sargasso Sea in order to explain what drives the observed dimethylsulfide (DMS) summer paradox: a summer DMS concentration maximum concurrent with a minimum in the biomass of phytoplankton, the producers of the DMS precursor dimethylsulfoniopropionate (DMSP). Several mechanisms have been postulated to explain this mismatch: a succession in phytoplankton species composition towards higher relative abundances of DMSP producers in summer; inhibition of bacterial DMS consumption by ultraviolet radiation (UVR); and direct DMS production by phytoplankton due to UVR‐induced oxidative stress. None of these hypothetical mechanisms, except for the first one, has been tested with a dynamic model. We have coupled a new sulfur cycle model that incorporates the latest knowledge on DMSP/DMS dynamics to a preexisting nitrogen/carbon‐based ecological model that explicitly simulates the microbial‐loop. This allows the role of bacteria in DMS production and consumption to be represented and quantified. The main improvements of DMOS with respect to previous DMSP/DMS models are the explicit inclusion of: solar‐radiation inhibition of bacterial sulfur uptakes; DMS exudation by phytoplankton caused by solar‐radiation‐induced stress; and uptake of dissolved DMSP by phytoplankton. We have conducted a series of modeling experiments where some of the DMOS sulfur paths are turned “off” or “on,” and the results on chlorophyll‐a, bacteria, DMS, and DMSP (particulate and dissolved) concentrations have been compared with climatological data of these same variables. The simulated rate of sulfur cycling processes are also compared with the scarce data available from previous works. All processes seem to play a role in driving DMS seasonality. Among them, however, solar‐radiation‐induced DMS exudation by phytoplankton stands out as the process without which the model is unable to produce realistic DMS simulations and reproduce the DMS summer paradox.
Cenozoic ice‐rafting history of the central Arctic Ocean: Terrigenous sands on the Lomonosov Ridge
Authors: St. John Kristen
Published: February 2008

The Cenozoic ice‐rafted debris (IRD) history of the central Arctic is reconstructed utilizing the terrigenous coarse sand fraction in IODP 302 cores from 0 to 273 meters composite depth. This Holocene−middle Eocene quantitative record of terrigenous sand accumulation on the Lomonosov Ridge, along with qualitative information on grain texture and composition, confirms the interpretation that ice initiation (sea ice and glacial ice) occurred ∼46 Ma in the Arctic, and provides a long‐term pattern of Arctic ice expansion and decay since the middle Eocene. IRD mass accumulation rates range from 0 to 0.13 g/cm2/ka in the middle Eocene and from 0 to 0.36 g/cm2/ka in the Neogene. IRD mass accumulation rate (MAR) maxima in the Miocene and Pliocene cooccur with either glacial initiation or intensification in the sub‐Arctic. The 46.25 Ma IRD onset in the central Arctic slightly precedes the earliest evidence of ice in the Antarctic, and compares in timing with a >1000 ppm decrease in atmospheric concentrations of CO2. The decline of pCO2 in the middle Eocene may have driven both poles across the temperature threshold that enabled the nucleation of glaciers on land and partial freezing of the surface Arctic Ocean, especially during times of low insolation.
Validated simulation of the ion extraction grid lifetime
Authors: Tartz M. , Hartmann E. , Neumann H.
Published: February 2008

This article describes the validation of our extraction grid erosion modeling for gridded ion thrusters. Experimental data from three life tests of different thruster sizes and operation conditions was available. For all test cases, an excellent agreement was achieved between experimental and simulated data, which increased the confidence in the lifetime predictions of the code for space missions.
Development of low energy ion beam system for space charge compensation experiments
Authors: Takeuchi Mitsuaki , Gotoh Yasuhito , Tsuji Hiroshi , Ishikawa Junzo , Sakai Shigeki
Published: February 2008

A low energy ion beam system for space charge compensation (SCC) experiments was developed and evaluated. This system was designed for observation of SCC of a positive ion beam with an electron beam. The system consisted of the ion source chamber and the SCC experiment chamber. The ion source chamber was equipped with the compact microwave ion source for low voltage extraction. Ion current at initial position of the analysis chamber was 84μA at extraction voltage of 500V, and satisfied a condition to observe the SCC effect clearly. In order to evaluate the SCC, we measured the arrival ion current by supplying thermionic electrons, which were extracted from a tungsten filament driven by ac voltage. As the electron supply, the arrival ion current increased from 40to68μA at the potential of filament of +3eV which produced the thermionic electron with extremely low energy extracted by space charge of the ion beam.
Matching from H− multicusp source to central region of a 100MeV compact cyclotron for high current injection
Authors: Yao Hongjuan , Zhang Tianjue , Lu Yinlong , Jia Xianlu , Guan Fengping , Lin Yuzheng
Published: February 2008

The way of matching the beam from an external ion source to the central region of the cyclotron CYCIAE-100 is presented in this article. The transverse acceptances of the central region were calculated, which provided the matching ellipse parameter requirements to the injection transport optics design. The optics of the injection line was simulated using TRANSOPTR, which allowed space charge effect calculation, and neutralization was taken into account.
New developments in multicusp H− ion sources for high energy accelerators (invited)
Authors: Peters J.
Published: February 2008

A functional description of multicusp H− sources is given, focusing on the fundamental elements of this source type: the discharge chamber, the extraction plasma region, and electron dumping. Design choices for these elements will be presented and compared. The options of rf and filament heating of the discharge chamber will be presented. Also, the construction opportunities in the extraction plasma region using filter field arrangements and collars are described. Three methods to dump the electrons at three different regions are discussed. In a brief history, the development of multicusp sources are described. The status of the new sources under development at DESY, SNS, and in Japan will be given in a table for all types of operational H− sources. Finally, the limits to the H− current and emittance as well as the Cs question will be discussed.
Electrostatic instability of electron beams in a planar E×B amplifier
Authors: Lee Myoung-Jae , Lee Hee J. , Chung Kyu-Sun
Published: February 2008

A wave equation is kinetically derived for an electron beam to describe the linear electrostatic perturbations propagating in the perpendicular direction with respect to the applied magnetic field in a planar E×B amplifier in which the operating electric field is inhomogeneous. For ωc>ωp, the massless guiding center limit is taken to obtain the wave equation for the analysis of the electrostatic instability encountered in the planar E×B amplifier. In this work the plasma density profile is assumed to be a step function with vacuum boundaries between the cathode and the anode. The growth rates of the perpendicular electrostatic wave are obtained for the parameters such as the location and the thickness of the electron beam as well as the wave number.
Self-consistent circuit model for plasma source ion implantation
Authors: Chung Kyoung-Jae , Jung Soon-Wook , Choe Jae-Myung , Kim Gon-Ho , Hwang Y. S.
Published: February 2008

A self-consistent circuit model which can describe the dynamic behavior of the entire pulsed system for plasma source ion implantation has been developed and verified with experiments. In the circuit model, one-dimensional fluid equations of plasma sheath have been numerically solved with self-consistent boundary conditions from the external circuit model including the pulsed power system. Experiments have been conducted by applying negative, high-voltage pulses up to −10kV with a capacitor-based pulse modulator to the planar target in contact with low-pressure argon plasma produced by radio-frequency power at 13.56MHz. The measured pulse voltage and current waveforms as well as the sheath motion have shown good agreements with the simulation results.
Test results of the proton injector of the Proton Engineering Frontier Project accelerator
Authors: Kwon Hyeok-Jung , Kim Han-Sung , Cho Yong-Sub
Published: February 2008

One of the goals of the Proton Engineering Frontier Project (PEFP) is to develop a 100MeV proton accelerator. As a low energy part, a 20MeV proton linac has been developed and tested. The 20MeV accelerator consists of a 50keV proton injector, a 3MeV radio frequency quadrupole, and a 20MeV drift tube linac. The proton injector includes a duoplasmatron type ion source and a low energy beam transport. After a preliminary test with 1mA peak current of the 20MeV accelerator, the proton injector was modified with the goal of increasing the 20MeV beam current up to 20mA peak value. The modifications include the reduction of the proton injector length, installation of a beam current monitor, and electron trap and supplement of the solenoid and steering magnets. In this paper, the modifications of the PEFP proton injector are described and initial test results are presented.
Physical and chemical sputterings of solid surfaces irradiated by ethanol cluster ion beams
Authors: Takaoka Gikan H. , Kawashita Masakazu , Okada Takeshi
Published: February 2008

In order to clarify the interactions of ethanol cluster ion beams with solid surfaces, sputtering processes for Si, SiO2 and Au surfaces were investigated by changing the acceleration voltage, the retarding voltage, and the incident angle. For the case of the normal incidence, the sputtered depth increased with increase of the acceleration voltage, and the sputtering ratio of Si to SiO2 was approximately 10 at an acceleration voltage of 9kV. Furthermore, with regard to the incident angle dependence, the sputtered depth for the Si surfaces had a maximum value at an incident angle between 10° and 60°. On the other hand, for the case of the physical sputtering by ethanol cluster ion irradiation on Au surfaces, the sputtered depth decreased with increase of the incident angle. With regard to the retarding voltage dependence, the sputtered depth had a maximum value for the Si surfaces, and the retarding voltage corresponding to the peak value changed according to the acceleration voltage applied. In addition, for the case of Au surfaces, the sputtered depth decreased with increase of the retarding voltage, and the physical sputtering was influenced by the minimum size of the cluster.
Incorporating postleap checks in tau-leaping
Authors: Anderson David F.
Published: February 2008

By explicitly representing the reaction times of discrete chemical systems as the firing times of independent, unit rate Poisson processes, we develop a new adaptive tau-leaping procedure. The procedure developed is novel in that accuracy is guaranteed by performing postleap checks. Because the representation we use separates the randomness of the model from the state of the system, we are able to perform the postleap checks in such a way that the statistics of the sample paths generated will not be biased by the rejections of leaps. Further, since any leap condition is ensured with a probability of one, the simulation method naturally avoids negative population values.
An electron cyclotron resonance ion source based low energy ion beam platform
Authors: Sun L. T. , Shang Y. , Ma B. H. , Zhang X. Z. , Feng Y. C. , Li X. X. , Wang H. , Guo X. H. , Song M. T. , Zhao H. Y. , Zhang Z. M. , Zhao H. W. , Xie D. Z.
Published: February 2008

To satisfy the requirements of surface and atomic physics study in the field of low energy multiple charge state ion incident experiments, a low energy (10eV∕q–20keV∕q) ion beam platform is under design at IMP. A simple test bench has been set up to test the ion beam deceleration systems. Considering virtues such as structure simplicity, easy handling, compactness, cost saving, etc., an all-permanent magnet ECRIS LAPECR1 [Lanzhou all-permanent magnet electron cyclotron resonance (ECR) ion source No. 1] working at 14.5GHz has been adopted to produce intense medium and low charge state ion beams. LAPECR1 source has already been ignited. Some intense low charge state ion beams have been produced on it, but the first test also reveals that many problems are existing on the ion beam transmission line. The ion beam transmission mismatches result in the depressed performance of LAPECR1, which will be discussed in this paper. To obtain ultralow energy ion beam, after being analyzed by a double-focusing analyzer magnet, the selected ion beam will be further decelerated by two afocal deceleration lens systems, which is still under design. This design has taken into consideration both ions slowing down and also ion beam focusing. In this paper, the conceptual design of deceleration system will be discussed.
Production of large area low-energy electron beams by extraction from a multidipolar plasma (abstract)
Authors: Lacoste A. , Béchu S. , Maulat O. , Pelletier J. , Arnal Y.
Published: February 2008

The production of large area low-energy electron beams by extraction from a low-pressure plasma is investigated. In order to extract electron beams of given energy from a plasma, the plasma potential, as referred to wall potential, must be stationary and independent of the extraction potential. The macroscopic neutrality of the plasma therefore requires that reactor walls are metallic and that extraction to reactor wall area ratio is less than the square root of the electron to ion mass ratio. These specifications are fulfilled in the source presented in this study where the plasma is produced in a cylindrical chamber by seven elementary dipolar sources distributed over the end flange of a cylindrical chamber opposite to the extraction grid. The plasma is sustained at electron cyclotron resonance by microwaves at 2.45GHz in the chamber whose wall potential (extraction potential) can be varied from 0to−200V with respect to the ground potential. The electron beam extracted from the plasma through the grid then expands in a second chamber whose walls are set at the ground potential. The details of the experimental arrangement built on the above specifications are presented and the characteristics of argon plasmas measured by using cylindrical Langmuir probes as a function of the extraction voltage. In the same way, electron beam characteristics in the ground potential chamber are also measured using both planar and cylindrical Langmuir probes. In particular, variations in plasma and beam potentials are simultaneously determined as a function of extraction voltage. Current-voltage extraction characteristics are also reported, which show that electron beams in the ampere range can be extracted between 0 and −200V. The experimental results are in good agreement with the expected electron source behavior.
Neutral beam injection with an improved accelerator for LHD
Authors: Tsumori K. , Osakabe M. , Kaneko O. , Takeiri Y. , Nagaoka K. , Oka Y. , Ikeda K. , Shibuya M. , Asano E. , Komada S. , Kondo T. , Sato M.
Published: February 2008

The beam profiles, port-through, rates and injection powers obtained with an improved accelerator with the multislot grounded grid are described. The accelerator has a combination of a steering grid with racetrack shaped aperture and multislot grounded grid to improve the beam optics. The optimal beam optics is obtained at the voltage ratio of 16.5–16.8, and the profiles are well fit by superposing multibeamlets with the divergent angles of 5.0 and 7.2mrad along the direction parallel to the long and short axes of the slots of grounded grid. By adopting the racetrack shaped steering grid, the port-through rate increases from 34% to 38%, and the maximum injection power reaches 6MW∕187keV.
Microscopic diffusion processes of NO on the Pt(997) surface
Authors: Tsukahara Noriyuki , Mukai Kozo , Yamashita Yoshiyuki , Yoshinobu Jun
Published: February 2008

The microscopic diffusion processes of NO molecules on Pt(997) at low coverage were investigated using time-resolved infrared reflection absorption spectroscopy (TR-IRAS). When NO molecules adsorb on Pt(997) at low temperature, each molecule transiently migrates on the surface from the first impact point to a possible adsorption site. At 11K, the molecules are trapped at four adsorption sites on Pt(997): the on-top sites on the (111) terrace (OT), the hollow sites on the (111) terrace (HT), the bridge sites at the step (BS) and the hollow sites at the step downstream (HS). Based on the initial population ratio for these sites, the mean lateral displacement by transient migration is estimated to be 4.1Å. By heating the surface to 45K, the HS species migrate up to the BS sites; the migration barrier is roughly estimated to be 120meV. In the temperature range from 70to77K, TR-IRAS measurements were carried out to observe the site change of OT species to the adjacent HT sites at isothermal conditions; the activation barrier and the preexponential factor are estimated to be 200meV and 2.0×1011s−1, respectively. In the temperature range from 100to110K, the HT species migrate across the terrace and finally reach the BS sites. The activation barrier between the HT sites and the preexponential factor are estimated to be 290meV and 6.5×1011s−1, respectively, from the TR-IRAS data together with kinetic Monte Carlo simulations. On the whole, the quantitative microscopic picture of NO migration on Pt(997) has been established.
Effect of intermittent convection movements on voltammogram and current transients
Authors: Ribeiro M. C. , Rego L. G. C. , D’Ajello P. C. T.
Published: February 2008

The influence of intermittent convection movements on electrochemical voltammograms is investigated. When the bath temperature rises to 315K, the voltammograms exhibit irregular plateaus that differ for independent voltammetry scans, even when the setup is maintained under exactly the same conditions. In this paper, we show that such behavior can be caused by convection movements that develop in the electrolytic cell as a consequence of velocity fluctuations, since no bubbles or regular convective patterns are observed at this temperature. Theoretical current-potential curves for the heterogeneous deposition of metals on silicon electrodes is derived from a model consisting of a one-dimensional balance equation that includes diffusion, convection, and reaction through a time-dependent boundary condition. We obtain the current density associated with the adsorption of particles on the surface and, through this expression, we consider the effect of constant convective velocities on voltammograms. Finally, we examine the effect of random convective movements, described by a Monte Carlo algorithm that takes into account the random temporal fluctuations around a null convective current. The model predicts accentuated fluctuations on the current profiles, especially on the current plateaus that correspond to a stationary current regime. The validity of the theoretical model is checked against experimental data.
Fragmentation of ionized doped helium nanodroplets: Theoretical evidence for a dopant ejection mechanism
Authors: Bonhommeau D. , Lewerenz M. , Halberstadt N.
Published: February 2008

We report a theoretical study of the effect induced by a helium nanodroplet environment on the fragmentation dynamics of a dopant. The dopant is an ionized neon cluster Nen+ (n=4−6) surrounded by a helium nanodroplet composed of 100 atoms. A newly designed mixed quantum/classical approach is used to take into account both the large helium cluster zero-point energy due to the light mass of the helium atoms and all the nonadiabatic couplings between the Nen+ potential-energy surfaces. The results reveal that the intermediate ionic dopant can be ejected from the droplet, possibly with some helium atoms still attached, thereby reducing the cooling power of the droplet. Energy relaxation by helium atom evaporation and dissociation, the other mechanism which has been used in most interpretations of doped helium cluster dynamics, also exhibits new features. The kinetic energy distribution of the neutral monomer fragments can be fitted to the sum of two Boltzmann distributions, one with a low kinetic energy and the other with a higher kinetic energy. This indicates that cooling by helium atom evaporation is more efficient than was believed so far, as suggested by recent experiments. The results also reveal the predominance of Ne2+ and HeqNe2+ fragments and the absence of bare Ne+ fragments, in agreement with available experimental data (obtained for larger helium nanodroplets). Moreover, the abundance in fragments with a trimeric neon core is found to increase with the increase in dopant size. Most of the fragmentation is achieved within 10 ps and the only subsequent dynamical process is the relaxation of hot intermediate HeqNe2+ species to Ne2+ by helium atom evaporation. The dependence of the ionic fragment distribution on the parent ion electronic state reached by ionization is also investigated. It reveals that HeqNe+ fragments are produced only from the highest electronic state, whereas HeqNe2+ fragments originate from all the electronic states. Surprisingly, the highest electronic states also lead to fragments that still contain the original ionic dopant species. A mechanism is conjectured to explain this fragmentation inhibition.
The effect of matrix structure on the diffusion of fluids in porous media
Authors: Sung Bong June , Yethiraj Arun
Published: February 2008

The effect of matrix structure on the transport properties of adsorbed fluids is studied using computer simulations and percolation theory. The model system consists of a fluid of hard spheres diffusing in a matrix of hard spheres fixed in space. Three different arrangements of the fixed spheres, random, templated, and polymeric, are investigated. For a given matrix volume fraction the diffusion coefficient of the fluid, D, is sensitive to the manner in which the matrix is constructed, with large differences between the three types of matrices. The matrix is mapped onto an effective lattice composed of vertices and bonds using a Voronoi tessellation method where the connectivity of bonds is determined using a geometric criterion, i.e., a bond is connected if a fluid particle can pass directly between the two pores the bond connects, and disconnected otherwise. The percolation threshold is then determined from the connectivity of the bonds. D displays universal scaling behavior in the reduced volume fraction, i.e., D∼(1−ϕm∕ϕc)γ, where ϕm is the matrix volume fraction and ϕc is the matrix volume fraction at the percolation threshold. We find that γ≈2.2, independent of matrix type, which is different from the result γ≈1.53 for diffusion in lattice models, but similar to that for conduction in Swiss cheese models. Lattice simulations with biased hopping probabilities are consistent with the continuous-space simulations, and this shows that the universal behavior of diffusion is sensitive to details of local dynamics.
Hyperspherical nuclear motion of H3+ and D3+ in the electronic triplet state, aΣu+3
Authors: Ferreira Tiago Mendes , Alijah Alexander , Varandas António J. C.
Published: February 2008

The potential energy surface of H3+ in the lowest electronic triplet state, aΣu+3, shows three equivalent minima at linear nuclear configurations. The vibrational levels of H3+ and D3+ on this surface can therefore be described as superimposed linear molecule states. Owing to such a superposition, each vibrational state characterized by quantum numbers of an isolated linear molecule obtains a one- and a two-dimensional component. The energy splittings between the two components have now been rationalized within a hyperspherical picture. It is shown that nuclear motion along the hyperangle ϕ mainly accounts for the splittings and provides upper bounds. This hyperspherical motion can be considered an extension of the antisymmetric stretching motion of the individual linear molecule.
Explicit state vector for Torres-Vega–Frederick phase space representation and its statistical behavior
Authors: Hu Li-yun , Fan Hong-yi , Lu Hai-liang
Published: February 2008

We find the explicit state vector for Torres-Vega–Frederick phase space representation [Go. Torres-Vega and J. H. Frederick, J. Chem. Phys. 98, 3103 (1993)], denoted by ∣Γ⟩. This set of states make up a complete and nonorthogonal representation. The Weyl ordered form of ∣Γ⟩⟨Γ∣ is derived, which can clearly exhibit the statistical behavior of marginal distribution of ∣Γ⟩⟨Γ∣. The minimum uncertainty relation for ∣Γ⟩ is demonstrated, which shows it being a coherent squeezed state.
Scintillator based energetic ion loss diagnostic for the National Spherical Torus Experiment
Authors: Darrow D. S.
Published: February 2008

A scintillator based energetic ion loss detector has been built and installed on the National Spherical Torus Experiment (NSTX) [Synakowski et al., Nucl. Fusion 43, 1653 (2000)] to measure the loss of neutral beam ions. The detector is able to resolve the pitch angle and gyroradius of the lost energetic ions. It has a wide acceptance range in pitch angle and energy, and is able to resolve the full, one-half, and one-third energy components of the 80keV D neutral beams up to the maximum toroidal magnetic field of NSTX. Multiple Faraday cups have been embedded behind the scintillator to allow easy absolute calibration of the diagnostic and to measure the energetic ion loss in several ranges of pitch angle with good time resolution. Several small, vacuum compatible lamps allow simple calibration of the scintillator position within the field of view of the diagnostic’s video camera.
Basis set sampling in the method of coupled coherent states: Coherent state swarms, trains, and pancakes
Authors: Shalashilin Dmitrii V. , Child Mark S.
Published: February 2008

The paper provides a systematic account of simple sampling techniques used in the multidimensional quantum dynamical method of coupled coherent states. For the sampling techniques based on a Gaussian distribution, it is noticed that faster convergence is achieved if “compression” of the basis set decreases as the basis size is increased. Good results are obtained for the autocorrelation functions of wave packets propagated in Henon-Heiles potentials with up to 32 degrees of freedom. Further test calculations are performed by employing trains of coherent states sampled on the same classical trajectory with successive time delays.
Configurational stability of patterned magnetic nanosystems
Authors: Pardavi-Horvath M. , Si P. E. , Ross C. A. , Castaño F. J. , Ng B. G. , Moralejo S.
Published: February 2008

The stability of the magnetic state of individual elements within patterned nanoarrays depends on the balance of interelement interactions and shape anisotropy. These effects were studied by ferromagnetic resonance and alternating gradient magnetometry to characterize large area hexagonal arrays of 20nm thick 190nm wide elliptical Permalloy elements with aspect ratios 1.6 and 3.6, made using interference lithography, electron beam evaporation, and lift-off processing. The in-plane anisotropy of short ellipses is 42mT, as compared to the 86mT in-plane anisotropy of ellipses twice as long. The out-of-plane anisotropy field is about 650mT, similar for both systems. Separation dependence of the interelement interactions was modeled numerically. The significant contribution from far neighbors to the interaction field is about 20mT, comparable to the switching field of soft magnetic materials.
The interaction between a monolayer of single-molecule magnets and a metal surface
Authors: Barraza-Lopez Salvador , Avery Michael C. , Park Kyungwha
Published: February 2008

We calculate within density functional theory (DFT) and the LSDA+U formalism the electronic properties of a nanostructure in which single-molecule magnets Mn12 are adsorbed via thiol groups onto the Au(111) surface. Our DFT calculation shows 1.23 electrons being transferred from the surface to the Mn12 molecule, dominated by the tail on the electronic charge distribution from the gold slab. LSDA+U calculations reveal that the on-site Coulomb repulsion U does not alter the direction of the electronic charge transfer obtained from DFT, because the gold Fermi level still lies above the lowest unoccupied molecular orbital (LUMO). The U term opens up the energy gap between the highest occupied molecular orbital (HOMO) and the LUMO for an isolated standard Mn12 but it minimally affects the gap for a sulfur-terminated Mn12.
High performance isotropic Sm–(Co,Fe)–C and Sm–(Co,Fe,Mn)–C magnets by melt spinning
Authors: Sultana D. , Gabay A. M. , Hadjipanayis G. C.
Published: February 2008

The magnetic properties and microstructure of melt-spun ribbons with a composition Smx(Co1−yMy)100−x−zCz for M=Fe or Fe+Mn and x=10–15, y=0–0.375, and z=0–6 have been studied systematically. The results indicate a strong dependence of the microstructure on the addition of C. The grain size decreases from few 100nm to below 20nm with addition of C. On the other hand, addition of Fe and Mn modifies both the saturation magnetization and the magnetocrystalline anisotropy. The Mn addition results in a coercivity of 11.7kOe for Sm12(Co0.75Fe0.185Mn0.065)86.5C1.5 ribbons spun at 40m∕s. The highest remanence of 102emu∕g was obtained in Sm12(Co0.75Fe0.25)83.5C4.5 ribbons spun at 50m∕s. A (BH)max of 14.7MGOe with a coercivity of 5.6kOe was obtained in as spun Sm13(Co0.75Fe0.25)82.5C4.5 ribbons.