Local Mixing Model Research Articles

Nanoplastic Transport in Soil via Bioturbation by Lumbricus terrestris.

Plastic pollution is increasingly perceived as an emerging threat to terrestrial environments, but the spatial and temporal dimension of plastic exposure in soils is poorly understood. Bioturbation displaces microplastics (>1 μm) in soils and likely also nanoplastics (<1 μm), but empirical evidence is lacking. We used a combination of methods that allowed us to not only quantify but to also understand the mechanisms of biologically driven transport of nanoplastics in microcosms with the deep-burrowing earthworm Lumbricus terrestris. We hypothesized that ingestion and subsurface excretion drives deep vertical transport of nanoplastics that subsequently accumulate in the drilosphere, i.e., burrow walls. Significant vertical transport of palladium-doped polystyrene nanoplastics (diameter 256 nm), traceable using elemental analysis, was observed and increased over 4 weeks. Nanoplastics were detected in depurated earthworms confirming their uptake without any detectable negative impact. Nanoplastics were indeed enriched in the drilosphere where cast material was visibly incorporated, and the reuse of initial burrows could be monitored via X-ray computed tomography. Moreover, the speed of nanoplastics transport to the deeper soil profile could not be explained with a local mixing model. Earthworms thus repeatedly ingested and excreted nanoplastics in the drilosphere calling for a more explicit inclusion of bioturbation in nanoplastic fate modeling under consideration of the dominant mechanism. Further investigation is required to quantify nanoplastic re-entrainment, such as during events of preferential flow in burrows.

Neutron characterization analysis of the negative ion-based neutral beam verification facility for CFETR

Neutral beam injection heating with negative ion source is one of the important auxiliary heating system to achieve the high parameter steady- state operation goal for CFETR. A Negative ion-based Neutral Beam Verification Facility (NNBVF), a conceptual design for testing the engineering feasibility of neutral beam injection systems, is proposed for the Chinese Fusion Engineering Test Reactor (CFETR). The neutral-beam injectors used for CFETR will operate with deuterium. Inevitably, a large number of neutrons are generated between beam deuterons and deuterons implanted in the calorimeter. Neutron source is one of the important input data for radiation analysis. The neutron characterization on the calorimeter surface were explored by using a multiaperture Gaussian divergence model and the local mixing model. The total neutron intensity reach up to 1.2 × 1014 n/s for 400 keV deuteron beam. Initial results revealed that the aspect ratio of the neutron beam profile decreases with increasing beam divergence. The shape of the neutron profile on the calorimeter surface remains the same for the beam density distributions. This implies the neutron intensity emitted from the target is suitable for the beam density distribution diagnostic applications.

Neutron measurements at the ELISE neutral beam test facility and implications for neutron based diagnostics at SPIDER.

Along the route to the development of a neutral beam injector for ITER, the Padua based Source for Production of Ion of Deuterium Extracted from Rf plasma (SPIDER) and megavolt ITER injector and concept advancement facilities will make use of neutron diagnostics to quantify the homogeneity of the neutral beam profile by measuring the map of the neutron emission from the beam dump with the close-contact neutron emission surface mapping (CNESM) system. Neutrons are here produced from beam-target reactions between the deuterium beam and the deuterons previously adsorbed in the calorimeter. In order to aid the interpretation of the diagnostic data, a dedicated experiment on neutron emission from beam-target reactions with beam parameters approaching those expected at SPIDER has been performed at the Extraction from a Large Ion Source Experiment (ELISE) neutral beam test facility. The time trace of neutron emission has been measured using a calibrated liquid scintillator detector at increasing power densities on the target. Compared to calculations based on the local mixing model, a systematic discrepancy was observed exceeding the statistical accuracy of the measurements and increasing as a linear function of the power density. The data are used to derive an empirical temperature dependent correction for applications to neutron measurements at SPIDER.

Experimental investigation of beam-target neutron emission at the ELISE neutral beam test facility

A dedicated experiment on beam-target neutron emission has been performed at the ELISE neutral beam test facility to investigate the accuracy of Local Mixing Model based calculations of neutron emission of relevance for the design of neutron diagnostics at the SPIDER neutral beam prototype. Compared to a previous experiment, we make use of fully developed infra-red and calorimetry diagnostics to include accurate data on beam profile and current in the calculations. Results show that variations of the neutron emission can be predicted with an accuracy better than 10%, which compares to 30% in our previous investigation. At the same time, convincing evidence of diffusion effects beyond the Local Mixing Model is identified in the data. Implications of these findings for neutron emission at SPIDER are discussed.

Neutron measurements from beam-target reactions at the ELISE neutral beam test facility.

Measurements of 2.5 MeV neutron emission from beam-target reactions performed at the ELISE neutral beam test facility are presented in this paper. The measurements are used to study the penetration of a deuterium beam in a copper dump, based on the observation of the time evolution of the neutron counting rate from beam-target reactions with a liquid scintillation detector. A calculation based on a local mixing model of deuterium deposition in the target up to a concentration of 20% at saturation is used to evaluate the expected neutron yield for comparison with data. The results are of relevance to understand neutron emission associated to beam penetration in a solid target, with applications to diagnostic systems for the SPIDER and MITICA Neutral Beam Injection prototypes.

A comparison of momentum mixing models for the planetary boundary layer

AbstractIn a convective planetary boundary layer (PBL), nonlocal closure PBL models represent the turbulent mixing as nonlocal mixing induced by large‐scale eddies in addition to local mixing between adjacent grid levels. Recently, the nonlocal mixing approach formulated with a counter‐gradient term has been developed for the momentum transport in many numerical models, such as the Weather Research and Forecasting (WRF) model and the UK Met Office Unified Model. This study reviews a few momentum mixing models, provides alternative mixing formulations to address the potential problem in momentum transport, and analyzes the wind and momentum flux profiles predicted by these momentum mixing models. The predictions for momentum mixing from two nonlocal closure PBL models, i.e., the Yonsei University (YSU) PBL model and the modified Frech and Mahrt (FMX) PBL model, and from one local closure PBL model (YSU‐local) are compared, using the WRF model. The YSU model, in which the nonlocal momentum flux is aligned with the surface wind shear, is found to often predict larger vertical wind shear than the local momentum mixing model YSU‐local in the interior of the convective PBL. In contrast, the FMX model produces more well‐mixed wind profiles that agree better with observations and large eddy simulations results than the YSU and YSU‐local models, especially when the sign of the bulk wind shear over the convective PBL is opposite to that of the surface wind shear.

Modeling Effects of Multinode Wells on Solute Transport

Long-screen wells or long open boreholes with intraborehole flow potentially provide pathways for contaminants to move from one location to another in a ground water flow system. Such wells also can perturb a flow field so that the well will not provide water samples that are representative of ground water quality a short distance away from the well. A methodology is presented to accurately and efficiently simulate solute transport in ground water systems that include wells longer than the grid spacing used in a simulation model of the system and hence are connected to multiple nodes of the grid. The methods are implemented in a MODFLOW-compatible solute-transport model and use MODFLOW's Multi-Node Well Package but are generic and can be readily implemented in other solute-transport models. For nonpumping multinode wells (used to simulate open boreholes or observation wells, for example) and for low-rate pumping wells (in which the flow between the well and the ground water system is not unidirectional), a simple routing and local mixing model was developed to calculate nodal concentrations within the borehole. For high-rate pumping multinode wells (either withdrawal or injection, in which flow between the well and the ground water system is in the same direction at all well nodes), complete and instantaneous mixing in the wellbore of all inflows is assumed.

Prediction of neutron source, tritium production and activation for long-pulse operation of the ITER neutral beam test facility

A local mixing model (LMM) (H.-D. Falter et al 1992 Proc. 17th Symp. on Fusion Technology (Rome, 1992) p 481) has been utilized to compute the evolution of the hydrogen isotope content within the implantation zone of the CuCrZr target material of the beam-stopping elements of the ITER neutral beam test facility (NBTF), together with the beam–target fusion reaction rate calculated by taking account of the slowing-down of the 1 MeV incoming projectile ion within the implantation layer. An important modification of the LMM code is to treat the tritium reaction product ions, resulting from D–D reactions, as a constituent of the incident beam. Although the treatment of tritium in the LMM is not ideal, this and other simplifying assumptions either do not significantly affect the predictions, or ensure conservatism in the results when used as input to the safety analysis of the facility. For example, it is shown that T → D ‘beam–target’ reactions always dominate over those of D → T, which overcomes the problem of uncertainty, in the model, of the distribution of tritium trapped within the implantation layer; in contrast there is little uncertainty that this region could approach deuterium saturation after <100 full-length (1 h duration) pulses. Using the computed sources as input, neutronics and activation calculations for the NBTF components have been carried out using the codes MCNP/FISPACT. As expected, long-pulse operational requirements lead to neutron activation and tritium production levels which have non-negligible but manageable radiological consequences.

Low-onset organic blue light emitting devices obtained by better interface control

We demonstrate that highly efficient single-layer light emitting devices (LED) can be realized by better control in the device production process, especially regarding the interface between the active material and the metal cathode. The device cross-section was investigated using Auger depth profiling and spectroscopy. Following this approach, LEDs with para-sexiphenyl (optical gap: 3.1 eV) as active medium and aluminum as cathode were realized, which emit blue light at a bias of only 3.5 V, when the aluminum deposition rate is drastically reduced, namely from 10 to below 1 Å/s. We find that the lower deposition rate of Al results in a decreased width of the interfacial region, where carbon, aluminum, and oxygen are intermixed. At the same time the relative oxygen concentration at the cathode interface is increased. However, the presence of oxygen does not lead to the formation of oxidized aluminum species, as verified by the Al local mixing model (LMM) Auger signal.

Simulation of transient wall pumping, fuelling effects and density control in tokamaks

A dynamical model has been developed which solves a time-dependent coupled system of equations for the (1) plasma density and (2) hydrogenic concentrations in the implantation layer of several representative plasma facing/limiting walls in tokamaks in order to examine the density behaviour during the transient phase of discharges as well as long pulse discharges, which are dominated by the wall pumping/fuelling characteristics. A 1-D plasma particle transport code including a model for the SOL and an analytical neutral transport model has been used to describe the plasma particle transport processes. The Local Mixing Model with some adaptations has been used to describe the behaviour of the hydrogenic concentrations in the walls. The new model significantly improves the simulation of a Tore Supra experiment with a transition from limiting the plasma on the outer limiter to limiting the plasma on the inner wall compared our previous, more simple model. Long pulse discharges in Tore Supra are examined by the more simplified model to identify the dominant mechanisms for the constant particle absorption mechanisms.

Modelling of hydrogen conditioning, retention and release in Tore Supra

A model based on a local mixing model has been previously developed at JET to explain the recovery of tritium after the first PTE experiment. This model is extended by a 0D plasma particle balance model and is applied to data from Tore Supra wall saturation experiments. With only two free parameters, representing the diffusion of hydrogen atoms and the volume recombination process between hydrogen atoms into molecules, the model can reproduce experimental data. The time evolution of the after-shot outgassing and the integral amount of particles recovered after the shot (assuming 13 m 2 of interacting surfaces between plasma and walls) are in good agreement with the experimental observations. The same set of parameters allows the model to simulate after-shot outgassing of five consecutive discharges. However, the model fails to predict the observed saturation of the walls by the plasma. Results from helium glow discharge (HeGD) can only be partially described. Good agreement with the experimental hydrogen release and its time evolution during HeGD is observed, but the model fails to describe the stability of a saturated graphite wall.

Measurement of H energy and flux by H→D isotope exchange in JFT-2M scrape-off layer

The energy and flux of H particles in the scrape-off layer (SOL) of JFT-2M were determined in H plasma discharges by measuring the retained amount of H incident on C edge probes as a function of number of discharges. The uncertainty in measuring H retention was avoided by using C samples implanted beforehand with D to a saturation level. The decrease in retained D was more easily measurable and was taken as the H retention. The validity of such replacement was examined with the local mixing model (LMM). By using a proper geometrical arrangement for probe samples the H + ions were separated from H 0 neutrals. At a specified halfway position in the SOL, about 30% of the total H density is found to consist of H + with a flux of (1.4±0.5)×10 16 H/cm 2 s while the remaining density (70%) is made up of H 0 having a flux of (3.3±1.2)×10 16 H/cm 2 s. In both cases, the particle energy is 30±9 eV.

Observation of helium gas discharge cleaning process for deuterium saturated graphite by A–U gauge

A gas analyzer based on the principle of ion backscattering has been developed to measure both pressures of helium and deuterium and is called A–U gauge. The A–U gauge is applied to observe a detrapping process of retained deuterium in a graphite (IG‐100 ) during rf‐glow discharge with helium gas. The detrapping behavior of D atoms from the graphite is discussed with a simplified local mixing model.

Study on hydrogen-isotope retention with collector probes in JFT-2M scrape-off layer

Hydrogen-isotope behavior in the scrape-off layer (SOL) of JFT-2M is investigated using the collector probe method. Using the Si probes, particles retained in the wall during the preceding Taylor-type discharge cleaning (TDC) are found to be reemitted during the subsequent ohmically heated discharges (OHDs) both with and without neutral beam injection of H 0. Hyrogen-isotope exchange observed in the C probes within the SOL of OHDs agrees with the local mixing model. However, it is found that the isotope exchange even occurs upon exposing the C probes to a TDC plasma at a relatively low temperature in spite of the low retention below the saturation level.

Isotopic exchange in hard amorphous carbonized layers

Hard amorphous films of a-C:H and a-C:D were deposited by a rf glow discharge in either CH4 or CD4 . By ion bombardment with protons and deuterons, the H/D exchange process was studied as function of the bombardment fluence by means of depth profile measurements. The local hydrogen and deuterium contents are not adding up to a constant ‘‘saturation’’ value: the local mixing model is not valid. Instead of that an initial depletion appears, which depends on the incident energy of the ions. At higher fluences, the total (H+D):C ratio tends to increase again, due to an increasing influence of the deposition process. This demonstrates a structural difference between a-C:H films and hydrogen-saturated layers of implanted carbon.

Meastri: A hybrid analytical/monte carlo code for particle transport in solids

We report on a hybrid analytic/Monte Carlo code for evaluating particle transport in solids. In this code, atomic concentrations in the near surface implanted region of a target material, { N k ( x, t)}, are propogated in time through the method of equivalent atomic stopping (MEAS). The implantation source profiles { P k ( x, t)}, initially obtained from the TRIM code, are propagated along with the { N k ( x, t)}. In addition, TRIM is invoked periodically to provide updated profiles { P k ( x, t)}, since the MEAS procedure includes only first order corrections to the { P k }. The code provides for simultaneous implantation of different atomic species and includes the local mixing model (LMM) for describing buildup to saturation and isotope exchange for implantation of hydrogen and/or other species. Since the MEAS procedure is analytic, both sputtering and diffusion are included in a natural way, and disparate time scales, e.g. for implantation, sputtering and diffusion, are easily accommodated. The code is particularly useful for high fluence implantation. Example calculations for simultaneous C and D implantation into graphite are compared with experimental data obtained from TFTR limiter tiles.

Ion-induced release of H and D implanted in Be, C, Si and SiC

Retention and ion-induced release of H and D implanted into Be, C, Si and SiC at the energy of 5–8 keV have been investigated at room temperature by elastic recoil detection analysis. Ion-induced release is induced both by low energy (5–20 keV) and high energy (2000 keV) 4He ions. For retention and isotopic replacement of H and D, the observed depth profiles show similar features as predicted by the Local Mixing Model. It is shown that the release behavior during 4He + irradiation at low energies is different from that for hydrogen bombardment. The release rate observed in C and SiC is much smaller than that in Be and Si. The release cross sections for high energy (2000 keV) 4He + bombardment are an order of magnitude larger than those for low energy bombardment (5–20 keV).

Hydrogen-ion-induced detrapping of implanted deuterium in graphite

The release of saturated 300 eV D + implants in graphite by 6–30 keV H + bombardment has been measured at temperatures between 295 and 473 K and fluences up to more than 3 × 10 18 H +-ions/cm 2. The decrease of retained D is found to be slower than exponential. Release rates decrease with increasing energy indicating that detrapping is primarily due to atomic collisons. A faster release is observed at higher target temperatures during H + bombardment and some enhancement persists if the target is annealed after deuterium implantation. The release of low energy implants by high energy ions cannot be described by the local mixing model. Instead, a model accounting for local molecular recombination and detrapping gives qualitative agreement with experimental dependence on H + energy. It cannot, however, explain the temperature dependence.

The local mixing model with sputtering/redeposition and projectile-implanted atom interaction

A previously discussed local mixing model (LMM) accurately describes the buildup of implanted hydrogen to saturation concentration and subsequent isotope exchange for implantation into a variety of materials of interest to the fusion community. Sputtering and surface deposition were not initially included in this model. We have now incorporated into the LMM the effects of sputtering and target-atom redepostion and have also included (in an approximate fashion) the slowing and scattering of implanted ions by previously implanted impurity species. This latter effect has important consequences for high fluence implants. Equations are derived which govern the evolution of the depth profiles of the implanted species and a formal solution is given for conditions which often are encountered in laboratory experiments. The time (fluence) dependence of the depth profile is discussed, and numerical results are compared with experiment for oxygen implantation into graphite at energies for which surface erosion is a significant effect. Solutions to the equations are also discussed for cases in which surface redeposition is significant. These latter solutions are pertinent to impurity and hydrogen isotope buildup in first wall components in operating fusion reactors.

高周波放電および直流グロー放電下のステンレス鋼真空壁における水素同位体交換脱離

The saturation and replacement of hydrogen isotopes implanted into 304 stainless steel vacuum wall at 300 K under RF discharge and RF-DC glow discharge has been studied from partial pressure measurements of desorbed isotopes. The bombarding ion flux on the wall are 12×1013 ions/cm2·s. The energy of H2+ or D2+ ion is determined by acceleration with space potential produced in discharge plasma, and is about 40 eV for RF discharge and 300 eV for RF-DC glow discharge. The hydrogen and deuterium saturation concentrations in the wall surface are 4.2×1015 and 1.3×1016 molecules/cm2 for ion bombardment of 40 eV and 300 eV, respectively. Nearly 90% of desorbed gas in isotopic exchange process is HD molecule. The desorption rate is expressed by inverse-exponential function with time. The time constant is proportional to isotope concentration and inversely proportional to bombarding ion flux.Experimental results show that the local mixing model (LMM) is still valid for such ion implantation in lower energy region less than 1 keV. But LMM must be corrected for explanation of the isotope exchange process under plasma ion bombardment at 40 eV. Two-component inverse-exponential functions are observed in the replacement. The function of component 2 is explained by adopting an effective trapping coefficient, although another function of component 1 can be described by LMM.