Particle Transport Effects Research Articles

Transport study on development of fuels and ash density profiles in start-up phase of JA DEMO using TASK code

This study has investigated the effect of particle transport of fuel ions (deuterium (D) and tritium (T)) and helium ash (He) as well as their heat transport on the nuclear fusion output in a DEMO reactor. The result was assessed and possible start-up scenarios for DEMO was discussed. In the study, the integrated tokamak modeling code, TASK/TR is used, and for turbulent transport model, the current diffusive ballooning mode (CDBM) has been utilized to investigate the hydrogen isotope effect on the fusion power output. Also, the effect of He build up has been discussed by introducing the mass-dependent enhancement of He transport. The result has suggested that efficient fueling of D and T to the core region of burning plasma with internal transport barrier is difficult without anomalous inward pinch, and helium build up is significant when only neoclassical and CDBM type of turbulent transport is considered for particle transport.

Particle Acceleration and Their Escape into the Heliosphere in Solar Flares with Open Magnetic Field

Energetic particle populations in the solar corona and in the heliosphere appear to have different characteristics even when produced in the same solar flare. It is not clear what causes this difference: properties of the acceleration region, the large-scale magnetic field configuration in the flare, or particle transport effects, such as scattering. In this study, we use a combination of magnetohydrodynamic and test-particle approaches to investigate magnetic reconnection, particle acceleration, and transport in two solar flares: an M-class flare on 2013 June 19, and an X-class flare on 2011 September 6. We show that in both events, the same regions are responsible for the acceleration of particles remaining in the coronal and being ejected toward the heliosphere. However, the magnetic field structure around the acceleration region acts as a filter, resulting in different characteristics (such as energy spectra) acquired by these two populations. We argue that this effect is an intrinsic property of particle acceleration in the current layers created by the interchange reconnection, and therefore, may be ubiquitous, particularly, in noneruptive solar flares with substantial particle emission into the heliosphere.

Magnetic connectivity and solar energetic proton event intensity profiles at deka-MeV energy

We present an analysis of the time-intensity profiles of 25 solar energetic proton events at 18.2 MeV, modelled by fitting an analytical function form (a modified Weibull function) to the observed intensities. Additionally relying on previous work that characterized the magnetic connectivity between the event-related solar flare and the observer in these events with three angular parameters, we investigate the fit function parameters, the connectivity parameters, and the iron-to-carbon ratio of the events for dependencies and correlations. We find that the fit parameter controlling the basic shape of the profile (parameter a) is not clearly dependent on the connectivity parameters or the Fe/C ratio, suggesting that the profile shapes of neither well and weakly connected nor generally “impulsive” and “gradual” events differ systematically during the early stages of the event at 1 AU. In contrast, the time scaling of the fit function (parameter b) is at least moderately correlated with both the magnetic connectivity parameters and the Fe/C ratio, in that well-connected and iron-rich events are typically shorter in relative duration than weakly connected and nominal-abundance events; intensity rise times display a similar correlation with the connectivity parameters. We interpret the former result as following from the combined effect of various transport processes acting on the particles in interplanetary space, while the latter is essentially consistent with established knowledge regarding the observed dependence of the time-intensity profile shapes of solar energetic particle events on their magnetic connectivity and heavy ion abundances. The desirability of modelling the particle transport effects in detail and extending the analysis to cover higher energies is indicated.

A spectroscopic inference and SOLPS-ITER comparison of flux-resolved edge plasma parameters in detachment experiments on TCV

This work presents the use of a collisional-radiative model to infer plasma parameters from 2D emissivities of several deuterium Balmer lines. The emissivities were obtained by MANTIS, an absolutely calibrated, ten-camera imaging polychromator with spatial resolution, up to 800 Hz frame rate, viewing the lower divertor tangentially. Our analysis of those image frames generates 2D maps of plasma parameters such as electron density, temperature, neutral atomic density and the reaction rates for ionisation, recombination and charge exchange as a function of time. The analysis is compared and validated against a SOLPS-ITER simulation accounting for drifts. The results are compared against the simulation in 2D, radial and poloidal profiles to probe the numerous effects of particle transport in the scrape-off layer, in particular in the approach to detached divertor leg conditions. The inferred inner divertor leg radial profiles of the electron density and temperature were consistent with the SOLPS-ITER predictions. A significant transport of particles to the private flux region is found experimentally, that is not captured in the simulation. The simulation diverges from the experiment at the outer divertor target, where the plasma emission appears to be consistent with the emission driven by plasma–molecule interactions. Our analysis also shows prospects for aiding the power exhaust control efforts by potentially providing an optical tracking of the particle balance in the divertor.

Multiband nonthermal radiative model of pulsar wind nebulae: Study of the effects of advection and diffusion

Aims. Nonthermal radiation properties of pulsar wind nebulae (PWNe) are studied in the frame of a time-dependent model with particle advection and diffusion. Methods. The dynamical and radiative evolution of a PWN was self-consistently solved in the model. The time-dependent lepton (electrons and positrons) equation is described by particle injection, advection, diffusion, adiabatic loss, and radiative loss. Nonthermal emission from a PWN is mainly produced by the relativistic leptons through synchrotron radiation and inverse Compton process. Results. The effect of particle transport including advection and diffusion was analyzed, showing that the particle transport process induces a decrease in the nebula flux, and that the total flux decreases with the increase in advection velocity and diffusion coefficient. Meanwhile, the particle transport processes may play an important role in modifying the spectrum of the emitted radiation at X-ray and TeV wavelengths, but only have slightly effects in radio and GeV bands. We applied our model to the three PWNe Crab nebula, 3C 58, and G54.1+0.3, and observed that the spectral energy distributions of photon emissions from the three PWNe are reproduced well. Our results indicate that (i) the particle cooling processes are dominated by adiabatic loss in lower-energy bands and synchrotron loss dominates for the higher-energy particles; and (ii) the particle transport processes are advection dominated, and a slow diffusion may occur within the Crab nebula, 3C 58, and G54.1+0.3.

Predicting the rotation profile in ITER

Determining the toroidal rotation for future tokamaks like ITER is a challenging and important problem. By combining empirical scalings for the intrinsic rotation at the top of the pedestal with the expected neutral beam torque and modeling of momentum transport, the toroidal rotation profile for ITER is predicted with TGYRO using TGLF (SAT0 and SAT1). On axis rotation exceeds 20 krad s−1 and the shear is significant enough to reduce turbulent transport and significantly increase confinement and fusion power when comparing to cases that ignore the effect of rotation. The prediction of the rotation at the top of the pedestal is made with increased confidence due to experiments and modeling in DIII-D that have determined the importance of fast-ion and neutral particle transport effects on intrinsic rotation at this location. In particular, the effect of neutral particles on momentum transport in the pedestal region is found to be insignificant.

On benchmarking of simulations of particle transport in ITER

We report results of benchmarking of core particle transport simulations by a collection of codes widely used in transport modelling of tokamak plasmas. Our analysis includes formulation of transport equations, difference between electron and ion solvers, comparison of modules of the pellet and edge gas fuelling on the ITER baseline scenario. During the first phase of benchmarking we address the particle transport effects in the stationary phase. Firstly, simulations are performed with identical sources, sinks, transport coefficients, and boundary conditions prescribed in the flattop H-mode phase. The transformation of ion particle transport equations is introduced so to directly compare their results to electron transport solvers. Secondly, the pellet fuelling models are benchmarked in various conditions to evaluate the dependency of the pellet deposition on the pellet volume, injection side, pedestal, and separatrix parameters. Thirdly, edge gas fuelling is benchmarked to assess sensitivities of source profile predictions to uncertainties in plasma conditions and detailed model assumptions. At the second phase, we address particle transport effects in the time-evolving plasma including the current ramp-up to the ramp-down phase. The ion and the electron solvers are benchmarked together. Differences between the simulation results of the solvers are investigated in terms of equilibrium, grid resolution, radial coordinate, radial grid distribution, and plasma volume evolution term. We found that the selection of the radial coordinate can yield prominent differences between the solvers mainly due to differences in the edge grid distribution. The simulations reveal that electron and ion solvers predict noticeably different density peaking for the same diffusion and pinch velocity while with the peaked profile of helium, expected in fusion reactors. The fuelling benchmarking shows that gas puffing is not efficient for core fuelling in H-modes and density control should be done by the high field side pellet injection in contrast to present machines.

Heat Transfer Modulation by Inertial Particles in Particle-Laden Turbulent Channel Flow

We study the effects of particle-turbulence interactions on heat transfer in a particle-laden turbulent channel flow using an Eulerian–Lagrangian simulation approach, with direct numerical simulation (DNS) for turbulence and Lagrangian tracking for particles. A two-way coupling model is employed in which the momentum and energy exchange between the discrete particles and the continuous fluid phase is fully taken into account. Our study focuses on the modulations of the temperature field and heat transfer process by inertial particles with different particle momentum Stokes numbers (St), which in a combination of the particle-to-fluid specific heat ratio and the Prandtl number results in different particle heat Stokes numbers. It is found that as St increases, while the turbulent heat flux decreases due to the suppression of wall-normal turbulence velocity fluctuation, the particle feedback heat flux increases significantly and results in an increase in the total heat flux. The particle thermal feedback effect is illustrated using the instantaneous structures and statistics of the flow and temperature fields. The mechanisms of heat transfer modulation by inertial particles are investigated in detail. The budget of turbulent heat flux is examined. Moreover, by taking advantage of the ability of numerical simulation to address different momentum and heat processes separately, we investigate in detail the two processes of particles affecting heat transfer for the first time, namely the direct effect of particle thermal feedback to the fluid (i.e., heat feedback) and the indirect effect of the modulation of turbulent velocity field induced by the particles (i.e., momentum feedback). It is found that the contribution of heat transfer from turbulent convection is reduced by both heat and momentum feedback due to the decrease of the turbulent heat flux. The contribution of heat transfer from particle transport effects is barely influenced by the momentum feedback, even if St is large and is mainly affected by the heat feedback. Our results indicate that both heat and momentum feedback are important when the particle inertia is large, suggesting that both feedback processes need to be taken into account in computation and modeling.

Thick Target Neutron Production on Aluminum and Copper by 40 MeV Deuterons**Supported by the Project of China Academy of Engineering Physics under Grant No 2013B0103015.

The thick target neutron yields (TTNYs) of deuteron-induced reaction on Al and Cu isotopes are analyzed by combining the improved nuclear models and particle transport effects. The modified Glauber model is employed mainly to produce the peak of double differential cross section for the breakup process, and the exciton model and the Hauser–Feshbach theory are used for the statistical processes. The thin-layer accumulation method is used to calculate the TTNYs considering the neutron attenuation effects in the target. The calculated results are compared with the existing experimental data, and the analysis method can predict the TTNY data well at the deuteron energy of 40 MeV.

An empirical model of ion plasma in the inner magnetosphere derived from CRRES/MICS measurements

AbstractWe describe an empirical model of energetic ion plasma (∼20–400 keV/q) that is constructed from measurements taken by the Magnetospheric Ion Composition Spectrometer (MICS) instrument that flew on the CRRES spacecraft. This is a unique data set in that it provides energetic ion composition in the near‐equatorial ring current region during a very active solar maximum. The model database is binned by energy, equatorial pitch angle, L shell, and magnetic local time and provides unidirectional, differential number fluxes of the major ionic constituents of the inner magnetosphere, such as protons (H+), singly charged oxygen (O+), and singly charged helium (He+). The H+ and O+ model fluxes are examined in detail and are consistent with well‐known particle transport effects (e.g., adiabatic heating). We also validate these model fluxes against a number of other ion plasma models that are available in the literature. The primary finding is the elevated levels of energetic O+ flux during the CRRES era. We attribute this to a solar cycle effect, related to the enhanced upwelling and oxygen outflow from the ionosphere that occurs during solar maximum, driven by elevated solar extreme ultraviolet radiation. We briefly discuss the implications that the enhanced O+ environment during the CRRES era may have for other results derived from CRRES observations (e.g., statistical wave distributions).

Energy dependence of SEP electron and proton onset times

AbstractWe study the large solar energetic particle (SEP) events that were detected by GOES in the >10 MeV energy channel during December 2006 to March 2014. We derive and compare solar particle release (SPR) times for the 0.25–10.4 MeV electrons and 10–100 MeV protons for the 28 SEP events. In the study, the electron SPR times are derived with the time‐shifting analysis (TSA) and the proton SPR times are derived using both the TSA and the velocity dispersion analysis (VDA). Electron anisotropies are computed to evaluate the amount of scattering for the events under study. Our main results include (1) near‐relativistic electrons and high‐energy protons are released at the same time within 8 min for most (16 of 23) SEP events. (2)There exists a good correlation between electron and proton acceleration, peak intensity, and intensity time profiles. (3) The TSA SPR times for 90.5 MeV and 57.4 MeV protons have maximum errors of 6 min and 10 min compared to the proton VDA release times, respectively, while the maximum error for 15.4 MeV protons can reach to 32 min. (4) For 7 low‐intensity events of the 23, large delays occurred for 6.5 MeV electrons and 90.5 MeV protons relative to 0.5 MeV electrons. Whether these delays are due to times needed for the evolving shock to be strengthened or due to particle transport effects remains unsolved.

Isotope effects on particle transport in the Compact Helical System

The hydrogen isotope effects of particle transport were studied in the hydrogen and deuterium dominant plasmas of the Compact Helical System (CHS). Longer decay time of electron density after the turning-off of the gas puffing was observed in the deuterium dominant plasma suggesting that the recycling was higher and/or the particle confinement was better in the deuterium dominant plasma. Density modulation experiments showed the quantitative difference of the particle transport coefficients. Density was scanned from 0.8 × 1019 m−3 to 4 × 1019 m−3 under the same magnetic field and almost the same heating power. In the low density regime (line averaged density < 2.5 × 1019 m−3), the lower particle diffusivity and the larger inwardly directed core convection velocity was observed in the deuterium dominant plasma, while in the high density regime (line averaged density >2.5 × 1019 m−3) no clear difference was observed. This result indicates that the isotope effects of particle transport exist only in the low density regime. Comparison with neoclassical transport coefficients showed that the difference of particle transport is likely to be due to the difference of turbulence driven anomalous transport. Linear character of the ion scale turbulence was studied. The smaller linear growth rate qualitatively agreed with the reduced particle transport in the deuterium dominant plasma of the low density regime.

A NEW APPROACH TO ASSESS BED STABILITY RELEVANT FOR INVERTEBRATE COMMUNITIES IN UPLAND STREAMS

ABSTRACTComposition and structure of lotic ecosystems can be affected by substrate instability. Consequently stream ecologists have used various methods to determine bed stability characteristics. However, the link between community composition and these measurements varies because benthic biota often responds to combinations of bed stability characteristics. This paper presents a protocol to determine reach‐scale stream bed stability in mountain streams which is relevant for invertebrate communities (Stream Bed Stability for Invertebrates, SBSI). The approach is calibrated on community composition response to bed stability but does not measure any single bed stability characteristic per se. It consists of 13 parameters that are assessed once at each reach with minimal instrumentation and low interference with the substrate. These 13 parameters cover aspects of sediment supply from banks, transport capacity and substrate erodibility as well as effects of particle transport on channel bottom structures, substrate assemblage and single grains. Application of the SBSI protocol improved the relationship between bed stability and community diversity compared to when conventional bed stability measures were employed. The SBSI protocol provides a cost‐effective and time‐effective assessment method for bed stability and its application can facilitate research on invertebrate community response to physical disturbance. Copyright © 2011 John Wiley &amp; Sons, Ltd.

The breakup of methane under ITER divertor hydrogen plasma conditions for carbon chemical erosion analysis with CH spectroscopy

Methane (CH4) was injected into the high density (ne ∼ 1020 m−3), low temperature (Te ∼ 1 eV) hydrogen plasma in Pilot-PSI to determine the CH A–X photon efficiency in this unexplored plasma regime. The effects of particle transport and particle reflection on the emission of directly excited CH under these plasma conditions were assessed with the 3D Monte Carlo code ERO. The simulations of the inverse photon efficiency showed a difference of ∼20% between full hydrocarbon sticking or no sticking (reflection). In addition it predicts that particle transport may lead to more than a factor of 10 increase. The measured inverse photon efficiency is however constant at 100 ± 30 for 0.1 < Te < 1.0 eV. The constancy is consistent with dissociative recombination of , and to produce excited CH instead of direct excitation. These results form a framework for in situ carbon erosion measurements in future fusion reactors such as ITER.

Effect of Particle Transport on the Measured Electron Cyclotron Current Drive Profile at High Relative Power Density

Recent experiments on the DIII-D tokamak have examined the effect of particle transport on the electron cyclotron current drive (ECCD) profile using measurements of the magnetic field pitch angles by motional Stark effect polarimetry. While previous ECCD studies on DIII-D did not observe any clear effects of transport, these new experiments at high ECCD power, low density, and radiation temperatures above 20 keV clearly demonstrate that the ECCD profile can be reduced and broadened compared to the Fokker-Planck code CQL3D predictions assuming no radial transport. A diffusion coefficient of [approximate]0.4 m2 /s is required in CQL3D to reproduce the experimental ECCD profile at high relative power densities, while smaller diffusion coefficients are needed at low relative power densities. This level of transport is comparable to the effective particle transport rate needed to maintain the density profile but an order of magnitude less than the electron thermal diffusivity. While radial transport of the current-carrying electrons is potentially detrimental for applications that rely on strong localization of the noninductive current, this effect should be negligible on ITER owing to its large size and low relative power density.

Large-eddy simulation of particle-laden turbulent flows

A large-eddy-based methodology for the simulation of turbulent sprays is discussed. The transport equations for the spatially filtered gas phase variables, in which source terms accounting for the droplet effects are added, are solved together with a probabilistic description of the liquid phase. The probabilistic approach for the liquid phase is based on the transport equation for the spatially filtered joint probability density function of the variables required in order to describe the state of the liquid phase. In this equation, unclosed terms representing the filtered Lagrangian rates of change of the variables describing the spray are present. General modelling ideas for subgrid-scale (SGS) effects are proposed. The capabilities of the approach and the validity of the closure models, with particular with respect to the SGS dispersion, are investigated through application to a dilute particle-laden turbulent mixing layer. It is demonstrated that the formulation is able to reproduce very closely the measured properties of both the continuous and dispersed phases. The large-eddy simulation (LES) results are also found to be entirely consistent with the experimentally observed characteristics of droplet–gas turbulence interactions. Consistent with direct numerical simulation (DNS) studies of isotropic turbulence laden with particles where the entire turbulence spectrum is found to be modulated by the presence of particles, the present investigation, which comprises the effects of particle transport upon the large-scale vortical structures of a turbulent shear flow, highlights what appears to be a selective behaviour; few large-scale frequencies gain energy whereas the remaining modes are damped.

Conversion of bulk kinetic energy into radiation in AGNs and GRBs: Particle transport effects

We investigate the spatial structure of collisionless collision fronts in relativistic outflows interacting with ambient material. As a result of the interaction, ambient particles are picked up by the outflow and generate transverse plasma waves via streaming instabilities. Pick-up particle transport under the influence of self-generated turbulence inside such interaction regions is studied. We extend our previous momentum space modeling to include also a spatial dimension. We find that the following possibilities are consistent with quasi-linear equations of particle transport and wave generation: (i) if background waves have small intensities inside the outflow region, leading to inefficient scattering across the pitch-angle, θ , of $90^\circ$, particles are isotropized in the backward hemisphere (relative to the outflow velocity vector) and self-generated waves have a steep, ${\propto} k^{-3}$ wavenumber spectrum; (ii) if background waves have large intensities, enabling particles to cross $\theta=90^\circ$, particles can be fully isotropized. In case (i), however, the calculated self-generated wave amplitudes are close to the magnitude of the ordered field for reasonable choices of model parameters, giving the particles a chance to be scattered across the resonance gap by non-resonant processes. If the resonance gap is filled, a large fraction of the pick-up particles is expected to return to the upstream region, and an ultra-relativistic shock wave is predicted to form in front of the outflow, where the two relativistic particle populations (ambient and reflected) mix and form a relativistic plasma. Reflection of pick-up protons decreases the $\pi^0$-decay luminosity of relativistic outflows, leading to a need to update parameters of previous modeling. An example of outflow parameters reproducing typical TeV-blazar observations is presented.

Collimation of PetaWatt laser-generated relativistic electron beams propagating through solid matter

Particle-in-cell simulations aimed at developing methods to control the relativistic electron beam blowup observed in recent laser-plasma experiments are described. By radially layering vacuum gaps and/or dissimilar materials with varying ionizability, a negative radial gradient in plasma density would be formed. This gradient results in confining fields that can, in principle, confine the hot electron column to nearly the laser injection spot size. Fully kinetic ion dynamics are included, to account for heavy particle transport effects across interfaces. Potential applications include radiography, electron beam focusing, and perhaps beam collimation for fast ignition. Experiments are presently being planned to test this concept.

Iron Convection Zones in B, A, and F Stars

Stellar models, including all effects of atomic diffusion and radiative accelerations, are evolved from the pre-main sequence to the giant branch for stars of 1.3 to 4.0 M☉, with metallicity ranging from Z0 = 0.01 to 0.03. It is shown that radiative accelerations lead to the accumulation of iron-peak elements around 200,000 K; this increases the opacity and causes the appearance of Fe convection zones when macroscopic motions are not rapid enough to wipe out the effects of particle transport. The behavior of Fe convection zones and conditions for their appearance are studied in detail. Iron-peak convection zones appear naturally in all solar metallicity models more massive than 1.5 M☉. In the 1.5 M☉ model, it is present only for a fraction of the main-sequence lifetime, but in models without turbulence of 1.7 M☉ and more, the Fe convection zone rapidly develops after arrival on the main-sequence and remains until its end. For a metallicity of Z = 0.01, an Fe convection zone appears even in a 1.3 M☉ model. Moreover, the interaction between the diffusion velocities of different species leads to an accumulation of heavy elements around the convective core, causing semiconvection. A detached semiconvection zone develops in the 1.5 M☉ model. Finally, the surface abundances are calculated using a number of turbulence models and compared to observations of τ UMa in order to show how abundance anomalies may be used to test various turbulence models; the gravity at which abundance anomalies should be expected to disappear is determined. It is shown that in Am stars, the Ca underabundance should disappear during evolution at the same gravity as iron-peak overabundances.

Magnetic fluctuation driven cross-field particle transport in the reversed-field pinch

Electrostatic and electromagnetic fluctuations generally cause cross-field particle transport in confined plasmas. Thus core localized turbulence must be kept at low levels for sufficient energy confinement in magnetic fusion plasmas. Reversed-field pinch (RFP) equilibria can, theoretically, be completely stable to ideal and resistive (tearing) magnetohydrodynamic (MHD) modes at zero beta. Unstable resistive interchange modes are, however, always present at experimentally relevant values of the poloidal beta βθ. An analytical quasilinear, ambipolar diffusion model is here used to model associated particle transport. The results indicate that core density fluctuations should not exceed a level of about 1% for plasmas of fusion interest. Parameters of experimentally relevant stationary states of the RFP were adjusted to minimize growth rates, using a fully resistive linearized MHD stability code. Density gradient effects are included through employing a parabolic density profile. The scaling of particle diffusion [D(r)∝λ2n0.5T/aB, where λ is the mode width] is such that the effects of particle transport are milder in present day RFP experiments than in future reactor-relevant plasmas.