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Methanol synthesis is carried out in a catalytic, packed bed reactor, where the shape and size of catalyst play a critical role in dictating overall reactor performance. In the current study, the effect of five different catalyst shapes including cylindrical, ring, trilobe, wagon wheel, and spherical on the reactor performance was investigated by conducting particle‐scale computational fluid dynamics (CFD) simulations. The predictions of pressure drop, velocity, temperature, reactant distribution and product yield were analyzed. When the performance of the simulated shapes was compared at the same tube Reynolds number of 50,000, internally contoured shapes (wagon wheel and ring) resulted in 40% higher pressure drops due to the tortuous flow path. However, the shape with internal void provided access for the reactant to reach the internal part of catalyst, resulting in higher yield produced per volume of catalyst. The wagon wheel shape produced 15% and 5% more yield per volume against the cylindrical shape and trilobe, respectively. The performance of the wagon wheel can be attributed to the lower diffusion limitation due to higher surface area available for reactant to penetrate the internal part of the catalyst. [ABSTRACT FROM AUTHOR]

This paper presents a method to enhance extended interaction oscillator (EIO) output power based on a dual-cavity parallel structure (DCPS). This stucture consists of two conventional ladder-line structures in parallel through a connecting structure, which improves the coupling efficiency between the cavities. The dual output power fusion structure employs an H-T type combiner as the output coupler, which can effectively combine the two input waves in phase to further increase the output power. The dispersion characteristics, coupling impedance, and field distribution of the DCPS are investigated through numerical and simulation calculations, and the optimal operating parameters and output structure are obtained by PIC simulation. At an operating voltage of 12.6 kV, current density of 200 A/ cm 2 , and longitudinal magnetic field of 0.5 T, the DCPS EIO exhibits an output power exceeding 600 W at a frequency of 140.6 GHz. This represents a nearly three-fold enhancement compared with the 195 W output of the conventional ladder-line EIO structure. These findings demonstrate the significant improvement in output power and interaction efficiency achieved by the DCPS for the EIO device. [ABSTRACT FROM AUTHOR]

Copyright of BHM Berg- und Hüttenmännische Monatshefte is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

C-band high-power microwave devices have important application prospects in communication, radar and other fields, and the demand for compact and lightweight devices is becoming more and more urgent in the industrial sector. A C-band high-efficiency coaxial transit-time oscillator (CTTO) with three-cavity buncher and dual-cavity extractor is proposed and investigated in theory and 2.5-dimensional particle-in-cell simulations, which is good for high power microwave generation devices at low frequency towards miniaturization and compactness. In order to avoid the transverse electromagnetic mode (TEM) leaking to the diode region, the reflector is designed and adopted in front of the buncher, which also can pre-modulate the beams and lead to the higher conversion efficiency. In this simulation, the typical performance of the CTTO reveals that microwaves with a power of 1.83 GW are generated at a frequency of 6.37 GHz when the beam voltage and current are 458 kV and 9.6 kA respectively, under a guiding magnetic field of 1.05 T. The corresponding power conversion efficiency is as high as 41.6%. [ABSTRACT FROM AUTHOR]

Particle trajectory and collision simulation is a critical step of the design and construction of novel particle accelerator components. However it requires a huge computational effort which can slow down the design process. We started from a sequential simulation program which is used to study an event called Multipacting. Our work explains the physical problem that is simulated and the implications it can have on the behavior of the components. Then we analyze the original program's operation to find the best options for parallelization. We first developed a parallel version of the Multipacting simulation and were able to accelerate the execution up to ∼ 35 × with 48 or 56 cores. In the best cases, parallelization efficiency was maintained up to 16 cores ( ∼ 95 %) and the speed-up plateaus at around 40–48 cores. When this first parallelization effort was tried for multi-power simulations, we found that parallelism was severely limited with a maximum of 20 × speed-up. For this reason, we introduced a new method to improve the parallelization efficiency for this second use case. This method uses a shared processor pool for all simulations of electrons (OnePool). OnePool improved scalability by pushing the speed-up to over 32 × . [ABSTRACT FROM AUTHOR]

This paper presents a method to enhance extended interaction oscillator (EIO) output power based on a dual-cavity parallel structure (DCPS). This stucture consists of two conventional ladder-line structures in parallel through a connecting structure, which improves the coupling efficiency between the cavities. The dual output power fusion structure employs an H-T type combiner as the output coupler, which can effectively combine the two input waves in phase to further increase the output power. The dispersion characteristics, coupling impedance, and field distribution of the DCPS are investigated through numerical and simulation calculations, and the optimal operating parameters and output structure are obtained by PIC simulation. At an operating voltage of 12.6 kV, current density of 200 A/cm2, and longitudinal magnetic field of 0.5 T, the DCPS EIO exhibits an output power exceeding 600 W at a frequency of 140.6 GHz. This represents a nearly three-fold enhancement compared with the 195 W output of the conventional ladder-line EIO structure. These findings demonstrate the significant improvement in output power and interaction efficiency achieved by the DCPS for the EIO device.

This paper proposes a particle-based nonlinear elastic object simulation technique for virtual surgery. Particle-based techniques are used to model and simulate nonlinear elastic objects, such as the skin and internal organs. This enables the simulation to consider various factors, such as location, direction, and depth, when making incisions in the organs. However, the issue with this method is that it can only simulate precisely cut tissue during incision. Objects with elasticity, such as tissue, require the generation of complex debris during incision. This paper proposes a particle-based elastic object simulation technique to model the debris from torn tissue when making an incision in the organs. It can predict where the tissue will tear based on the maximum shear stress (MSS) theory and Tresca's yield criterion when the force applied to the tissue exceeds the maximum stress. Newly generated particles at the predicted location are remeshed with nearby particles. We verified the superiority of our proposed method over traditional particle-based methods by accurately representing more complex debris and comparing the results of incisions in the same area of the body. This allows various incision types, such as stab wounds and lacerations, to be simulated. [ABSTRACT FROM AUTHOR]

The breakdown of CF3I gas at low pressure is of significant importance for applications in fields such as aerospace and microelectronics. However, the DC low-pressure breakdown characteristics of CF3I remain underexplored. In this work, we utilize a one-dimensional implicit particle-in-cell/Monte Carlo collision (PIC/MCC) algorithm to investigate the complete DC breakdown process of low-pressure CF3I. Our model accounts for ion–molecule collisions, recombination reactions, and external circuit influences. The breakdown process is delineated into three stages: before breakdown, breakdown, and after breakdown. In the before-breakdown stage, both the density and energy of particles are low. In the breakdown stage, the rapid increase in electron density and energy accelerates ionization reactions, leading to successful breakdown. The circuit behavior transitions from capacitive to resistive, sharing voltage with the external resistance. In the after-breakdown stage, continued positive ion growth leads to the formation of a thin anode sheath and a negative plasma potential. Energy production, including heating power and secondary electron emission (SEE) power, balances with energy loss through collision and boundary absorption. Specifically, 62% of the total heating power comes from positive ions, 1.5% from negative ions, and approximately 85% of electron energy is lost via boundary absorption. Finally, we compare the Paschen curves of CF3I with those of SF6, providing insights that are beneficial for the application of CF3I as an SF6 alternative.

The grooved-wheel fertilizer machine is one of the most widely used pieces of fertilization equipment. However, detailed information on the fertilizer filling status and the mechanism of particle interactions during the operation of the grooved wheel remains limited. To delve into the underlying mechanisms through which working parameters affect fertilization performance, this study, building upon prior research, conducted a qualitative analysis and numerical investigation of fertilizer particles using the Discrete Element Method. The analysis examined the impact of three working parameters, namely the grooved-wheel speed, working length, and forward speed of the fertilization equipment, on the morphology, forces, and kinetic properties of the fertilizer particles. Combining this analysis with orthogonal experimental research, we optimized the aforementioned working parameters. Both simulation and benchtop experimental results indicate that the grooved-wheel speed and working length influence the fertilizer filling status, while the forward speed of the equipment has no effect on the filling status. The connection between fertilizer force and kinetic changes is influenced by particle-free space. The lowest coefficient of variation (CV) for fertilization uniformity was observed at the grooved-wheel speed of 53.64 r/min, the wheel working length of 33.45 mm, and the forward speed of 0.7–1 m/s. The research findings contribute to a better understanding of the influencing mechanism of particle movement and fertilization outcomes in the operation of grooved-wheel fertilizer spreaders. This understanding enables precise control of the fertilizer application process, facilitating accurate and efficient fertilization. As a result, it enhances fertilizer utilization rates and reduces agricultural costs. [ABSTRACT FROM AUTHOR]

How do soft granular materials (or dense amorphous systems) respond to externally applied deformations at different rates and for different system sizes? This long-standing question was intensively studied for shear deformations but only more recently for isotropic deformations, like compression–decompression cycles. For moderate strain rates, in the solid-like state, above jamming, the system appears to evolve more or less smoothly in time/strain, whereas for slow enough deformations, the material flips intermittently between the elastic, reversible base state and plastic, dynamic “events.” Only during the latter events, the microstructure re-arranges irreversibly. The reversible base state involves both affine and non-affine deformations, while the events are purely non-affine. The system size and rate dependence of the events are studied, providing reference data for comparison in future studies evaluating materials like hydrogel particles.

Explosive reactive armor (ERA) is currently being actively developed as a protective system for mobile devices against ballistic threats such as kinetic energy penetrators and shaped-charge jets. Considering mobility, the aim is to design a protection system with a minimal amount of required mass. The efficiency of an ERA is sensitive to the impact position and the timing of the detonation. Therefore, different designs have to be tested for several impact scenarios to identify the best design. Since analytical models are not predicting the behavior of the ERA accurately enough and experiments, as well as numerical simulations, are too time-consuming, a data-driven model to estimate the displacements and deformation of plates of an ERA system is proposed here. The ground truth for the artificial neural network (ANN) is numerical simulation results that are validated with experiments. The ANN approximates the plate positions for different materials, plate sizes, and detonation point positions with sufficient accuracy in real-time. In a future investigation, the results from the model can be used to estimate the interaction of the ERA with a given threat. Then, a measure for the effectiveness of an ERA can be calculated. Finally, an optimal ERA can be designed and analyzed for any possible impact scenario in negligible time.

Copyright of BHM Berg- und Hüttenmännische Monatshefte is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We have performed a series of simulation runs for whistler‐mode wave‐particle interaction in a parabolic magnetic field with 12 different frequencies of triggering waves and 3 different plasma frequencies specifying cold plasma densities. Under a given plasma condition, a specific frequency range of the triggering wave exists that can generate rising‐tone emissions. The generation region of rising‐tone emission shifts upstream. The velocity of the wave generation region is dependent on duration of the subpacket, which is controlled by the formation of the resonant current in the generation region. When the source velocity, which is a sum of the resonance and group velocities, is approximately the same as the velocity of the wave generation region, a long‐sustaining rising‐tone emission is generated. When the spatial and temporal gap between subpackets exists due to damping phase of short subpacket generation, resonant electrons in the gap of the subpackets are carried at the resonance velocity to the upstream region, and the velocity of the wave generation region becomes large in magnitude. When formation of resonant currents is delayed, the velocity of the generation region becomes smaller than the source velocity in magnitude. Below one quarter of the cyclotron frequency, coalescence of subpackets takes place, suppressing formation of the resonant current in the generation region. Since gradual upstream shift of the generation region is necessary for the wave to grow locally, the source velocity should be a small negative value. Plain Language Summary: The generation process of whistler‐mode chorus emissions has been a mystery for a long time, while its detailed nonlinear processes are being clarified by particle simulations in recent years. Although it is generally understood that chorus generation regions are close to the magnetic equator, the exact location and process of the wave generation have not been identified yet. It has been observed that a rising‐tone chorus emission consists of many subpackets with gradually increasing frequencies. A new subpacket is generated by a foregoing wave packet, and the process is repeated. Instead of the naturally growing wave packet, we inject a wave with a constant frequency to trigger new emissions. We have conducted many simulation runs with different parameters to identify the location and the motion of the generation region. We have found that the generation region always moves upstream from the triggering wave. These characteristics of triggered emissions are valuable information for understanding chorus emissions observed by spacecraft. Key Points: The generation region of rising‐tone emission below half the cyclotron frequency moves upstream as the frequency increasesThe generation region of a long rising‐tone emission moves with a velocity, which is the sum of the resonance velocity and group velocityThe velocity of the wave packet generation region is controlled by the formation of the resonant current [ABSTRACT FROM AUTHOR]

Abstract We perform an electromagnetic particle simulation of triggered emissions in a uniform magnetic field for understanding of nonlinear wave–particle interaction in the vicinity of the magnetic equator. A finite length of a whistler-mode triggering wave packet with a constant frequency is injected by oscillating an external current at the equator. We find that the first subpacket of triggered emissions is generated in the homogeneous magnetic field. By analyzing resonant currents and resonant electron dynamics in the simulation, we find that the formation of an electron hole in a velocity phase space results in resonant currents, and the currents cause wave amplification and frequency increase. We obtain the interaction time of counter-streaming resonant electrons in a triggering wave packet with a finite width. By changing the duration time of the triggering pulse, we evaluate the interaction time necessary for formation of an electron hole. We conduct 4 runs with different duration times of the triggering pulse, 980, 230, 105, 40 $$\Omega _e^{-1}$$ Ω e - 1 , which correspond to cases with interaction times, 370%, 86%, 39%, and 15% of the nonlinear trapping period, respectively. We find generation of triggered emissions in the three cases of 370%, 86%, and 39%, which agrees with the conventional nonlinear model that the nonlinear transition time, which is necessary for formation of resonant currents, is about a quarter of the nonlinear trapping period. Graphic Abstract

We perform a self‐consistent one‐dimensional electromagnetic particle simulation with a uniform magnetic field and open boundaries. The plasma environment consists of cold isotropic electrons, energetic electrons, and immobile ions. The energetic electrons are initialized with a subtracted‐Maxwellian distribution with temperature anisotropy. By oscillating external currents with a constant frequency 0.2 fce, where fce is the electron cyclotron frequency, a whistler‐mode wave is injected as a triggering wave from the center of the simulation system, and we investigated the process of interactions between the triggering wave and energetic electrons. We find that both rising‐tone and falling‐tone emissions are triggered through the formation of an electron hole and an electron hill in the velocity phase space consisting of a parallel velocity and the gyro‐phase angle of the perpendicular velocities. The rising‐tone emission varies from 0.2 fce to 0.4 fce, while the falling‐tone varies from 0.2 fce to 0.15 fce. The generation region of the rising‐tone triggered emission starts near the injection point of the triggering wave and moves upstream generating new subpackets. The generation region of the falling‐tone triggered emission also moves upstream generating new subpackets. The simultaneous formation of the electron hole and hill is identified by separating small and large wavenumber components corresponding to lower and higher frequencies, respectively, by applying the discrete Fourier transformation to the waveforms in space. Based on the simulation results of the whistler‐mode triggered emissions, we conclude that the mechanism of frequency variation of whistler‐mode chorus emissions works even in a uniform magnetic field. Plain Language Summary: We perform a particle‐in‐cell simulation for the generation of whistler‐mode waves in a homogeneous magnetized plasma. In addition to cold electrons supporting the propagation of whistler‐mode waves, we assume energetic electrons that can interact with the waves through cyclotron resonance. We find both rising‐tone and falling‐tone waves are excited from a triggering wave with a constant frequency. We analyze the velocity distribution function of energetic electrons and find the formation of both electron holes and hills in the velocity phase space. The mechanism of frequency variation and nonlinear wave growth of whistler‐mode chorus emissions works even in a uniform magnetic field. Key Points: Rising‐tone and falling‐tone emissions are generated simultaneously in a homogeneous magnetic fieldElectron holes and hills are formed in velocity phase space during the generation of rising and falling tone emissions, respectivelyGeneration points of both emissions move upstream from the triggering wave [ABSTRACT FROM AUTHOR]

Explosive reactive armor (ERA) is currently being actively developed as a protective system for mobile devices against ballistic threats such as kinetic energy penetrators and shaped-charge jets. Considering mobility, the aim is to design a protection system with a minimal amount of required mass. The efficiency of an ERA is sensitive to the impact position and the timing of the detonation. Therefore, different designs have to be tested for several impact scenarios to identify the best design. Since analytical models are not predicting the behavior of the ERA accurately enough and experiments, as well as numerical simulations, are too time-consuming, a data-driven model to estimate the displacements and deformation of plates of an ERA system is proposed here. The ground truth for the artificial neural network (ANN) is numerical simulation results that are validated with experiments. The ANN approximates the plate positions for different materials, plate sizes, and detonation point positions with sufficient accuracy in real-time. In a future investigation, the results from the model can be used to estimate the interaction of the ERA with a given threat. Then, a measure for the effectiveness of an ERA can be calculated. Finally, an optimal ERA can be designed and analyzed for any possible impact scenario in negligible time. [ABSTRACT FROM AUTHOR]

The grooved-wheel fertilizer machine is one of the most widely used pieces of fertilization equipment. However, detailed information on the fertilizer filling status and the mechanism of particle interactions during the operation of the grooved wheel remains limited. To delve into the underlying mechanisms through which working parameters affect fertilization performance, this study, building upon prior research, conducted a qualitative analysis and numerical investigation of fertilizer particles using the Discrete Element Method. The analysis examined the impact of three working parameters, namely the grooved-wheel speed, working length, and forward speed of the fertilization equipment, on the morphology, forces, and kinetic properties of the fertilizer particles. Combining this analysis with orthogonal experimental research, we optimized the aforementioned working parameters. Both simulation and benchtop experimental results indicate that the grooved-wheel speed and working length influence the fertilizer filling status, while the forward speed of the equipment has no effect on the filling status. The connection between fertilizer force and kinetic changes is influenced by particle-free space. The lowest coefficient of variation (CV) for fertilization uniformity was observed at the grooved-wheel speed of 53.64 r/min, the wheel working length of 33.45 mm, and the forward speed of 0.7–1 m/s. The research findings contribute to a better understanding of the influencing mechanism of particle movement and fertilization outcomes in the operation of grooved-wheel fertilizer spreaders. This understanding enables precise control of the fertilizer application process, facilitating accurate and efficient fertilization. As a result, it enhances fertilizer utilization rates and reduces agricultural costs.

Abstract We conduct electromagnetic particle simulations to examine the applicability of nonlinear wave growth theory to the generation process of plasmaspheric hiss. We firstly vary the gradient of the background magnetic field from a realistic model to a rather steep gradient model. Under such variation, the threshold amplitude in the nonlinear theory increases quickly and the overlap between threshold and optimum amplitude disappears correspondingly, the nonlinear process is suppressed. In the simulations, as we enlarge the gradient coefficient of the background magnetic field, waves generated near the equator do not grow through propagation. By examining the range of suitable values of inhomogeneity factor S (i.e., $$|S|

Whistler mode chorus, characterized by frequency chirping, is an important type of waves in planetary magnetospheres. To investigate the role of nonlinear wave particle interactions in excitation of chorus, analysis of electron phase space dynamics is required. While electron phase hole associated with rising tone chorus has been well studied, the phase space structure corresponding to falling tone chorus is less understood. Here, we investigate in detail the electron phase space dynamics in a spontaneously generated falling tone chorus with a parabolic type magnetic field, where field strength decreases away from the equator. We demonstrate the formation and evolution of electron phase space clump from downstream to upstream regions, and show that the variation of frequency chirping rate is caused by the movement of the source region. The results are consistent with the recently proposed Trap‐Release‐Amplify model, and should be useful to understand the mechanism of chorus frequency chirping. Plain Language Summary: Chorus is a type of important electromagnetic waves frequently observed in planetary magnetospheres. It is named after its sound while played through a loudspeaker. The particular sound is related to frequency chirping, meaning the wave frequency increases or decreases rapidly with time. Scientists have been debating the frequency chirping mechanism for more than half a century. One important conclusion emerging from previous theoretical studies is about the existence of two types of phase space structures: the phase space hole and the phase space clump associated with rising‐tone and falling‐tone chorus, respectively. While the former has been verified by many computer simulations, the latter has not been well studied. In this work, we use first‐principle computer simulations to investigate, for the first time, the formation and evolution of phase space clump in excitation of spontaneous falling tone chorus. We also propose the variation of chirping rate within chorus element is related to the movement of source region, where phase space structures are released from the wave packet. We find that our simulation results agree very well with the recently proposed "Trap‐Release‐Amplify" model of chorus. Key Points: We use a Particle‐In‐Cell simulation to demonstrate the formation of phase space clump in excitation of falling tone chorusWe prove that electron phase space clump evolves in a way consistent with the Trap‐Release‐Amplify modelOur results show that the variation of chirping rate within an element could be explained by the movement of source region [ABSTRACT FROM AUTHOR]

Abstract Whistler mode chorus, characterized by frequency chirping, is an important type of waves in planetary magnetospheres. To investigate the role of nonlinear wave particle interactions in excitation of chorus, analysis of electron phase space dynamics is required. While electron phase hole associated with rising tone chorus has been well studied, the phase space structure corresponding to falling tone chorus is less understood. Here, we investigate in detail the electron phase space dynamics in a spontaneously generated falling tone chorus with a parabolic type magnetic field, where field strength decreases away from the equator. We demonstrate the formation and evolution of electron phase space clump from downstream to upstream regions, and show that the variation of frequency chirping rate is caused by the movement of the source region. The results are consistent with the recently proposed Trap‐Release‐Amplify model, and should be useful to understand the mechanism of chorus frequency chirping.

We perform an electromagnetic particle simulation of triggered emissions in a uniform magnetic field for understanding of nonlinear wave–particle interaction in the vicinity of the magnetic equator. A finite length of a whistler-mode triggering wave packet with a constant frequency is injected by oscillating an external current at the equator. We find that the first subpacket of triggered emissions is generated in the homogeneous magnetic field. By analyzing resonant currents and resonant electron dynamics in the simulation, we find that the formation of an electron hole in a velocity phase space results in resonant currents, and the currents cause wave amplification and frequency increase. We obtain the interaction time of counter-streaming resonant electrons in a triggering wave packet with a finite width. By changing the duration time of the triggering pulse, we evaluate the interaction time necessary for formation of an electron hole. We conduct 4 runs with different duration times of the triggering pulse, 980, 230, 105, 40 Ω e - 1 , which correspond to cases with interaction times, 370%, 86%, 39%, and 15% of the nonlinear trapping period, respectively. We find generation of triggered emissions in the three cases of 370%, 86%, and 39%, which agrees with the conventional nonlinear model that the nonlinear transition time, which is necessary for formation of resonant currents, is about a quarter of the nonlinear trapping period. [ABSTRACT FROM AUTHOR]

A capillary suspension is a ternary system consisting of one solid and two immiscible liquids. As a small amount of one of the fluids, also known as the secondary fluid, is present in between the particles to form a sample-spanning network, the suspension experiences dramatic changes in the rheological properties. In this paper, a particle simulation on the capillary suspension is proposed by introducing the coarse-grained interaction between fluid droplets and solid particles. The shape of the free surface between two fluid phases is assumed as a sphere, which can be partially superposed on the solid surface, while the volume occupied by the fluid droplet is preserved. By using the method, the formation of the capillary suspension is simulated under various conditions to achieve a sample-spanning network. The structure and flow properties of the capillary suspension are investigated at the size ratio of 0.4 and secondary fluid concentration of 2 vol%. Under low dimensionless shear rates less than 0.5, the simulated flow properties show a sign of local freezing, and the local properties of the frozen zone are isolated from the bulk properties and further investigated. The local stress increases linearly with the local strain in the frozen zone, exhibiting an elastic behavior. This indicates that the material preserves its structure over the imposed strain, and the difference between imposed and apparent strain is interpreted as the effect of yield stress. Finally, the yield stress is calculated as 5.7 in dimensionless form, similar to that reported in previous studies. [ABSTRACT FROM AUTHOR]

In this paper, the effects of the pulse rise-time in the performance of the electron heating in the interaction of intense laser pulse with under-dense plasma have been investigated using the particle-in-cell (PIC) simulation code. In this way, two laser pulses with a length of 200 fs and different pulse rise-time of 80 fs and 40 fs have been selected. The laser intensity and the plasma density are I=1018 w/cm2 and n=0.02 ncr (ncr is the plasma critical density), respectively. To correctly characterize the local plasma temperature in different regions of plasma, the stress tensor elements have been used. Our results show that the electron heating and the temperature of the electrons are strongly dependent on the laser pulse rise-time and increase significantly with decreasing the pulse rise-time. In other words, when the pulse rise-time is initially smooth (80 fs), the scattering fields can provide the necessary condition for stochastic heating. Whereas, in the quickly rising pulses (40 fs), the electron heating is initiated by the wave breaking. In the following, the effect of increasing the laser pulse intensity on the electron heating is investigated for the pulses with different rise-times, hence these effects are more severe for the pulses with the short rise-time.

Heat transfer enhancement by suspending nano-sized particles in a liquid, called nanofluid, has attracted considerable attention over the past few decades. The underlying mechanisms are an area of active research. The present study focused on the inhomogeneous and non-isotropic aspects of the thermophysical properties of nanofluids. A non-equilibrium molecular dynamics simulation was conducted, such that the temperature gradient was applied directly to the simulation domain where various particles were located, and local and directional characteristics of the heat transfer were investigated. The ordered structure in the liquid region next to the liquid-solid atomic interface played a critical negative role in altering the thermal properties that could not be predicted through the simple composite model of Maxwell. The formed liquid structure was sensitive to the surroundings, and the size, shape, and multi-particle arrangement affected the performance accordingly. Two nano-particles oscillating in the temperature-gradient direction were significantly effective in enhancing heat transfer. [ABSTRACT FROM AUTHOR]

• Utilized Hilbert space-filling curve and serialization for data transfer. • Works with any models including PIC, DSMC and CFD with non-uniform mesh. • Over-decomposition generally reduced the simulation run-time. • Greater benefit in simulations with a highly non-uniform particle distribution. An electric propulsion plasma plume simulation employs the Particle-in-Cell (PIC) model for heavy species (i.e. ions and neutral atoms) and yields a highly non-uniform particle distribution at steady state. For parallel simulations, partitioning the computational domain uniformly and evenly assigning the subdomains to MPI processes would never lead to properly balanced loads across MPI processes. For this reason, a patch-based dynamic load balancing method with over-decomposition and Hilbert space-filling curve has been implemented into the Thermophysics Universal Research Framework (TURF). The data transfer is accomplished by utilizing TURF's serialization and deserialization feature, in which the tree hierarchical object structure is copied into a contiguous memory block of data, and the tree hierarchy is reconstructed after the data transfer. This generalized dynamic load balancing approach allows for the same routine to be used for any models besides the PIC model in TURF (e.g. fluid, continuum kinetic, hybrid models, etc). This paper first introduces individual pieces of TURF that enable dynamic load balancing when they are combined. Then, the dynamic load balancing method is demonstrated in three different particle simulations, showing performance gains when the number of subdomains is larger than that of MPI processes. The benefit is even more notable in simulations with highly non-uniform particle distributions at steady state, commonly seen in plasma plume simulations. [ABSTRACT FROM AUTHOR]

We conduct electromagnetic particle simulations in a uniform magnetic environment to verify the nonlinear wave growth process of plasmaspheric hiss in the equatorial plasmasphere. The satisfaction of the separability criterion for coexisting multiple frequency waves in the initial stage of wavenumber‐time evolution declares that wave packets are coherent and capable of growing nonlinearly. Spatial and temporal evolutions of two typical modes located in wavenumber‐time evolution demonstrate the consistency among wave growths, frequency variations, and inhomogeneity factor S in coherent wave packets, showing that rising and falling tones occur at negative and positive S values, respectively, and an obvious wave growth happens in a reasonable range of S satisfying the second‐order resonance condition. Wave packets extracted from wave fields in space and time by a frequency band‐pass filter confirm a good agreement between the nonlinear theory and simulation results. The nonlinear growth rates of the extracted wave packets possess similar magnitudes to the growth rates of wave packets in the simulation, and they are much greater than the theoretical linear growth rate, indicating that the nonlinear process is essential in the generation of plasmaspheric hiss. Plain Language Summary: We have conducted a one‐dimensional electromagnetic particle simulation of whistler‐mode waves instability driven by minor energetic electrons with temperature anisotropy in a uniform magnetic field. We find generation of many short wave packets with rising and falling tone frequencies starting from different frequencies. These wave packets are mostly coherent satisfying the separability criteria in frequency and wavenumbers. Namely, the nonlinear trapping potentials due to coherent wave packets do not overlap each other in the velocity phase space. By applying band‐pass filter, we extracted each wave packet, and calculated the wave amplitude, frequency, the inhomogeneity factor of the second‐order cyclotron resonance, and resonant currents. We find reasonable agreement between the growth rates of the wave packets in the simulation and the theoretical nonlinear growth rates, which are much greater than the linear growth rates. The simulation result confirms that the coherent nonlinear wave growth process due to frequency variation is the essential process in generating whistler‐mode hiss emissions locally in the equatorial plasmasphere. Key Points: Coherent whistler‐mode hiss wave packets can grow from thermal fluctuations in the equatorial plasmasphereNonlinear wave growth processes due to frequency variation take place with the coherent hiss wave packetsThe nonlinear growth rates of hiss with rising and falling tones are much greater than the linear growth rates [ABSTRACT FROM AUTHOR]

Focusing on ring-shaped ion velocity distributions with a finite width formed in magnetic reconnection in the presence of a guide magnetic field, intriguing properties such as the formation mechanism, a significant change in the shape, and necessary conditions for the change are investigated by means of theory and simulations. The width of a ring velocity distribution predominantly originates from velocity variations of seed particles for the pickup-like process. A function exactly representing a ring with a width is analytically formulated, assuming a steady supply of seed particles satisfying a Maxwellian velocity distribution and a mixing of gyration phases. The formulated function indicates that when the ring width is larger than a criterion, the local minimum of the ring’s center is changed into the maximum, and the shape is transformed into a mountain shape. Such a mountain-like distribution is defined as “a pseudo-Maxwellian distribution,” because it is almost indistinguishable in shape from a genuine Maxwellian distribution. Actually, particle simulations demonstrate that mountain-shaped ion velocity distributions are formed during magnetic reconnection with a guide magnetic field, and it is nearly concluded that they are pseudo-Maxwellian distributions. Moreover, two types of evidence for pseudo-Maxwellian distributions are shown by simulations. One is to analyze the dependence of the distribution shape on the guide magnetic field, which is explored by the particle simulation. In cases of slightly different values of the guide field, vague shapes of rings with a width are observed as ion velocity distributions. The other is to observe velocity distributions under a hypothetical condition of an artificial zero temperature in the upstream by utilizing a test particle simulation. In the test particle simulation, ring-shaped distributions with a width are clearly seen, because the velocity variations in the upstream are reduced. From the two types of evidence, it is definitely confirmed that the mountain-shaped distributions found in the particle simulations are pseudo-Maxwellian distribution. These results imply that pseudo-Maxwellian distributions would be created for various cases of guide field magnetic reconnection.

Topological data analysis tools enjoy increasing popularity in a wide range of applications, such as Computer graphics, Image analysis, Machine learning, and Astronomy for extracting information. However, due to computational complexity, processing large numbers of samples of higher dimensionality quickly becomes infeasible. This contribution is twofold: We present an efficient novel sub-sampling strategy inspired by Coulomb's law to decrease the number of data points in d -dimensional point clouds while preserving its homology. The method is not only capable of reducing the memory and computation time needed for the construction of different types of simplicial complexes but also preserves the size of the voids in d -dimensions, which is crucial e.g. for astronomical applications. Furthermore, we propose a technique to construct a probabilistic description of the border of significant cycles and cavities inside the point cloud. We demonstrate and empirically compare the strategy in several synthetic scenarios and an astronomical particle simulation of a dwarf galaxy for the detection of superbubbles (supernova signatures). [ABSTRACT FROM AUTHOR]

We conduct electromagnetic particle simulations to examine the applicability of nonlinear wave growth theory to the generation process of plasmaspheric hiss. We firstly vary the gradient of the background magnetic field from a realistic model to a rather steep gradient model. Under such variation, the threshold amplitude in the nonlinear theory increases quickly and the overlap between threshold and optimum amplitude disappears correspondingly, the nonlinear process is suppressed. In the simulations, as we enlarge the gradient coefficient of the background magnetic field, waves generated near the equator do not grow through propagation. By examining the range of suitable values of inhomogeneity factor S (i.e., | S | < 2 ), we find the generation of wave packets is limited to the equatorial region when the background field is steep, showing a good agreement with what is indicated by critical distance in the theory. We then check the dependence of generation of hiss emissions on different hot electron densities. Since the overlap between threshold and optimum amplitude vanishes, the nonlinear process is weakened when hot electron density becomes smaller. In the simulation results, we find similar wave structures in all density cases, yet with different magnitudes. The existence of suitable S values implies that the nonlinear process occurs even at a low level of hot electron density. However, by examining J E that closely relates to the wave growth, we find energy conveyed from particles to waves is much limited in small density cases. Therefore, the nonlinear process is suppressed when hot electron density is small, which agrees with the theoretical analysis. [ABSTRACT FROM AUTHOR]

Particle simulation has been widely used in studying plasmas. The technique follows the motion of a large assembly of charged particles in their self-consistent electric and magnetic fields. Plasmons, collective oscillations of the free electrons in conducting media such as metals, are connected to plasmas by very similar physics, in particular, the notion of collective charge oscillations. In many cases of interest, plasmons are theoretically characterized by solving the classical Maxwell’s equations, where the electromagnetic responses can be described by bulk permittivity. That approach pays more attention to fields rather than motion of electrons. In this work, however, we apply the particle simulation method to model the kinetics of plasmons, by updating both particle position and momentum (Newton–Lorentz equation) and electromagnetic fields (Ampere and Faraday laws) that are connected by current. Particle simulation of plasmons can offer insights and information that supplement those gained by traditional experimental and theoretical approaches. Specifically, we present two case studies to show its capabilities of modeling single-electron excitation of plasmons, tracing instantaneous movements of electrons to elucidate the physical dynamics of plasmons, and revealing electron spill-out effects of ultrasmall nanoparticles approaching the quantum limit. These preliminary demonstrations open the door to realistic particle simulations of plasmons.

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Collisionless magnetic reconnection occurs ubiquitously in space plasma environments and plays an important role in energy conversion therein. In collisionless magnetic reconnection, the reconnection site is usually unsteady and ejects reconnection fronts away from it. Using two‐dimensional particle‐in‐cell simulations, we study the energy budgets from the collisionless magnetic reconnection site to reconnection fronts. It is concluded that the reconnection rate cannot well reflect energy conversion of nonsteady state magnetic reconnection because energy conversion occurs predominantly at the reconnection fronts, whereas the reconnection rate can only represent the energy conversion at the reconnection site. We clarify the connection between the reconnection site and the reconnection fronts in terms of energy conversion. The reconnection site functions as a trigger and energy source that generates the outflow of Poynting flux, bulk kinetic energy flux, and enthalpy flux forming the reconnection fronts that move downstream. The well‐developed reconnection fronts are no longer related to the reconnection site. The energy income at the reconnection fronts is mainly the Poynting flux from their top and bottom boundaries, most of which is transformed to P¯⋅V flux flowing downstream out of the moving front through the work by the electric field. The work done by the electric force is compensated with the work done by the thermal pressure gradient, which guarantees that the released magnetic energy is mostly converted to thermal energy. Key Points: Energy budgets from collisionless magnetic reconnection site to reconnection fronts are studied through two‐dimensional particle‐in‐cell simulationsThe reconnection fronts that dominate the energy conversion are generated from the reconnection siteEnergy conversion at the well‐developed reconnection front is unrelated to the reconnection site. The Poynting flux inflow is mainly converted to enthalpy flux [ABSTRACT FROM AUTHOR]

Biochar, as a soil additive, will inevitably affect the transformation process of soil and water interactions. Biochar plays an important role in reducing soil degradation. The capillary water in soil affects the stability of the soil structure. The characteristics of the capillary force between the soil and biochar particles are investigated in this study. Based on an advanced capillary force measurement system, biochar and soil particles are simplified as spherical particles of equal size. The image of the liquid bridge between different particles is recorded by a high-speed camera (AOSVI-HD202WF). Based on digital image processing technology, the fractal dimension of the image is calculated, and the influence of the liquid volume and particle properties is analyzed. The results indicate that the capillary force between the particles decreases with increase in interparticle distance. Due to the increase of the liquid volume, the initial capillary force between the biochar and soil particles is increased from 3.6×10−4 to 4.4×10−4 N, and the fracture distance of the liquid bridge is increased from 0.41 to 0.70 mm, respectively, for a liquid volume from 0.8 to 2.4 ul. The fractal dimension of morphology of the liquid bridge between the particles increased and decreased in a cycle (C). It is found that the variation of the liquid bridge between the particles is stable with a high liquid volume (≥2.4 ul). It is also found that the cycle period of the liquid bridge between the biochar and soil particles is not affected by the liquid volume. The calculated capillary forces between the biochar and soil particles by using a Laplace-Young equation have been well verified by the experimental data. Biochar has an effect on the stability of the capillary force of the liquid bridge between the soil particles. Biochar-soil particles can be effectively combined to improve the capillary force and water retention capacity of the soil. [ABSTRACT FROM AUTHOR]

Diamond wire sawing has been developed to reduce the cutting loss when cutting silicon wafers from ingots. The surface of silicon solar cells must be flawless in order to achieve the highest possible efficiency. However, the surface is damaged during sawing. The extent of the damage depends primarily on the material removal mode. Under certain conditions, the generally brittle material can be machined in ductile mode, whereby considerably fewer cracks occur in the surface than with brittle material removal. In the presented paper, a numerical model is developed in order to support the optimisation of the machining process regarding the transition between ductile and brittle material removal. The simulations are performed with an GPU-accelerated in-house developed code using mesh-free methods which easily handle large deformations while classic methods like FEM would require intensive remeshing. The Johnson-Cook flow stress model is implemented and used to evaluate the applicability of a model for ductile material behaviour in the transition zone between ductile and brittle removal mode. The simulation results are compared with results obtained from single grain scratch experiments using a real, non-idealised grain geometry as present in the diamond wire sawing process. [ABSTRACT FROM AUTHOR]

For X-ray tubes in lab based computed micro-tomography systems, off-focal X rays are those emitted from the source but not produced at the primary focal spot of the electron beam. They can be attributed to both electron and photon scatter within the X-ray tube. Off-focal X rays can represent a significant fraction of the total X-ray flux and lead to artefacts in the radiographs, and reconstructed tomographic volumes, degrading contrast and resolution. Therefore, they are an undesirable feature in industrial and medical X-ray CT scanners. In this work, a general model of an X-ray tube with a transmission target has been developed in TOPAS, a Monte Carlo (MC) simulation extension to GEANT4. MC simulations enable: (1) the analysis of the origin of various types of off-focal X rays; (2) quantification of the prevalence of each type for a specific tube geometry and (3) replication and understanding of experimental results affected by off-focal X rays. Our MC analysis herein shows that the amount of off-focal X rays can represent up to 25% of total X-ray flux. The most prevalent off-focal X rays were found to be the X rays created by the electrons back-scattered from the target into the aperture and generating X rays. [ABSTRACT FROM AUTHOR]

A technique called the Direct Simulation Monte Carlo (DSMC) method is used for simulating laminar one-dimensional hydrogen-air flames with an aim to establish the feasibility of molecular-level simulations for combustion using the Quantum-Kinetic (QK) reaction model. In DSMC, simulation particles are employed which represent many molecules with similar properties. The DSMC method effectively solves the Boltzmann equation by decoupling the motion of molecules into two phases: a Move phase and a Collision phase. Chemistry is treated using the Quantum-Kinetic model in which the total energy exchange resulting from molecular collision determines the outcome of the chemical reactions. A major advantage of DSMC is that it avoids using continuum Arrhenius reaction rates or simplified gradient laws for the diffusivities of mass, momentum and heat. Instead, any such laws and their parameters emerge naturally from the DSMC results. Simulations of one-dimensional hydrogen-air flames using the DSMC method with a detailed chemical scheme show promising results for molecular-level simulations of combustion. An adjustment to the activation energy of a key reaction is made in order to achieve the correct flame speed. The species and temperature profiles from DSMC show reasonable agreement with those obtained from Direct Numerical Simulation (DNS) using a conventional continuum approach. Likewise, the molecular diffusivity of hydrogen and oxygen also show reasonable agreement with the DNS results, although differences in the results still exist, especially for oxygen diffusivity. These are expected to improve with further development in the DSMC method for combustion. [ABSTRACT FROM AUTHOR]

The low-voltage compact gyrotron is suitable for industrial applications. However, the beam-wave interaction efficiency is low in conventional low-voltage gyrotron. To improve the whole tube efficiency, a compact depressed collector is introduced in developing a 75 GHz low-voltage compact gyrotron. The compact depressed collector is directly connected to the output waveguide. It is grounded and isolated with the cavity by a ceramic ring which is easy to be connected with the application system. The design of the original tube electron beam voltage and electron beam current are 10 kV and 1.2 A. In the particle-in-cell (PIC) simulation, the operating mode is TE0,1 and the generated power is 1.2 kW operated at the frequency of 75.5 GHz, which corresponds to an electron efficiency of 10%. When the depressed collector is performed and the electron reflux is under 5%, the efficiency of the whole tube can reach 30%, and when the reflux rate is controlled at about 15%, the efficiency of the whole tube can reach 50%. The dissipation power would be sufficiently reduced. [ABSTRACT FROM AUTHOR]

The charging effect generated by the accumulation of negative charges on a mask surface strongly limits the perfect pattern transfer from the mask to the substrate during the plasma etching process, which leads to the deformation of etched features and severely damages the mask pattern. This study first verified that various placements of the holes will result in different distributions of the electric filed (E-field) and the etching rate. This work next shows that the electron angular distribution (EAD) is closely related to the damage to the mask pattern. Based on a reliable modeling framework, the effects of changing the EAD on the distributions of spatial E-field and the etching rate were examined focusing on two adjacent and asymmetrically-shaped mask holes. Specifically, both the E-field strength and the etching rate around the edge of the two mask holes can be greatly reduced by narrowing the EAD, meanwhile, the number of electrons penetrating into the bottom of two holes will be significantly increased. These results will reduce the mask pattern damage and improve the etching of high-aspect-ratio features into a substrate. In the cases of different EADs, the simulated evolution rates of these two mask holes and the E-field strength inside the holes verify these conclusions. The mechanism is discussed in detail. This work will greatly help to optimize the etching technique. [ABSTRACT FROM AUTHOR]

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We perform a one‐dimensional electromagnetic full particle simulation for triggered falling‐tone emissions in the Earth's magnetosphere. The equatorial region of the magnetosphere is modeled with a parabolic magnetic field approximation. The short whistler‐mode waves with a large amplitude are excited and propagate poleward from an artificial current oscillating with a constant frequency and amplitude. Following the excited waves, clear emissions are triggered with a falling frequency. Without the inhomogeneity of the background magnetic field, no triggered emission appears. The falling tone has several subpackets of amplitude and decreases the frequency in a stepwise manner. The positive resonant current formed by resonant electrons in the direction of the wave magnetic field clearly shows that an electron hill is formed in the phase space and causes the frequency decrease. The entrapping of the resonant electrons at the front of the packets and the decrease of the amplitude at the end of packets are essential for the generation of falling‐tone emissions. Each wavefront of the emission has a strongly negative resonant current −JE, which results in the wave growth. In the formation process of the resonant currents, we investigate the inhomogeneous factor S, which controls the nonlinear motion of the resonant electrons interacting with waves. The factor S consists of two terms, a frequency sweep rate and a gradient of the background magnetic field. The resonant current JE in the wave packet changes its sign from negative to positive as the packet moves away from the equator, terminating the wave growth. Key Points: We simulate triggered falling‐tone emissions by electromagnetic full particle‐in‐cell calculationThe wave growth with falling frequency is caused by the formation of electron hill induced by a triggering waveThe electron hill is formed by entrapping of electrons at low pitch angles and variation of the wave amplitude [ABSTRACT FROM AUTHOR]

The perfect pattern transfer from mask to substrate during the plasma‐etching process is strongly limited by the charging effect on the mask surface, which is increased by the accumulation of negative charges on the surface. These are mainly caused by high‐velocity isotropic electrons impinging on the mask surface faster than ions. These anisotropic ions thus bombard the undesired locations of the mask under the influence of the electric field (E‐field) established by electrons. This problem leads to significant damages to the mask pattern and causes deformations of etched features due to failure pattern transfer. This study examined that electron angular distribution (EAD; relative to the vertical direction, which can be regulated by voltage waveform tailoring) displays a close relationship with the mask pattern damage. Based on a modeling framework that consists of a surface etching module, a surface charging module, and a profile evolution module, the effects of changing the EAD on distributions of spatial E‐field and etching rate were studied focusing on an isolated rough mask hole surface. It is revealed that by narrowing the EAD shape, the E‐field strength and the etching rate around the mask hole edge can be reduced strongly, meanwhile, the number of electrons penetrating into the bottom of the trench can be greatly increased. These developments will supposedly reduce the mask‐pattern damage and improve the etching of high‐aspect‐ratio (HAR) features into the substrate. The simulated evolution rates of profile of a rough mask hole and the profile of E‐field strength inside the hole under various EAD shapes verify the above conclusions. The mechanism behind these results was analyzed systematically. This study provides a significant point for further investigation into the optimization of the etching technique. [ABSTRACT FROM AUTHOR]