Calculation of the stability ratio of suspensions using Emulsion Stability Simulations

According to Emulsion Stability Simulations (ESS), the flocculation of two non-deformable drops in the primary minimum of the interaction potential, necessarily leads to their coalescence. This property is used here for the evaluation of the stability ratio (W) of solid particles, interacting with the same inter-particle potential as the one of non-deformable droplets. Two different methodologies are used. The first one consists on the repeated evaluation of the coalescence time between two particles. The second one consists on the estimation of the time required for a decrease in the number of aggregates of the dispersion equal to n₀/2 (where n₀ is the initial number of aggregates). The results of the simulations are contrasted with the stability ratio of an anionic latex suspension subject to several ionic strengths (400-1000 mM). The first methodology is far more efficient for the evaluation of W although it misses the development of the aggregates and their growth. Absolute coagulation rates (kf) can also be obtained using one N-particle simulation for the calculation of the fast flocculation rate , and several two-particle simulations for the evaluation of W. This combined procedure is also more efficient than the N-particle evaluation of .     

A transport simulation code for inertial confinement fusion relevant laser-plasma interaction

We present a code for the simulation of laser-plasma interaction processes relevant for applications in inertial confinement fusion. The code consists of a fully nonlinear hydrodynamics in two spatial dimensions using a Lagrangian, discontinuous Galerkin-type approach, a paraxial treatment of the laser field and a spectral treatment of the dominant non-local transport terms. The code is fully parallelized using MPI in order to be able to simulate macroscopic plasmas.One example of a fully nonlinear evolution of a laser beam in an underdense plasma is presented for the conditions previewed for the future MegaJoule laser project.     

High permeability nano-crystalline FeSiBTaAg ribbons obtained by direct casting

Nano-crystalline soft magnetic ribbons, being extensively used as magnetic cores for switching power supplies, have been invariantly obtained by melt-spinning followed by post-annealing. Reported herewith are the attainment, by direct-casting without annealing, of nano-crystalline Fe_77.4−xSi_15.5B₇Ta_xAg_0.1 (x = 1, 2) ribbons with superior soft magnetic properties (named TAGMET after the addition of Ta and Ag). The grain size of nano-crystalline -FeSi, from 20 to 30 nm, varies with composition and quenching speeds. As-spun Fe_75.4Si_15.5B₇Ta₂Ag_0.1 ribbons consisting of 25 nm nano-crystals exhibit a saturation magnetization of 157 emu/g (1.45 T), an effective permeability of 56,000 at 1 kHz, and coercivity, 8 A/m. With the combination of easier manufacturing process and excellent soft magnetic properties, this alloy is competitive in industrial applications versus the well-known FINEMET.     

dHybrid: A massively parallel code for hybrid simulations of space plasmas

A massively parallel simulation code, called dHybrid, has been developed to perform global scale studies of space plasma interactions. This code is based on an explicit hybrid model; the numerical stability and parallel scalability of the code are studied. A stabilization method for the explicit algorithm, for regions of near zero density, is proposed. Three-dimensional hybrid simulations of the interaction of the solar wind with unmagnetized artificial objects are presented, with a focus on the expansion of a plasma cloud into the solar wind, which creates a diamagnetic cavity and drives the Interplanetary Magnetic Field out of the expansion region. The dynamics of this system can provide insights into other similar scenarios, such as the interaction of the solar wind with unmagnetized planets.     

Block-structured grids in Lagrangian 3D edge plasma transport simulations

Distinct from conventional Eulerian 2D fluid solvers, applied routinely to magnetic fusion edge plasma studies, complex 3D magnetic topologies are currently treated by the geometrically more flexible Lagrangian schemes, supplemented by Monte Carlo procedures for higher order derivatives (dissipative terms due to diffusion processes) and sources. These particle based algorithms are combined with a field line reconstruction technique for dealing with partially ergodic magnetic fields, involving field aligned regular grids. A generalization to block-structured grids is carried out, which greatly enhances the applicability range of present 3D fusion plasma edge codes, in particular also to poloidally magnetic diverted configurations, as currently envisaged for the largest magnetic fusion device under construction: ITER.     

Development of crystal texture in R-lean RFeCoNbB (R = Nd,Pr) alloy during melt spinning processes

The formation of crystal texture of R₂Fe₁₄B nanocrystals in R–Fe–B (R = rare earth) alloys with low R content (<11.8 at.%) is the most critical issue for the development of anisotropic nanocomposite magnets. In the present study, we succeeded in yielding a strong crystal texture for (Nd,Pr)₂Fe₁₄B nanocrystals during the melt spinning processes of Nd_3.6Pr_5.4Fe₈₀Co₃NbB₇ by effectively employing the seeding effect of α-Fe nanocrystal texture. The (Nd,Pr)₂Fe₁₄B nanocrystals produced from the R-lean alloy at a wheel speed of 18 m/s show a strong (0 0 l) texture parallel to the ribbon plane, which yields a high remanence M_r = 0.78M_s and a large energy product (BH)_max = 25.2 MGOe for the α-Fe/(Nd,Pr)₂Fe₁₄B nanocomposite ribbons. The present study provides a promising approach to prepare anisotropic nanocomposite magnets from R-lean alloys.     

On a semi-implicit scheme for spectral simulations of dispersive magnetohydrodynamics

The effectiveness of a semi-implicit (SI) temporal scheme is discussed in the context of the dispersive magnetohydrodynamics where, due to the whistler modes, stability of explicit algorithms requires a time step decreasing quadratically as the resolution is linearly increased. After analyzing the effects of this scheme on the Alfvén-wave dispersion relation, spectral simulations of nonlinear initial value problems where small-scale dispersion has a main effect on the global dynamics are presented. Permitting a moderate, albeit significant, increase of the time step for a minor additional cost relatively to explicit schemes, the SI algorithm provides an efficient tool in situations, such as turbulent regimes, where the time steps making fully implicit schemes efficient are too large to ensure a satisfactory accuracy.     

Magnetic properties of the 1/1 approximant Ag42In42Gd16 to the icosahedral quasicrystal Ag-In-Gd

The structural, magnetic, and ¹⁵⁵Gd Mössbauer spectral properties of the 1/1 approximant Ag₄₂In₄₂Gd₁₆ to an icosahedral quasicrystal Ag-In-Gd are reported. Based on dc magnetic susceptibility measurements, it is shown that the studied compound develops no long-range magnetic order in the temperature range 1.8-300 K. The dc zero-field-cooled and field-cooled susceptibility data indicate that the 1/1 approximant Ag₄₂In₄₂Gd₁₆ is a spin glasss with freezing temperature T_f = 3.6(1) K. This is further confirmed by the analysis of the frequency dependence of T_f using the Vogel-Fulcher law and the dynamic scaling behavior near T_f. It is argued that the spin freezing process is a true equilibrium phase transition rather than a nonequilibrium phenomenon. The large frustration parameter of the studied compound indicates that it belongs to a category of strongly geometrically frustrated magnets. The ¹⁵⁵Gd Mössbauer spectra of the 1/1 approximant Ag₄₂In₄₂Gd₁₆ confirm that the Gd spins are frozen at 1.5 K and are fluctuating at 4.6 K. The Debye temperature of the 1/1 approximant Ag₄₂In₄₂Gd₁₆ is 200(1) K.     

Constructing plasma response models from full toroidal magnetohydrodynamic computations

We explore accurate and efficient algorithms for constructing plasma response models, based on the computed data using a full toroidal MHD stability code MARS-F. These response models are used to study feedback stabilization of resistive wall modes for fusion plasmas. Three approaches are discussed and compared. A direct full-model computation offers the most accurate response, unfortunately without producing analytical expressions for the response model. The pole-residue expansion methods yield analytical and asymptotically rigorous response models. A low-order Padé approximation serves as a model reduction technique that simplifies the controller design, while keeping a reasonable accuracy for the response models. From the computational viewpoint, the most efficient approaches are the pole-residue expansion based on eigenfunction projection, and the low-order Padé approximation.     

ORCO: A numerical tool to study the radial diffusion of runaway electrons in tokamaks

ORCO is a new code that estimates the spatial structure of the radial diffusion coefficient for runaway electrons in tokamak geometry. In real experiments, the location of these electrons can be detected by measuring the time evolution of their fast electron bremsstrahlung (FEB) emissivities, usually integrated along several lines of view. ORCO uses a Levenberg-Marquardt algorithm to adjust the free parameters of a generalized transport model to best reproduce the time evolution of these temporal traces. A possible future application for this type of calculations is to use them as indirect probes to test theoretical models of turbulent transport driven by stochastic magnetic fields in tokamaks.