Measuring the complex impedance of a superconducting magnet as a function of frequency provides valuable insight into its electrodynamics. In particular, the characteristic features of some ...non-conform behaviour, such as an insulation fault, may be easier to assess when performing impedance measurements rather than observing time-domain signals. A physics-driven equivalent circuit model of a superconducting magnet has been recently developed, whose parameters are derived using solely measured geometric and material properties. This contribution describes its validation against impedance measurements of a spare LHC superconducting main dipole, performed at the CERN magnet test facility. The proposed model includes lumped-elements capturing individual physical phenomena, such as superconducting filament magnetization, inter-filament and inter-strand coupling currents, eddy currents in the strand copper matrix and various magnet components, and stray capacitances. It is possible to predict the impact of different physical effects in different frequency ranges and compare simulations to experimental results. It is shown that the validated model can accurately reproduce the magnet's impedance in a frequency range up to 5 kHz in the different conditions considered.
A numerical model for the adhesion of osteoblasts on titanium micropillar structures is suggested, and a function representing the concentration level of the adhesion on the pillars is constructed ...based on experimental observation. The introduction of this function helps a well‐known bio‐chemo‐mechanical model to better predict the formation of actin in osteoblasts when they are laid on arrays of titanium micro‐pillars of various size attached to silicon substrate. A parameter study suggests that each pillar is associated with a different pattern of adhesion. Our finding emphasises a capability of the bio‐chemo‐mechanical model that it can well explain the strong influence of the boundary condition on the formation of actin within the cells.
A mathematical description for an inhomogeneous cell adhesion was proposed to be used with a well‐known bio‐chemo‐mechanical model that has been widely applied in cell mechanics. The augmented model could predict the distribution of actin in osteoblasts when they are laid on arrays of titanium micropillars of various size attached to silicon substrate.
Introduction Deep brain stimulation (DBS) is a common therapy for the late-stage treatment of Parkinson’s disease. In order to explore clinically relevant questions, DBS is studied both in vivo in ...animal models and in silico in computational models. Objectives Creating an anatomically realistic volume conductor model (VCM) of the rat brain allows for a close interrelationship of in vivo and in silico models. Thus, an optimization of the stimulation field distribution and an improvement of DBS in animal models is facilitated. Various quantities affect the resulting spatial distribution of the stimulation field in a VCM, namely the specific topology of the compartments and the electric properties of brain tissue (e.g., its anisotropy). Methods Based on the quasi-static modeling approach Laplace equation div( σ )grad(Φ) = 0 is solved, with σ as electric conductivity and Φ as electric potential. The potential distribution evolved by DBS is computed with COMSOL Multiphysics® (Comsol). With the digital rat brain atlas in Papp et al., 2014 , the target region for DBS is assembled as three-dimensional structures. Modeled grey matter nuclei are the subthalamic nucleus (STN) and the entopeduncular nucleus (EPN). In the cubic region of interest (3 mm edge length) all white matter structures and cerebrospinal fluid cavities are included. Whilst the electrical conductivities of grey matter may be assumed as isotropic, for white matter an anisotropic conductivity should be assigned. By assuming that white matter has a ten times higher longitudinal conductivity along the axonal fibers than in transverse direction ( Tuch et al., 2001 ), the conductivity tensor is defined. The orientation of the anisotropic properties follows curvilinear coordinates. These coordinates are computed by using the diffusion method implemented in Comsol by solving Laplace’s equation, −grad(U) = 0, where the gradient of the solution U forms the first basis vector of the coordinate system. Thereby the anisotropy follows the shape of the white matter fiber bundles. The bipolar electrode model and the stimulation parameters are based on the in vivo model in Badstübner, 2013. Results In this model, an anisotropic conductivity following curvilinear coordinates is implemented in the corticofugal pathway (CP). In Fig. 1 the periprosthetic potential either for the fully isotropic or partially anisotropic model is compared. Here, the relative difference of the potential Φ is the difference between a baseline model, with a homogeneous conductivity in all structures, and both heterogeneous models. The relative difference varies up to 15% and 60% in the isotropic and anisotropic case, respectively. Conclusion Our results suggest that neglecting an anatomically realistic anisotropy would lead to incorrect assessments of the neuronal activation.
Microelectrode arrays serve as an indispensable tool in electro-physiological research to study the electrical activity of neural cells, enabling measurements of single cell as well as network ...communication analysis. Recent experimental studies have reported that the neuronal geometry has an influence on electrical signaling and extracellular recordings. However, the corresponding mechanisms are not yet fully understood and require further investigation. Allowing systematic parameter studies, computational modeling provides the opportunity to examine the underlying effects that influence extracellular potentials. In this letter, we present an in silico single cell model to analyze the effect of geometrical variability on the extracellular electric potentials. We describe finite element models of a single neuron with varying geometric complexity in three-dimensional space. The electric potential generation of the neuron is modeled using Hodgkin-Huxley equations. The signal propagation is described with electro-quasi-static equations, and results are compared with corresponding cable equation descriptions. Our results show that both the geometric dimensions and the distribution of ion channels of a neuron are critical factors that significantly influence both the amplitude and shape of extracellular potentials.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This paper reports on comprehensive efforts on uncertainty quantification and global sensitivity analysis for accelerator cavity design. As a case study object the TESLA shaped superconducting ...cavities, as produced for the European X-ray Free Electron Laser (EXFEL), are selected. The choice for these cavities is explained by the available measurement data that can be leveraged to substantiate the simulation model. Each step of the manufacturing chain is documented together with the involved uncertainties. Several of these steps are mimicked on the simulation side, e.g., by introducing a random eigenvalue problem. The uncertainties are then quantified numerically and in particular the sensitivities give valuable insight into the system behavior. We also compare these findings to purely statistical studies carried out for the manufactured cavities. More advanced, adaptive, surrogate modeling techniques are adopted, which are crucial to incorporate a large number of uncertain parameters. The main contribution is the detailed comparison and fusion of measurement results for the EXFEL cavities on the one hand and simulation based uncertainty studies on the other hand. After introducing the quantities of physical interest for accelerator cavities and the Maxwell eigenvalue problem, the details on the manufacturing of the EXFEL cavities and measurements are reported. This is followed by uncertainty modeling with quantification studies.
Introduction: Electrical stimulation has been used as a promising approach in bone repair for several decades. However, the therapeutic use is hampered by inconsistent results due to a lack of ...standardized application protocols. Recently, electrical stimulation has been considered for the improvement of the osseointegration of dental and endoprosthetic implants. Methods: In a pilot study, the suitability of a specifically developed device for electrical stimulation in situ was assessed. Here, the impact of alternating electric fields on implant osseointegration was tested in a gap model using New Zealand White Rabbits. Stimulation parameters were transmitted to the device via a radio transceiver, thus allowing for real-time monitoring and, if required, variations of stimulation parameters. The effect of electrical stimulation on implant osseointegration was quantified by the bone-implant contact (BIC) assessed by histomorphometric (2D) and µCT (3D) analysis. Results: Direct stimulation with an alternating electric potential of 150 mV and 20 Hz for three times a day (45 min per unit) resulted in improved osseointegration of the triangular titanium implants in the tibiae of the rabbits. The ratio of bone area in histomorphometry (2D analysis) and bone volume (3D analysis) around the implant were significantly increased after stimulation compared to the untreated controls at sacrifice 84 days after implantation. Conclusion: The developed experimental design of an electrical stimulation system, which was directly located in the defect zone of rabbit tibiae, provided feedback regarding the integrity of the stimulation device throughout an experiment and would allow variations in the stimulation parameters in future studies. Within this study, electrical stimulation resulted in enhanced implant osseointegration. However, direct electrical stimulation of bone tissue requires the definition of dose-response curves and optimal duration of treatment, which should be the subject of subsequent studies.
The Z-pole option of FCC-ee is an Ampere class machine with a beam current of 1.39 A. Due to high HOM power and strong HOM damping requirements, the present baseline of FCC-ee considers a single-cell ...cavity at 400 MHz. In this paper, different HOM damping schemes are compared for the Z-pole operating scenario with the aim of lowering the parasitic longitudinal and transverse impedance. The HOM power for each damping scheme is also calculated.
The aim of this study was to examine the influence of uncertainty of the material properties of brain tissue on the probabilistic voltage response and the probabilistic volume of tissue activated ...(VTA) in a volume conductor model of deep brain stimulation. To quantify the uncertainties of the desired quantities without changing the deterministic model, a nonintrusive projection method was used by approximating these quantities by a polynomial expansion on a multidimensional basis known as polynomial chaos. The coefficients of this expansion were computed with a multidimensional quadrature on sparse Smolyak grids. The deterministic model combines a finite element model based on a digital brain atlas and a multicompartmental model of mammalian nerve fibers. The material properties of brain tissue were modeled as uniform random parameters using data from several experimental studies. Different magnitudes of uncertainty in the material properties were computed to allow predictions on the resulting uncertainties in the desired quantities. The results showed a major contribution of the uncertainties in the electrical conductivity values of brain tissue on the voltage response as well as on the predicted VTA, while the influence of the uncertainties in the relative permittivity was negligible.
Contact between a charged metal surface and an electrolyte implies a particular ion distribution near the charged surface, i.e. the electrical double layer. In this mini review, different mean-field ...models of relative (effective) permittivity are described within a simple lattice model, where the orientational ordering of water dipoles in the saturation regime is taken into account. The Langevin-Poisson-Boltzmann (LPB) model of spatial variation of the relative permittivity for point-like ions is described and compared to a more general Langevin-Bikerman (LB) model of spatial variation of permittivity for finite-sized ions. The Bikerman model and the Poisson-Boltzmann model are derived as limiting cases. It is shown that near the charged surface, the relative permittivity decreases due to depletion of water molecules (volume-excluded effect) and orientational ordering of water dipoles (saturation effect). At the end, the LPB and LB models are generalised by also taking into account the cavity field.
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