arXiv subjects

Kiwan Park

Publications and source records attributed to Kiwan Park.

At least 19 recordsLinked to original sources

arXiv2026

Connecting Dynamo Theory with DNS Data: A Computational Analysis of {\alpha} and \b{eta} Effects

We investigate the influence of current helicity on the turbulent magnetic diffusivity $\beta$ using three complementary derivations of the $\alpha$ and $\beta$ coefficients, based on the large-scale magnetic field $\overline{\mathbf{B}}$, the turbulent velocity $\mathbf{u}$, and the turbulent magnetic field $\mathbf{b}$. Applying these coefficients to raw DNS data, we reconstruct $\overline{\mathbf{B}}$ and compare the results with the original simulations. In the kinematic regime all models agree well with the DNS data. In the nonlinear regime, however, $\beta_{\mathrm{vv-vw}}$ alone produces unbounded growth of $\overline{\mathbf{B}}$. Including the contribution from turbulent magnetic fields ($\beta_{\mathrm{bb+jb}}$) suppresses this unphysical growth and restores agreement with the DNS results. We find that kinetic helicity drives $\beta$ more negative, while current helicity shifts it back toward zero. Weighted combinations of the coefficients further show that the $\beta$ effect dominates the evolution of $\overline{\mathbf{B}}$ throughout, whereas the $\alpha$ effect becomes important mainly for sustaining the field in the nonlinear regime. The corresponding IDL analysis scripts are provided to facilitate practical implementation of the theoretical models.

astro-ph.SR↗

arXiv2025

Magnetic Field Amplification and Reconstruction in Rotating Astrophysical Plasmas: Verifying the Roles of $α$ and $β$ in Dynamo Action

We investigated the $α$ and $β$ effects in a rotating spherical plasma system relevant to astrophysical environments. These coefficients were derived using three different approaches based on the large-scale magnetic field $\overline{\mathbf{B}}$, turbulent velocity $\mathbf{u}$, and turbulent magnetic field $\mathbf{b}$, yielding $α_{\mathrm{EM-HM}}$, $β_{\mathrm{EM-HM}}$, $β_{\mathrm{vv-vw}}$, and $β_{\mathrm{bb+jb}}$. Using raw data from direct numerical simulations (DNS), we constructed the magnetic induction equation incorporating the $α$ and $β$ coefficients. We then reproduced the $\overline{\mathbf{B}}$ field and compared the results with the DNS data. In the kinematic regime, where $\overline{\mathbf{B}}$ is weak, all models exhibit good agreement with the DNS results. However, in the nonlinear regime, the $\overline{\mathbf{B}}$ field, reproduced using $β_{\mathrm{vv-vw}}$, deviates from the DNS and exhibits unbounded growth. To address this discrepancy, we added $β_{\mathrm{bb+jb}}$, which represents the contribution of turbulent magnetic fields, to $β_{\mathrm{vv-vw}}$. This addition suppresses the divergent growth of $\overline{\mathbf{B}}$ in the nonlinear regime. We then assessed the actual influence of $α$ and $β$ on the evolution of $\overline{\mathbf{B}}$ by applying weighted combinations of the two coefficients. Our results show that magnetic $β$ diffusion plays a dominant role throughout the entire process. In contrast, the $α$ effect is minor in the kinematic regime but becomes essential for sustaining the $\overline{\mathbf{B}}$ field in the nonlinear regime. We also discussed the underlying physical mechanism responsible for this behavior.

physics.plasm-ph↗

arXiv2025

$α$ Effect and Magnetic Diffusivity $β$ in Helical Plasma under Turbulence Growth

We investigate the transport coefficients $α$ and $β$ in plasma systems with varying Reynolds numbers while maintaining a unit magnetic Prandtl number. {The $α$ and $β$ tensors parameterize the turbulent electromotive force (EMF) in terms of the large-scale magnetic field ${\bf \overline{B}}$ and current density ${\bf \overline{}}$ as follows : $\langle {\bf u}\times {\bf b} \rangle = α{\bf \overline{B}}-β{\nabla\times \bf \overline{B}}$.} In astrophysical plasmas, high fluid Reynolds numbers ($Re$) and magnetic Reynolds numbers ($Re_\mathrm{M}$) drive turbulence, where $Re$ governs flow dynamics and $Re_\mathrm{M}$ controls magnetic field evolution. The coefficients $α_{\text{semi}}$ and $β_{\text{semi}}$ are obtained from large-scale magnetic field data as estimates of the $α$ and $β$ tensors, while $β_{\text{theo}}$ is derived from turbulent kinetic energy data. The reconstructed large-scale field $\overline{B}$ agrees with simulations, confirming consistency among $α$, $β$, and $\overline{B}$ in weakly nonlinear regimes. This highlights the need to incorporate magnetic effects under strong nonlinearity. To clarify $α$ and $β$, we introduce a field structure model, identifying $α$ as the electrodynamic induction effect and $β$ as the fluid-like diffusion effect. The agreement between our method and direct simulations suggests that plasma turbulence and magnetic interactions can be analyzed using fundamental physical quantities. Moreover, $α_{\text{semi}}$ and $β_{\text{semi}}$, which successfully reproduce the numerically obtained magnetic field, provide a benchmark for future theoretical studies.

physics.plasm-ph↗

arXiv2024

Effect of Turbulent Kinetic Helicity on Diffusive beta effect for Large Scale Dynamo

We investigated a plasma system with kinematic viscosity ($ν= 0.006$) and magnetic diffusivity ($η= 0.006$), driven by helical kinetic energy, to study the dynamics of energy and helicity in magnetic diffusion. Using the numerical data obtained, we explored methods to determine the $α$ and $β$ coefficients that linearize the nonlinear electromotive force (EMF) and the dynamo process. Initially, we applied conventional statistical approaches such as mean-field theory (MFT), direct interaction approximation (DIA), and eddy-damped quasinormal Markovian (EDQNM) closure. We then proposed a simpler alternative method using large-scale magnetic data and turbulent kinetic data to calculate $α$ and $β$. Our findings show that while $α$ qualitatively aligns with theoretical predictions, $β$ remains negative, indicating an inverse cascade of energy through magnetic diffusion. This deviated from conventional models and was further analyzed using a recursive method in the second moment identity, revealing that small-scale kinetic helicity couples with large-scale current density to transport energy inversely. We validated our method by reproducing the numerically calculated data. The consistency between our method and direct numerical simulations (DNS) suggests that the negative diffusion process in plasma has a physical basis.

astro-ph.SR↗

arXiv2024

Analytical approach to the design of RF photoinjector

The objective of this thesis is to ascertain the dimensions of an RF 2.856GHz photoinjector through a combination of analytical and computational approaches. The phase velocity within a single cavity exceeds 'c', rendering it inadequate for storing the requisite energy for beam acceleration. To surmount this limitation, we aim to devise a multi-celled cavity design. However, the alterations in electromagnetic fields and resonant frequency within the multi-celled cavity are intricate and sensitive, presenting challenges in obtaining precise dimensions solely via computer simulations. Prior to numerical methods, it is essential to analyze the photoinjector using theoretical frameworks. We employ perturbation theory and the construction of an equivalent circuit to elucidate the underlying physics of the photoinjector and the electrical oscillations within the cell structure. Detailed analytical methods for the equivalent circuit are explored. Through theoretical analysis, the dimensions and simulation outcomes can be determined quantitatively.

physics.plasm-ph↗

arXiv2024

Magnetic Effect on Potential Barrier for Nucleosynthesis II

We investigate the impact of magnetic fields on the potential barrier between two interacting nuclei. We addressed this by solving the Boltzmann equation and Maxwell's theory in the presence of a magnetic field, resulting in the determination of magnetized permittivity. Additionally, we derived the magnetized Debye potential, which combines the conventional Debye potential with an additional magnetic component. We then compared the Boltzmann approach with the Debye method. Both methods consistently demonstrate that magnetic fields increase permittivity. This enhanced permittivity leads to a reduction in the potential barrier, consequently increasing the reaction rate for nucleosynthesis. Furthermore, the dependence on temperature and electron density in each approach is consistent. Our findings suggest that magnetized plasmas, which have existed since the Big Bang, have played a crucial role in nucleosynthesis.

astro-ph.CO↗

arXiv2023

Magnetic Effect on the Potential Barrier for Nucleosynthesis

We demonstrated that a weak magnetic field can increase the permittivity, leading to a reduction in the potential barrier within the Debye sphere consisting of electrons and a nucleus. By solving the Boltzmann equation with the inclusion of the magnetic field, we obtained the magnetized permittivity. The resulting enhanced permittivity field inversely decreases the potential barrier, thereby increasing the reaction rate between two fusing nuclei. We compared this Boltzmann kinetic approach with the Debye potential method. We found that they are qualitatively consistent. Further, we also derived the magnetized Debye potential composed of the conventional term with a new magnetic effect. Both approaches indicate that magnetized plasmas, which have existed since the Big Bang, have ultimately influenced permittivity, potential barrier, and nucleosynthesis.

physics.plasm-ph↗

arXiv2022

Effects of electromagnetic fluctuations in plasmas on solar neutrino fluxes

We explore the effects of electromagnetic (EM) fluctuations in plasmas on solar neutrino fluxes exploiting the fluctuation-dissipation theorem. We find that the EM spectrum in the solar core is enhanced by the EM fluctuations due to the high density of the Sun, which increases the radiation energy density and pressure. By the EM fluctuations involving the modified radiation formula, the central temperature decreases when the central pressure of the Sun is fixed. With a help of the empirical relation between central temperature and neutrino fluxes deduced from the numerical solar models, we present the change in each of the solar neutrino fluxes by the EM fluctuations. We also discuss the enhanced radiation pressure and energy density by the EM fluctuations for other astronomical objects.

astro-ph.SR↗

arXiv2021

Dynamical Screening Effects on Big Bang Nucleosynthesis

A moving ion in plasma creates a deformed electric potential depending on the ion velocity, which leads to the distinct screening effect compared to the standard static Salpeter formula. In this paper, adopting the test charge method, we explore the dynamical screening effects on big bang nucleosynthesis (BBN). We find that the high temperature in the early universe causes the ion velocity to be faster than the solar condition so that the electric potential is effectively polarized. However, the low density of background plasma components significantly suppresses the dynamical screening effects on thermonuclear reaction rates during the BBN epoch. We compare our results with several thermonuclear reaction rates for solar fusion considering the dynamical screening effects. Also, we discuss the additional plasma properties in other astrophysical sites for the possible expansion from the present calculation in the future.

nucl-th↗

arXiv2021

Negative Turbulent Magnetic Diffusivity $β$ effect in a Magnetically Forced System

We have studied the large scale dynamo process forced with helical magnetic energy. The magnetically driven dynamo is not so well studied as kinetically forced dynamo. It has been thought to represent the amplification of magnetic field in the stellar corona, accretion disk, or plasma lab. However, the interaction between the helical magnetic field and plasma is a more fundamental phenomenon that can be extended to the early Universe. The scale-invariant helical magnetic field not only explains the currently observed large scale astrophysical magnetic fields but also has information on the horizon scale in the early Universe. The interaction between magnetic field and plasma is inherently non-linear, making its mechanism difficult to understand. But, if the plasma system is driven with helical field, the process can be linearized with alpha&betaand large scale magnetic field. Conventionally, alpha effect is thought to transfer magnetic field to the large scale regime, and betaeffect diffuses magnetic field. However, these conclusions are based on the incompletely derived alpha&beta. To get the exact profiles of evolving alpha&\b{eta}, we solved a coupled semi-analytic equation set and applied the result to simulation data for the large scale magnetic helicity and magnetic energy. Our result shows that the averaged alpha effect decreases before making a significant contribution to the amplification of large scale B field. Rather, betaeffect, which keeps negative, de facto plays a key role in the amplification of large scale B field with Laplacian. And, this negative diffusivity accounts for the attenuation of plasma kinetic energy

physics.plasm-ph↗

arXiv2019

Negative Magnetic Diffusivity beta replacing alpha effect in Helical Dynamo

The alpha effect is known to be an indispensable energy source of the poloidal magnetic field in the sun or planet. However, the alpha effect is quenched as the magnetic field grows due to the conservation of magnetic helicity. With these conventional understanding, what indeed generates and sustains the observed poloidal magnetic field remains a mystery. To solve this contradiction between theory and real nature, we derived a semi-analytic representation of alpha and beta using large scale magnetic helicity and energy. Applying the simulation data to alpha and beta, we found that the negative beta effect is a promising substitution of the quenched alpha effect. However, since the negative beta effect contradicts the conventional dynamo theory, we derived the new beta expression referring to the field structure model. This analytic result with the field relation between velocity U and magnetic field B shows that the beta effect in the helical system is not a fixed one. Rather, it plays a variable and dynamic role in dynamo depending on the interaction between the poloidal velocity field and relative strength of the large scale magnetic field.

astro-ph.SR↗

arXiv2018

Principle of Helical \& Nonhelical Dynamo and $α$ effect in Field Structure model

We explain the (non)helical dynamo process using a field-structure model based on magnetic induction equation in an intuitive way. We show how nonhelical kinetic energy converts into magnetic energy and cascades toward smaller eddies in a mechanically forced plasma system. Also, we show how helical magnetic energy is inversely cascaded ($α$ effect) toward large scale magnetic eddies in a mechanically or magnetically forced system. We, then, compare the simulation results with the model qualitatively for the verification of the model. In addition to these intuitive and numerical approaches, we show how to get $α$ and $β$ coefficient semi-analytically from the temporally evolving large scale magnetic energy and magnetic helicity.

astro-ph.CO↗

arXiv2017

On the Inverse Transfer of (Non-)Helical Magnetic Energy in a Decaying Magnetohydrodynamic Turbulence

In our conventional understanding, large-scale magnetic fields are thought to originate from an inverse cascade in the presence of magnetic helicity, differential rotation, or a magneto-rotational instability. However, as recent simulations have given strong indications that an inverse cascade (transfer) may occur even in the absence of magnetic helicity, the physical origin of this inverse cascade is still not fully understood. We here present two simulations of freely decaying helical \& non-helical magnetohydrodynamic (MHD) turbulence. We verified the inverse transfer of helical and non-helical magnetic fields in both cases, but we found the underlying physical principles to be fundamentally different. In the former case, the helical magnetic component leads to an inverse cascade of magnetic energy. We derived a semi analytic formula for the evolution of large scale magnetic field using $α$ coefficient and compared it with the simulation data. But in the latter case, the $α$ effect, including other conventional dynamo theories, are not suitable to describe the inverse transfer of non-helical magnetic. To obtain a better understanding of the physics at work here, we introduced a `field structure model' based on the magnetic induction equation in the presence of inhomogeneities. This model illustrates how the curl of the electromotiveforce (EMF) leads to the build up of a large-scale magnetic field without the requirement of magnetic helicity. And we applied a Quasi Normal approximation to the inverse transfer of magnetic energy.

physics.plasm-ph↗

arXiv2016

Dynamo model for the inverse transfer of magnetic energy in a nonhelical decaying magnetohydrodynamic turbulence

The inverse cascade of magnetic energy occurs when helicity or rotational instability exists in the magnetohydrodynamic (MHD) system. This well known phenomenon has been considered as a basis for the large scale magnetic field in universe. However nonhelical magnetic energy in a decaying MHD system also migrates toward the large scale, which holds vital clues to the origin of large scale magnetic field in a quiescent astrophysical system. Zeldovich's rope dynamo model is considered as a basic and symbolistic model of magnetic field amplification. However, the rope model assuming specific external forces like buoyancy or Coriolis force is not appropriate for a decaying turbulent system without any external force. So we suggest a new dynamo model based on magnetic induction equation excluding a forcing source. This model shows the expansion and growth of magnetic field (flux) is basically the redistribution of energy in the system. The transfer of magnetic energy is in fact a successive induction of magnetic field resulted from the interaction between the fluid motion and seed magnetic field. We also discuss about an analytic theorem based on the scaling invariant MHD equation.

physics.plasm-ph↗

arXiv2015

Evolution of Kinetic and Magnetic Energy in Intra Cluster Media

Intra Cluster Media (ICMs) located at galaxy clusters is in the state of hot, tenuous, magnetized, and highly ionized X-ray emitting plasmas. This overall collisionless, viscous, and conductive magnetohydrodynamic (MHD) turbulence in ICM is simulated using hyper and physical magnetic diffusivity. The results show that fluctuating random plasma motion amplifies the magnetic field, which cascades toward the diffusivity scale passing through the viscous scale. The kinetic eddies in the subviscous scale are driven and constrained by the magnetic tension which finally gets balanced with the highly damping effect of the kinetic eddies. However, the saturated kinetic energy spectrum is deeper than that of the incompressible or compressible hydrodynamics fluid. To explain this unusual field profile we set up two simultaneous differential equations for the kinetic and magnetic energy spectrum using an Eddy Damped Quasi Normal Markovianized (EDQNM) approximation. The analytic solution tells us that the magnetic energy in addition to the viscous damping effect constrains the plasma motion leading to the power spectra: kinetic energy spectrum $E_V^k\sim k^{-3}$ and corresponding representative magnetic energy spectrum $E_M^k\sim k^{-1/2}$. Also the comparison of simulation results with different resolutions and magnetic diffusivities implies the role of small scale magnetic energy in dynamo.

physics.plasm-ph↗

arXiv2014

Influence of small scale $E_M$ and $H_M$ on the growth of large scale magnetic field

We investigated the influence of small scale magnetic energy ($E_M$) and magnetic helicity ($H_M$) on the growth rate ($γ$) of large scale magnetic field ($\overline{\bf B}$). $H_M$ that plays a key role in MHD dynamo is a topological concept describing the structural properties of magnetic fields. So, it is not possible to differentiate the intrinsic properties of $H_M$ from the influence of $E_M$, and vice versa. However, to understand MHD dynamo the features of helical and nonhelical magnetic field should be made clear. For this, we made a detour: we gave each simulation set its own initial condition ($IC$, same $E_M$(0) and specific $H_M$(0) at $k_f=5$), and then drove the system with positive helical kinetic energy($k_f=5$). According to the simulation results, $E_M$(0), whether or not helical, increases the growth rate of $\overline{\bf B}$. The positive $H_M$(0) boosts the increased growth rate, but the negative $H_M$(0) decreases it. To explain these results two coupled equations of $H_M$ and $E_M$ were derived and solved using a simple approximate method. The equations imply that helical magnetic field generates the whole (helical and nonhelical) magnetic field but quenches itself. Nonhelical magnetic field also generates the whole magnetic field but quenches itself. The initially given $E_M$(0) modifies the electromotive force ($\langle {\bf v}{\bf \times} {\bf b}\rangle$, $EMF$) and generates new terms. The effects of these terms depend on the magnetic diffusivity $η$, position of initial conditions $k_f$, and time. But the influence disappears as time passes ($\sim e^{-ηk_f^2 t}$), so the saturated magnetic fields are independent of the initial conditions.

physics.plasm-ph↗

arXiv2014

Influence of small scale magnetic energy and helicity on the growth of large scale magnetic field

The influence of initially given small scale magnetic energy($E_M(0)$) and helicity($H_M(0)$) on the magnetohydrodynamics(MHD) dynamo was investigated. Equations for $E_M$(t), $H_M$(t), and electromotive force($\langle {\bf v}\times {\bf b}\rangle$, $EMF$) were derived and solved. The solutions indicate small scale magnetic field(${\bf b}_i$) caused by $E_M$(0) modifies $EMF$ and generates additional terms of which effect depends on magnetic diffusivity $η$, position of initial conditions($IC$s) $k_f$, and time ($\sim e^{-ηk_f^2 t}$). ${\bf b}_i$ increases the inverse cascade of energy resulting in the enhanced growth of large scale magnetic field($\overline{\bf B}$). Simulation data show that $E_M$(0) in small scale boosts the growth rate, which also proportionally depends on $H_M(0)$. If $E_M$(0) is the same, positive $H_M(0)$ is more effective for MHD dynamo than negative $H_M(0)$ is. It was discussed why large scale magnetic helicity should have the opposite sign of the injected kinetic helicity.

astro-ph.SR↗

arXiv2013

Influence of initial conditions on the large-scale dynamo growth rate

To investigate the effect of energy and helicity on the growth of magnetic field, helical kinetic forcing was applied to the magnetohydrodynamic(MHD) system that had a specific distribution of energy and helicity as initial conditions. Simulation results show the saturation of a system is not influenced by the initial conditions, but the growth rate of large scale magnetic field is proportionally dependent on the initial large scale magnetic energy and helicity. It is already known that the helical component of small scale magnetic field(i.e., current helicity $<{\bf j}\cdot {\bf b}>$) quenches the growth of large scale magnetic field. However, $<{\bf j}\cdot {\bf b}>$ can also boost the growth of large scale magnetic field by changing its sign and magnitude. In addition, simulation shows the nonhelical magnetic field can suppress the velocity field through Lorentz force. Comparison of the profiles of evolving magnetic and kinetic energy indicates that kinetic energy migrates backward when the external energy flows into the three dimensional MHD system, which means the velocity field may play a preceding role in the very early MHD dynamo stage.

astro-ph.EP↗