The Experts below are selected from a list of 261 Experts worldwide ranked by ideXlab platform
Paradela I Perez - One of the best experts on this subject based on the ideXlab platform.
-
assessment of particle and heat loads to the upper open divertor in asdex upgrade in favourable and unfavourable Toroidal Magnetic Field directions
Nuclear materials and energy, 2019Co-Authors: Paradela I Perez, M Groth, M Wischmeier, A Scarabosio, D Brida, P David, D Silvagni, D Coster, T LuntAbstract:Abstract Pairs of ASDEX Upgrade L-mode discharges with the Toroidal Magnetic Field, BT, in the forward and reverse directions have been used to study the impact of neoclassical drifts on the divertor plasma conditions and detachment. The evolution of the peak heat flux and the total power loads onto both the outer and the inner targets depends significantly on the Toroidal Field direction: increasing the core plasma density affects mainly the heat loads in the BT 0 (favourable). Ion saturation current measurements show similar trends to those of the IR heat flux data. These discrepancies are not only caused by drifts but also by different levels of radiated power in the core, thus the power across the separatrix, Psep. Tomographic reconstructions show that Psep is not constant within the entire dataset. Finally, at I p = 0.8 MA , a significant reduction of the peak heat flux is observed at both targets for both Field directions. On the other hand, at I p = 0.6 MA , a reduction of the peak heat flux is only observed for BT I p = 0.8 MA .
Bernard R. Durney - One of the best experts on this subject based on the ideXlab platform.
-
The Energy Lost by Differential Rotation in the Generation of the Solar Toroidal Magnetic Field
Solar Physics, 2000Co-Authors: Bernard R. DurneyAbstract:The integrals, I_i(t) = ∫_GL u_i j × B _ i d v over the volume GL are calculated in a dynamo model of the Babcock–Leighton type studied earlier. Here, GL is the generating layer for the solar Toroidal Magnetic Field, located at the base of the solar convection zone (SCZ); i=r, θ, φ, stands for the radial, latitudinal, and azimuthal coordinates respectively; j = (4π)^-1 ∇ × B , where B is the Magnetic Field; u_r,u_θ are the components of the meridional motion, and u_φ is the differential rotation. During a ten-year cycle the energy ∫_cycle I_φ(t)d t needs to be supplied to the azimuthal flow in the GL to compensate for the energy losses due to the Lorentz force. The calculations proceed as follows: for every time step, the maximum value of |B_φ| in the GL is computed. If this value exceeds B_cr (a prescribed Field) then there is eruption of a flux tube that rises radially, and reaches the surface at a latitude corresponding to the maximum of |B_φ| (the time of rise is neglected). This flux tube generates a bipolar Magnetic region, which is replaced by its equivalent axisymmetric configuration, a Magnetic ring doublet. The erupted flux can be multiplied by a factor F_t, i.e., by the number of eruptions per time step. The model is marginally stable and the ensemble of eruptions acts as the source for the poloidal Field. The arbitrary parameters B_cr and F_t are determined by matching the flux of a typical solar active region, and of the total erupted flux in a cycle, respectively. If E(B) is the energy, in the GL, of the Toroidal Magnetic Field B_φ = B sin θ cos θ, B (constant), then the numerical calculations show that the energy that needs to be supplied to the differential rotation during a ten-year cycle is of the order of E(B_cr), which is considerably smaller than the kinetic energy of differential rotation in the GL. Assuming that these results can be extrapolated to larger values of B_cr, Magnetic Fields ≈10^4 G, could be generated in the upper section of the tachocline that lies below the SCZ (designated by UT). The energy required to generate these 10^4 G Fields during a cycle is of the order of the kinetic energy in the UT.
-
on a babcock leighton solar dynamo model with a deep seated generating layer for the Toroidal Magnetic Field iv
The Astrophysical Journal, 1997Co-Authors: Bernard R. DurneyAbstract:The study is continued of a dynamo model of the Babcock-Leighton type (i.e., the surface eruptions of Toroidal Magnetic Field are the source for the poloidal Field) with a thin, deep seated layer (GL), for the generation of the Toroidal Field, B. The partial differential equations satisfied by B and by the vector potential for the poloidal Field are integrated in time with the help of a second order time- and space-centered finite different scheme. Axial symmetry is assumed; the gradient of the angular velocity in the GL is such that within this layer a transition to uniform rotation takes place; the meridional motion, transporting the poloidal Field to the GL, is poleward and about 3 m s-1 at the surface; the radial diffusivity ηr equals 5 × 109 cm2 s-1, and the horizontal diffusivity ηθ is adjusted to achieve marginal stability. The initial conditions are: a negligible poloidal Field, and a maximum value of |B| in the GL equal to 1.5 × Bcr, where Bcr is a prescribed Field. For every time step the maximum value of |B| in the GL is computed. If this value exceeds Bcr, then there is eruption of a flux tube (at the latitude corresponding to this maximum) that rises radially to the surface. Only one eruption is allowed per time step (Δt) and B in the GL is unchanged as a consequence of the eruption. The ensemble of eruptions is the source for the poloidal Field, i.e., no use is made of a mean Field equation relating the poloidal with the Toroidal Field. For a given value of Δt, and since the problem is linear, the solutions scale with Bcr. Therefore, the equations need to be solved for one value of Bcr only. Since only one eruption is allowed per time step, the dependence of the solutions on Δt needs to be studied. Let Ft be an arbitrary numerical factor (= 3 for example) and compare the solutions of the equations for (Bcr, Δt) and (Bcr, Δt/3). It is clear that there will be 3 times as many eruptions in the second case (with the shorter time step) than in the first case. However, if the erupted flux in case one is multiplied by 3, then the solutions for this case become nearly identical to those of case two (Δt is shorter than any typical time of the system, and the difference due to the unequal time steps is negligible). Therefore, varying the time step is equivalent to keeping Δt fixed while multiplying the erupted flux by an appropriate factor. In the numerical calculations Δt was set equal to 105 s. The factor Ft can then be interpreted as the number of eruptions per 105 s. The integration of the equations shows that there is a transition in the nature of the solutions for Ft ≈ 2.5. For Ft 2.5, the eruptions occur for θ greater than ≈ π/4, where θ is the polar angle. Furthermore, for Ft 2.5. The factor Ft is an arbitrary parameter in the model and an appeal to observations is necessary. We set Bcr = 103 G. In the model, the Magnetic flux of erupting Magnetic tubes, is then about 3 × 1021 G, of the order of the solar values. For this value of Bcr and for the value of Ft (≈ 2.5) at which the transition takes place, the total erupted flux in 10 years is about 0.85 × 1025 Mx in remarkable agreement with the total erupted flux during a solar cycle. Concerning the dynamo models studied here, a major drawback encountered in previous papers has been the eruptions at high latitudes, which entail unrealistically large values for the radial Magnetic Field at the poles. The results of this paper provide a major step forward in the resolution of this difficulty.
-
On a Babcock-Leighton dynamo model with a deep-seated generating layer for the Toroidal Magnetic Field, II
Solar Physics, 1996Co-Authors: Bernard R. DurneyAbstract:In a previous paper (Paper I), we studied a dynamo model of the Babcock-Leighton type (i.e., the surface eruptions of Toroidal Magnetic Field are the source for the poloidal Field) that included a thin, deep seated, generating layer ( GL ) for the Toroidal Field, Bφ. Meridional motions (of the order of 12 m s^−1 at the surface), rising at the equator and sinking at the poles were essential for the dynamo action. The induction equation was solved by approximating the latitudinal dependence of the Fields by Legendre polynomials. No solutions were found with Φ_ p = Φ_ f where Φ_ p and Φ_ f are the fluxes for the preceding and following spot, respectively. The solutions presented in Paper I, had Φ_ p = −0.5 Φ_ f , were oscillatory in time, and large radial Fields, Bτ, were present at the surface. Here, we resume the study of Paper I with a different numerical approach allowing for a much higher resolution in θ , the polar angle. The time dependent partial differential equations for the Toroidal and poloidal Field are solved with the help of a second order, time and space centered, finite difference scheme. Oscillatory solutions with Φ_ p = Φ_ f are found for various values of the meridional motions and diffusivity coefficients. The surface values of Bτ, while considerably smaller than those of Paper I, are still unacceptably large, specially at the poles. The reason can be traced to the eruption of Toroidal Field at high latitudes. It appears that in order to obtain small values for the radial Field in the polar regions, high latitude sources ( θ smaller than π/4, say), must reach their maximum below the surface. Weaker meridional motions near the poles than in the equatorial region are also suggested.
T Lunt - One of the best experts on this subject based on the ideXlab platform.
-
assessment of particle and heat loads to the upper open divertor in asdex upgrade in favourable and unfavourable Toroidal Magnetic Field directions
Nuclear materials and energy, 2019Co-Authors: Paradela I Perez, M Groth, M Wischmeier, A Scarabosio, D Brida, P David, D Silvagni, D Coster, T LuntAbstract:Abstract Pairs of ASDEX Upgrade L-mode discharges with the Toroidal Magnetic Field, BT, in the forward and reverse directions have been used to study the impact of neoclassical drifts on the divertor plasma conditions and detachment. The evolution of the peak heat flux and the total power loads onto both the outer and the inner targets depends significantly on the Toroidal Field direction: increasing the core plasma density affects mainly the heat loads in the BT 0 (favourable). Ion saturation current measurements show similar trends to those of the IR heat flux data. These discrepancies are not only caused by drifts but also by different levels of radiated power in the core, thus the power across the separatrix, Psep. Tomographic reconstructions show that Psep is not constant within the entire dataset. Finally, at I p = 0.8 MA , a significant reduction of the peak heat flux is observed at both targets for both Field directions. On the other hand, at I p = 0.6 MA , a reduction of the peak heat flux is only observed for BT I p = 0.8 MA .
Steven J. Pearce - One of the best experts on this subject based on the ideXlab platform.
-
Steady state Toroidal Magnetic Field at Earth's core-mantle boundary
Journal of Geophysical Research, 1991Co-Authors: Eugene H. Levy, Steven J. PearceAbstract:Measurements of the DC electrical potential near the top of Earth's mantle have been extrapolated into the deep mantle in order to estimate the strength of the Toroidal Magnetic Field component at the core-mantle interface. Recent measurements have been interpreted as indicating that at the core-mantle interface, the Magnetic Toroidal and poloidal Field components are approximately equal in magnitude. A motivation for such measurements is to obtain an estimate of the strength of the Toroidal Magnetic Field in the core, a quantity important to our understanding of the geoMagnetic Field's dynamo generation. Through the use of several simple and idealized calculations, this paper discusses the theoretical relationship between the amplitude of the Toroidal Magnetic Field at the core-mantle boundary and the actual amplitude within the core. Under physical conditions believed to characterize the core and mantle a low value of the Field at the core-mantle boundary is an inevitable consequence of the electrical conductivity contrast between core and lower mantle material; this conclusion is largely independent of details of the mantle's conductivity profile. Sample calculations, based on simplifying assumptions, are also used to explore the extrapolation of the Toroidal Field amplitude into the core. Even with a very low inferred value of the Toroidal Field amplitude at the core-mantle boundary, (∼ few gauss), the Toroidal Field amplitude within the core could be consistent with a magnetohydrodynamic dynamo dominated by nonuniform rotation and having a strong Toroidal Magnetic Field.
Kazem Faghei - One of the best experts on this subject based on the ideXlab platform.
-
Time dependence of advection-dominated accretion flow with a Toroidal Magnetic Field
Monthly Notices of the Royal Astronomical Society, 2009Co-Authors: A. R. Khesali, Kazem FagheiAbstract:The present study examines the self-similar evolution of advection-dominated accretion flow (ADAF) in the presence of a Toroidal Magnetic Field. In this research, it was assumed that angular momentum transport is due to viscous turbulence and the α-prescription was used for the kinematic coefficient of viscosity. The flow does not have a good cooling efficiency and so a fraction of energy accretes along with matter on to the central object. The effect of a Toroidal Magnetic Field on such a system with regard to the dynamical behaviour was investigated. In order to solve the integrated equations that govern the dynamical behaviour of the accretion flow, a self-similar solution was used. The solution provides some insights into the dynamics of quasi-spherical accretion flow, and avoids many of the strictures of steady self-similar solutions. The solutions show that the behaviour of the physical quantities in a dynamical ADAF is different from that for a steady accretion flow or a disc using a polytropic approach. The effect of the Toroidal Magnetic Field is considered using additional variable β (= P mag /P gas , where P mag and P gas are the Magnetic and gas pressure, respectively). Also, to consider the effect of advection in such systems, the advection parameter f, which stands for the fraction of energy that accretes by matter on to the central object, was introduced. The solution indicates a transonic point in the accretion flow for all selected values of f and β. Also, by increasing the strength of the Magnetic Field and the degree of advection, the radial thickness of the disc decreases and the disc compresses. The model implies that the flow has differential rotation and is sub-Keplerian at small radii and super-Keplerian at large radii, and that different results were obtained using a polytropic accretion flow. The β parameter obtained was a function of position, and increases with increasing radii. Also, the behaviour of ADAF in a large Toroidal Magnetic Field implies that different results are obtained using steady self-similar models in large Magnetic Fields.
-
Time-Dependent of Accretion Flow with Toroidal Magnetic Field
Monthly Notices of the Royal Astronomical Society, 2008Co-Authors: A. R. Khesali, Kazem FagheiAbstract:In the present study time evolution of quasi-spherical polytropic accretion flow with Toroidal Magnetic Field is investigated. The study especially focused the astrophysically important case in which the adiabatic exponent $\gamma=5/3$. In this scenario, it was assumed that the angular momentum transport is due to viscous turbulence and used $\alpha$-prescription for kinematic coefficient of viscosity. The equations of accretion flow are solved in a simplified one-dimensional model that neglects the latitudinal dependence of the flow. In order to solve the integrated equations which govern the dynamical behavior of the accretion flow, self-similar solution was used. The solution provides some insight into the dynamics of quasi-spherical accretion flow and avoids many of the strictures of the steady self-similar solution. The effect of the Toroidal Magnetic Field is considered with additional variable $\beta[=p_{mag}/p_{gas}]$, where $p_{mag}$ and $p_{gas}$ are the Magnetic and gas pressure, respectively. The solution indicates a transonic point in the accretion flow, that this point approaches to central object by adding strength of the Magnetic Field. Also, by adding strength of the Magnetic Field, the radial-thickness of the disk decreases and the disk compresses. It was analytically indicated that the radial velocity is only a function of Alfv'en velocity. The model implies that the flow has differential rotation and is sub-Keplerian at all radii.
