Computer modeling of linear theta pinch machines

In this paper we investigate, by means of two-dimensional magnetohydrodynamic simulations, the impact of temperature-dependent resistivity and thermal conduction on the development of plasmoid instabilities in reconnecting current sheets in the solar corona. We find that the plasma temperature in the current sheet region increases with time and it becomes greater than that in the inflow region. As secondary magnetic islands appear, the highest temperature is not always found at the reconnection $X$-points, but also inside the secondary islands. One of the effects of anisotropic thermal conduction is to decrease the temperature of the reconnecting $X-$points and transfer the heat into the $O-$points, the plasmoids, where it gets trapped. In the cases with temperature-dependent magnetic diffusivity, $\eta \sim T^{-3/2}$, the decrease in plasma temperature at the $X-$points leads to: (i) increase in the magnetic diffusivity until the characteristic time for magnetic diffusion becomes comparable to that of thermal conduction; (ii) increase in the reconnection rate; and, (iii) more efficient conversion of magnetic energy into thermal energy and kinetic energy of bulk motions. These results provide further explanation of the rapid release of magnetic energy into heat and kinetic energy seen during flares and coronal mass ejections. In this work, we demonstrate that the consideration of anisotropic thermal conduction and Spitzer-type, temperature-dependent magnetic diffusivity, as in the real solar corona, are crucially important for explaining the occurrence of fast reconnection during solar eruptions.

A computational study of operating regimes for poloidal divertors

Energy line density, E/sub l/ = ..integral..(F/2)nk(T/sub e/ + T/sub i/)dA, has proved to be one of the easiest (and most critical) measurements made on the Scylla IV-P, 5-m linear theta pinch. A quantitative evaluation of end-stoppering techniques is made available by extracting the line energy containment time, tau/sub E/sub l//, from the time history of E/sub l/. To model E/sub l/(t) properly and, hence, determine the details of energy loss mechanisms (streaming and thermal conduction) requires separate measurements of n and T. A simple measurement of line density, n/sub l/ = ..integral..ndA, is proposed, in which a single laser beam is passed repeatedly through the plasma column along different chords. A discussion is presented of the measurement accuracy as a function of the number of passes.

Observation of nearly undisturbed penetration of the Porcupine dense (nb, ≫ nion) and fast (υb = 1.7 · 104ms−1) heavy (xenon, mi = 131 mp) ion beam into the magnetized ionospheric plasma after termination of the short (τm < 1 ms) adiabatic phase requires very fast magnetic field diffusion into the beam. Collisional diffusion and instability of a sharp boundary between the magnetic field and plasma yield diffusion times too long to account for the observations. The diffusion is therefore attributed to a transverse electron drift current‐driven electrostatic instability excited by the diamagnetic current flowing in the boundary layer between the injected beam and the ambient field. In the low‐drift regime applicable to the final stage of diffusion in the outer part of the layer, electron acoustic waves are excited. In the initial strong current regime of the inner part of the layer the modified two‐stream instability becomes unstable. The anomalous collision frequencies produced in both cases by entirely different saturation mechanisms turn out to be of the order of the local lower‐hybrid (LH) frequency in the dense xenon plasma and are high enough to yield the required fast magnetic field diffusion rate. Energetically, only a very small fraction of beam energy is dissipated in the diffusion process so that no remarkable deceleration of the ion beam is observed, thus giving a satisfactory explanation of the negligible beam energy losses.

Thermal Conductivity and Lattice Vibrational Modes

Several closed configurations consisting of interconnected linear theta pinches are proposed which may considerably reduce their end losses.

Experiments of GIT-4 facility are shown that Rayleigh-Taylor instabilities destroy the cylindrical gas shell at low mass and the X-ray yield falls sharply. Double gas puff is the efficient load designs for plasma radiation source. A double gas puff can be efficiently only if the magnetic field diffusion is absent at the onset of the current. The problem of the skin layer production in the outer shell was decided using magnetron discharges. As a result a good reproducibility of K-shell generation was achieved for Ar double gas puff. The final plasma column demonstrated the absence of bright spots and produced 0.3 kJ/cm of the K-shell yield at the 1.7 MA current.

The saturation of the hydromagnetic Rayleigh–Taylor instability is caused by the reduction of driving current in the bubble region between the spikes formed as the instability develops. For short wavelengths linear magnetic field diffusion provides the necessary smoothing of the magnetic field to reduce the driving force. For wavelengths longer than the magnetic field diffusion length, the current is shorted through material which expands into the bubble region. This initially low density accumulates in the bubble and eventually provides a source of sufficiently high conductivity plasma which reduces the magnetic field penetration to the front of the bubble. Simple analytic models have been developed to verify and and quantify these predictions. These models have been compared with two-dimensional magnetohydrodynamic calculations for imploding plasma shells and give good agreement with these more detailed simulations.

Investigations of magnetic field diffusion and plasma resistivity in 12-cm-diameter triple-gas-puff Ar <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z</i> -pinch implosions were carried out by using planar laser-induced fluorescence (PLIF), a laser shearing interferometer (LSI), and a laser wavefront analyzer (LWA) on a 3.5-MA 200-ns generator. The PLIF measurements gave the initial Ar gas distributions. The implosion velocity and electron density profiles were measured from LWA and/or LSI. From these, the implosion plasma sheath thickness, ion density, mean ion charge states, temperatures, and implosion velocity are obtained, which allows us to calculate the classical plasma resistivity. A 1-D analytic magnetic field diffusion model is constructed and used to predict the imploding plasma sheath thickness and its resistivity. Based on comparisons of the experimental measurements and the diffusion model prediction, we found out that plasma resistivity is enhanced by the cross-field diffusion above the classical value, as high as 60 times the Spitzer's value. Details are given in this paper.

Finite difference time domain algorithm for electromagnetic problems involving material movement

Decompose the electron and phonon thermal transport of intermetallic compounds NiAl and Ni3Al by first-principles calculations

Te-doped Mg2Si (Mg2Si:Tem, m = 0, 0.01, 0.02, 0.03, 0.05) alloys were synthesized by a solid-state reaction and mechanical alloying. The electronic transport properties (Hall coefficient, carrier concentration, and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity, and figure of merit) were examined. Mg2Si was synthesized successfully by a solid-state reaction at 673 K for 6 h, and Te-doped Mg2Si powders were obtained by mechanical alloying for 24 h. The alloys were fully consolidated by hot-pressing at 1073 K for 1 h. All the Mg2Si:Tem samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased and the absolute value of the Seebeck coefficient decreased with increasing Te content, because Te doping increased the electron concentration considerably from 1016 cm−3 to 1018 cm−3. The thermal conductivity did not change significantly on Te doping, due to the much larger contribution of lattice thermal conductivity over the electronic thermal conductivity. Thermal conduction in Te-doped Mg2Si was due primarily to lattice vibrations (phonons). The thermoelectric figure of merit of intrinsic Mg2Si was improved by Te doping.

Context.Turbulent diffusion of large-scale flows and magnetic fields plays a major role in many astrophysical systems, such as stellar convection zones and accretion discs.Aims.Our goal is to compute turbulent viscosity and magnetic diffusivity which are relevant for diffusing large-scale flows and magnetic fields, respectively. We also aim to compute their ratio, which is the turbulent magnetic Prandtl number, Pmt, for isotropically forced homogeneous turbulence.Methods.We used simulations of forced turbulence in fully periodic cubes composed of isothermal gas with an imposed large-scale sinusoidal shear flow. Turbulent viscosity was computed either from the resulting Reynolds stress or from the decay rate of the large-scale flow. Turbulent magnetic diffusivity was computed using the test-field method for a microphysical magnetic Prandtl number of unity. The scale dependence of the coefficients was studied by varying the wavenumber of the imposed sinusoidal shear and test fields.Results.We find that turbulent viscosity and magnetic diffusivity are in general of the same order of magnitude. Furthermore, the turbulent viscosity depends on the fluid Reynolds number (Re) and scale separation ratio of turbulence. The scale dependence of the turbulent viscosity is found to be well approximated by a Lorentzian. These results are similar to those obtained earlier for the turbulent magnetic diffusivity. The results for the turbulent transport coefficients appear to converge at sufficiently high values of Re and the scale separation ratio. However, a weak trend is found even at the largest values of Re, suggesting that the turbulence is not in the fully developed regime. The turbulent magnetic Prandtl number converges to a value that is slightly below unity for large Re. For small Re we find values between 0.5 and 0.6 but the data are insufficient to draw conclusions regarding asymptotics. We demonstrate that our results are independent of the correlation time of the forcing function.Conclusions.The turbulent magnetic diffusivity is, in general, consistently higher than the turbulent viscosity, which is in qualitative agreement with analytic theories. However, the actual value of Pmtfound from the simulations (≈0.9−0.95) at large Re and large scale separation ratio is higher than any of the analytic predictions (0.4−0.8).

The topic of this paper is the distribution of magnetic fields inside the rails of the electromagnetic railgun RAFIRA located at the ISL. The magnetic field pulse characteristics are measured using colossal magnetoresistance-B-scalar sensors placed at different depths inside the rails of the accelerator. During launch the muzzle velocity reached up to 1.4 km/s, the electrical shot energy is about 1.2 MJ and the projectile mass was 140 g. The obtained results are analyzed using two models based on analytic solutions of Maxwell's equations. The first model considers the 1-D magnetic field diffusion in the direction perpendicular to the rails. The second model includes convection and simulates the 2-D behavior of the magnetic field distribution in three regions: the armature, the contact zone between rail and armature and the rail behind the armature. Additionally, 2-D and 3-D quasistationary finite element models are developed using Comsol Multiphysics. Excellent agreement is found between the 3-D simulation results and the measurements of magnetic diffusion.

In low-collisionality plasmas, velocity-space instabilities are a key mechanism providing an effective collisionality for the plasma. We use particle-in-cell (PIC) simulations to study the interplay between electron- and ion-scale velocity-space instabilities and their effect on electron pressure anisotropy, viscous heating, and thermal conduction. The adiabatic invariance of the magnetic moment in low-collisionality plasmas leads to pressure anisotropy, , if the magnetic field is amplified ( and denote the pressure of species j (electron, ion) perpendicular and parallel to ). If the resulting anisotropy is large enough, it can in turn trigger small-scale plasma instabilities. Our PIC simulations explore the nonlinear regime of the mirror, IC, and electron whistler instabilities, through continuous amplification of the magnetic field by an imposed shear in the plasma. In the regime ( ), the saturated electron pressure anisotropy, , is determined mainly by the (electron-lengthscale) whistler marginal stability condition, with a modest factor of ∼1.5–2 decrease due to the trapping of electrons into ion-lengthscale mirrors. We explicitly calculate the mean free path of the electrons and ions along the mean magnetic field and provide a simple physical prescription for the mean free path and thermal conductivity in low-collisionality β j ≳ 1 plasmas. Our results imply that velocity-space instabilities likely decrease the thermal conductivity of plasma in the outer parts of massive, hot, galaxy clusters. We also discuss the implications of our results for electron heating and thermal conduction in low-collisionality accretion flows onto black holes, including Sgr A* in the Galactic Center.