Length Of Magnetic Field Lines Research Articles

Analysis of peak heat load on the blanket module for JA DEMO

Plasma heat flux in the peripheral plasma reaches the first wall (FW) along a magnetic field line, and it sometimes causes several MW/m2 orders of magnitude high heat flux concentration at narrow region, such as the edge of the blanket module. Thus, to assess and to reduce the heat load is key issue in the DEMO design activity. In this research, a new heat load analysis code is introduced based on the e-folding model. In this code, the decay length λ is changed depending on wall connection length of magnetic field line (defined as the length of magnetic field line from the wall to the wall), and parallel heat flux q// is calculated in each flux tube. This code can calculate the FW heat load simulating actual blanket module shapes. The 0.23 MW/m2 peak heat load at inner midplane in the case of the ideal FW (without gaps between blanket modules) is increased to 22 MW/m2 at toroidal module edge in the case of box shaped module. To shadow the edge and reduce such peak heat load, toroidal, and poloidal roof shaping is applied. Required roof height is analyzed from this code calculation. After shaping, peak heat load is reduced to 1.2 MW/m2. This value is under the allowable value 1.5 MW/m2, and in this case, surface temperature is also less than allowable temperature.

Helically self-organized pinches: dynamical regimes and magnetic chaos healing

This paper deals with helical self-organization in current-carrying toroidal pinches for the magnetic confinement of fusion plasmas. We perform our study in the framework of 3D nonlinear visco-resistive magnetofluid modelling, where a large set of simulations is now available. A global picture is derived about how visco-resistive transport coefficients and magnetic boundary conditions rule the self-organized helical states for the reversed-field pinch configuration. Decreasing visco-resistive dissipation causes a transition from steady ohmic helical equilibria to intermittent states (sawtoothing activity), while selected helical magnetic perturbations applied at the boundary favor steady global quasi-helical solutions. The sawtoothing frequency decreases together with visco-resistive dissipation, while sawtoothing amplitude decreases when applying non-resonant magnetic perturbations. Simulations of the tokamak configuration allow us to draw a tight parallelism with reversed-field pinches: a similar role of dissipation and magnetic boundary conditions on the dynamics of the internal kink mode is found, with decreasing sawtoothing frequency and amplitude by decreasing dissipation and by applying suitable helical boundary conditions. Magnetic chaos healing is the topological feature of the transition from intermittent to quasi-quiescent helical states in reversed-field pinches. Bundles of Lagrangian coherent structures (LCS) in the weakly stochastic region surrounding the helical core constitute the skeleton of chaos healing, and behave as barriers to the transport of magnetic field lines. In this work, they are detected during the whole temporal evolution and it is proved that they can withstand the nonlinear dynamics even during sawtoothing activity. Furthermore, we show that LCS are connected to regions with strong gradients of the connection length of magnetic field lines to the edge. This provides a further indication of their possible role in the formation of electron temperature barriers. As a final result it is shown that a reasonable value of plasma rotation can further enhance the intensity of the dominant helical mode.

Magnetohydrodynamic waves in braided magnetic fields

Aims. We investigate the propagation of transverse magnetohydrodynamic (MHD) wave fronts through a coronal plasma containing a braided magnetic field. Methods. We performed a series of three dimensional MHD simulations in which a small amplitude, transverse velocity perturbation is introduced into a complex magnetic field. We analysed the deformation of the wave fronts as the perturbation propagates through the braided magnetic structures and explore the nature of Alfvénic wave phase mixing in this regime. We considered the effects of viscous dissipation in a weakly non-ideal plasma and evaluate the effects of field complexity on wave energy dissipation. Results. Spatial gradients in the local Alfvén speed and variations in the length of magnetic field lines ensure that small scales form throughout the propagating wave front due to phase mixing. Additionally, the presence of complex, intricate current sheets associated with the background field locally modifies the polarisation of the wave front. The combination of these two effects enhances the rate of viscous dissipation, particularly in more complex field configurations. Unlike in classical phase mixing configurations, the greater spatial extent of Alfvén speed gradients ensures that wave energy is deposited over a larger cross-section of the magnetic structure. Further, the complexity of the background magnetic field ensures that small gradients in a wave driver can map to large gradients within the coronal plasma. Conclusions. The phase mixing of transverse MHD waves in a complex magnetic field will progress throughout the braided volume. As a result, in a non-ideal regime wave energy will be dissipated over a greater cross-section than in classical phase mixing models. The formation rate of small spatial scales in a propagating wave front is a function of the complexity of the background magnetic field. As such, if the coronal field is sufficiently complex it remains plausible that phase mixing induced wave heating can contribute to maintaining the observed temperatures. Furthermore, the weak compressibility of the transverse wave and the observed phase mixing pattern may provide seismological information about the nature of the background plasma.

Electron acceleration associated with the magnetic flux pileup regions in the near‐Earth plasma sheet: A multicase study

AbstractUsing the Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations, we study electron acceleration (<30 keV) in the magnetic flux pileup regions (FPRs) in the near‐Earth plasma sheet (X ~ –10 RE). We present three cases of FRPs associated with dipolarization fronts and substorm dipolarization. Based on the characteristics of the magnetic field, we defined the magnetic field enhancement region (MFER) as the magnetic field with significant ramp that is usually observed near the dipolarization front boundary layer. On the other side, the increased magnetic field without a significant ramp is the rest of a FPR. Our results show that betatron acceleration dominates for 10–30 keV electrons inside the MFER, whereas Fermi acceleration dominates for 10–30 keV electrons inside the rest of the FPR. Betatron acceleration is caused by the enhancement of the local magnetic field, whereas Fermi acceleration is related to the shrinking length of magnetic field line. These accelerated electrons inside the FPRs in the near‐Earth tail play a potentially important role in the evolution of the Earth's electron radiation belt and substorms.

THEMIS observations of electron acceleration associated with the evolution of substorm dipolarization in the near‐Earth tail

AbstractWe present the evolution of dipolarizations in the near‐Earth tail during a substorm on 15 March 2009, based on the two‐point measurements in the nightside plasma sheet at X ~ −8.0 RE. The earthward‐moving dipolarization, the magnetic flux pileup, and the tailward‐moving dipolarization were observed. For the 30–200 keV electrons, betatron acceleration was the dominant process, which was caused by the much larger gradient of the magnetic field there during the earthward‐moving dipolarization or by a local compression of the magnetic field during the magnetic flux pileup and tailward‐moving dipolarization. These near‐perpendicular distributions for the 30–200 keV electrons are interpreted as produced by a two‐step acceleration: Electrons were first accelerated in the dipolarization fronts in the midtail or the near‐Earth tail and then were further accelerated near the tail current disruption region. For the more than 200 keV electrons, Fermi acceleration was the dominant process, which was caused by the shrinking length of magnetic field line during the tailward‐moving dipolarization. The source region of the more than 200 keV electrons may be near the tail current disruption region, but these electrons were accelerated locally. These field‐aligned electrons can precipitate into the ionosphere and form the discrete auroral arcs. Two parallel arcs were clearly observed around the substorm onset: one propagated equatorward, another propagated poleward. We suggest that the earthward‐moving dipolarization, the magnetic flux pileup, and the tailward‐moving dipolarization near the tail current disruption region can well explain the auroral evolution around the substorm onset.

Broadening of rectified potential structures induced by rf currents in a magnetized plasma: Application to ITER scrape-off-layer

Measurements show that the vicinity of powered ion cyclotron range of frequency (ICRF) antennae is biased positively with respect to its environment [J. Gunn et al., Proc. 22nd IAEA Fusion Energy Conference, Geneva 2008, EX/P6-32]. This is attributed to radio-frequency (rf) sheath rectification. The radial penetration of these direct current (dc) potentials from ICRF launchers into the tokamak scrape-off layer (SOL) determines the power deposition on the walls and especially on the antenna structure, which is a key point for long time clean discharges. Within independent flux tube models of rf sheath rectification the radial penetration of dc potentials is determined by the skin depth x0=c/ωpe for the slow wave. When self-consistent exchange of transverse rf current is allowed between neighboring flux tubes, such a structure can be broadened radially up to a characteristic transverse length L. Broadening arises as soon as L>x0. A linear modeling of the process gives a first evaluation of the theoretical length L=(L∥ρci/2)1/2. Within the “flute assumption,” it scales with the length L∥ of open flux tubes and the ion Larmor radius Ωci. This trend has been confirmed by nonlinear fluid simulations using the SEM code taking into account nonlinearities of the sheath dynamics. Parametric regimes are outlined where broadening or nonlinearity arise. Langmuir probe measurements on Tore Supra suggest that the observed broadening is lower than predicted by the code. This suggests that actual rf current exchanges probably do not occur over the whole length of magnetic field lines but only on a fraction of it. This “effective parallel magnetic connection length” L∥eff is estimated from the measurements. The model is finally applied to several potential maps generated by an ITER antenna, with different plasma parameters depending on possible SOL scenarios in ITER, and “reasonable assumptions” about L∥eff. It comes out that L ranges between 1 and 10 cm depending on local L∥eff and on typical ITER plasma parameters.

Transverse Oscillations of Flowing Prominence Threads Observed with Hinode

Recent observations with the Hinode Solar Optical Telescope display an active region prominence whose fine threads oscillate in the vertical direction as they move along a path parallel to the photosphere. A seismological analysis of this event is carried out by taking advantage of the small radius of these structures compared to the total length of magnetic field lines, i.e., by using the thin-tube approximation. This analysis reveals that the oscillatory period is only slightly modified by the existence of the flow and that the difference between the period of a flowing thread and a static one is below the error bars of these observations. Moreover, although it is not possible to obtain values of the physical parameters, a lower bound for the Alfvén speed (ranging between 120 and 350 km s−1) is obtained for each of the threads. Such Alfvén speeds agree with the intense magnetic fields and large densities usually found in active region prominences.

Length of magnetic field lines in turbulent plasmas.

An estimation of the length of any magnetic field line in a two-dimensional periodic magnetohydrodynamic problem is provided. This is done by using some classical function theory results on the analytic extension of the vector potential. The essential parameter, the maximum of this extension, may be analyzed in the case of turbulent plasmas by admitting the Iroshnikov-Kraichnan statistics, establishing in this way a relation between the length of any magnetic field line and the energy dissipation scale. (c) 2002 American Institute of Physics.

Bounds on the length of magnetic field lines in a two-dimensional plasma.

Magnetic field lines in ideal turbulent plasmas tend to become quite complicated and their length to grow in time. Diffusivity allows for reconnection and possible shortening, but this fact has not so far been rigorously quantified. We show that in a two-dimensional diffusive plasma the mean length of field lines stays bounded for all time. Moreover, these estimates are local, in the sense that the mean values of magnetic field and velocity in the neighborhood of a ball determine bounds for length within the ball, without recourse to external magnitudes.

Current Sheets in Two-Dimensional Turbulent Plasmas

It is found that the mean length of magnetic field lines in a two-dimensional plasma is a lower bound of the total current in it, and this estimate does not depend on the value of the resistivity. In a turbulent plasma, the current is often localized in current sheets (of finite thickness). Since the length of curves transported by chaotic flows tends to grow exponentially in time, the same must happen to the current density at these sheets. This fact was previously known by numerical methods.

EFFECTS OF PINNING ANISOTROPY ON YBCO FILM CURRENT-VOLTAGE CHARACTERISTICS

The dissipative properties of (103)/(031) oriented films YBCO films grown on (110) SrTiO3 substrates were investigated in the vortex creep motion regime and compared with characteristics of c-axis oriented films deposited on both and (001)SrTiO3 and (001)NdGaO3 substrates. The current-voltage characteristics were measured at different temperatures, using a proper geometrical bridge configuration, able to test space anisotropy. The in-plane anisotropy due to the layered structure of the superconductor was clearly observed in the (103)/(031) oriented films. Experimental data are consistent with a thermal activated mechanism of vortex motion on a periodic pinning potential, attributed to intrinsic layer pinning. The intrinsic pinning energy per unit length of magnetic field line is evaluated as U0≈400 KeV/cm, in the low temperature limit. Consistently with theory, this energy is weakly dependent on temperature, the U/KBT ratio fading out at T ≈ Tc, where flux flow mechanism is activated for vortex motion.

Using energetic electrons to probe the topology of the October 18–20, 1995 magnetic cloud

We present ∼0.1–10 2 keV electron observations from the 3-D Plasma and Energetic Particles experiment of a magnetic cloud which passed by the WIND spacecraft on October 18 – 20, 1995. Although the magnetic field exhibits the smooth, continuous rotation signature of a helical flux rope, the electron observations show numerous abrupt discontinuities in flux level and anisotropy within the cloud. In addition, five solar impulsive electron events, accompanied by solar type III radio bursts, were observed while the cloud was passing by the spacecraft. The electron/type III events showed that some of the cloud field lines were magnetically connected to solar active region 7912. Analysis of the velocity dispersion in the arrival times of the solar impulsive electrons provides the length of magnetic field lines. The field line length is found to be longest (∼4 AU) near the exterior with a minimum (∼1.5 AU) near the center of the cloud, consistent with a helical flux rope model. We interpret the discontinuities in electron fluxes as evidence of disconnection of magnetic field lines from the Sun.