The Effect of the Hall Term on the Nonlinear Evolution of the Magnetorotational Instability: II. Saturation Level and Critical Magnetic Reynolds Number

The nonlinear evolution of the magnetorotational instability (MRI) in weakly ionized accretion disks, including the effect of the Hall term and ohmic dissipation, is investigated using local three-dimensional MHD simulations and various initial magnetic field geometries. When the magnetic Reynolds number, Re_M \equiv v_A^2 / \eta \Omega (where v_A is the Alfven speed, \eta the magnetic diffusivity, and \Omega the angular frequency), is initially larger than a critical value Re_{M, crit}, the MRI evolves into MHD turbulence in which angular momentum is transported efficiently by the Maxwell stress. If Re_M < Re_{M, crit}, however, ohmic dissipation suppresses the MRI, and the stress is reduced by several orders of magnitude. The critical value is in the range of 1 - 30 depending on the initial field configuration. The Hall effect does not modify the critical magnetic Reynolds number by much, but enhances the saturation level of the Maxwell stress by a factor of a few. We show that the saturation level of the MRI is characterized by v_{Az}^2 / \eta \Omega, where v_{Az} is the Alfven speed in the nonlinear regime along the vertical component of the field. The condition for turbulence and significant transport is given by v_{Az}^2 / \eta \Omega \gtrsim 1, and this critical value is independent of the strength and geometry of the magnetic field or the size of the Hall term. If the magnetic field strength in an accretion disk can be estimated observationally, and the magnetic Reynolds number v_A^2 / \eta \Omega is larger than about 30, this would imply the MRI is operating in the disk.Comment: 43 pages, 8 tables, 20 figures, accepted for publication in ApJ, postscript version also available from http://www.astro.umd.edu/~sano/publications

Seiberg-Witten map and Galilean symmetry violation in a non-commutative planar system

An effective U(1) gauge invariant theory is constructed for a non-commutative Schrodinger field coupled to a background U(1)_{\star} gauge field in 2+1-dimensions using first order Seiberg-Witten map. We show that this effective theory can be cast in the form of usual Schrodinger action with interaction terms of noncommutative origin provided the gauge field is of ``background'' type with constant magnetic field. The Galilean symmetry is investigated and a violation is found in the boost sector. We also consider the problem of Hall conductivity in this framework.Comment: REVTeX, 4 pages, Title changed, Paper shortened, Appendix removed, A new section on Galilean symmetry adde

Comparison of single-phase matrix converter and H-bridge converter for radio frequency induction heating

This paper compares the newly developed single-phase matrix converter and the more conventional H- bridge converter for radio frequency induction heating. Both the converters exhibit unity power factor, very low total harmonic distortion at the utility supply interface, good controllability under soft switching condition for a wide range of power, and high efficiencies, whilst still having simple structures. A novel switching control pattern has been proposed for the matrix converter in order to maintain the comparable performance to the H-bridge converter. Simulation and experimental results for both converters are presented. Comparisons between two converters have confirmed the excellent performance of the proposed matrix converter

A three-phase to single-phase matrix converter for high-frequency induction heating

The paper describes a new three-phase to single-phase matrix converter featuring unity input power factor, very low input total harmonic distortion, and soft-switching over the full power range, for high frequency induction heating applications. A variable output pulse density modulation scheme has been proposed for stable operation of the converter, with the notable feature of requiring no on-line calculations for the synthesis of three-phase input current system. Practical issues in realising the converter, viz. line frequency synchronisation and output current circulation, are described. Good agreement between simulation and experimental results confirm the benefits of the proposed converter

Single phase matrix converter for radio frequency induction heating

Conventional converters for radio frequency induction heating usually follow an AC-DC-AC structure, which can exhibit non-unity power factor and introduce large harmonic currents into the utility supply. The need for a direct converter for radio frequency induction heating, featuring unity power factor, and sinusoidal input current, has motivated the development of a single phase matrix converter as an induction heater. A novel commutation strategy is therefore required to ensure smooth operation of the converter whilst creating a high frequency output under soft switching conditions. The operating principle and features of the proposed converter are described here, and experimentally verifie

Nanoengineered Curie Temperature in Laterally-Patterned Ferromagnetic Semiconductor Heterostructures

We demonstrate the manipulation of the Curie temperature of buried layers of the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the effect of annealing. Patterning the GaAs-capped ferromagnetic layers into nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As layers and thus allows the removal of Mn interstitials using annealing. This leads to an enhanced Curie temperature and reduced resistivity compared to unpatterned samples. For a fixed annealing time, the enhancement of the Curie temperature is larger for narrower nanowires.Comment: Submitted to Applied Physics Letters (minor corrections

A Local One-Zone Model of MHD Turbulence in Dwarf Nova Disks

The evolution of the magnetorotational instability (MRI) during the transition from outburst to quiescence in a dwarf nova disk is investigated using three-dimensional MHD simulations. The shearing box approximation is adopted for the analysis, so that the efficiency of angular momentum transport is studied in a small local patch of the disk: this is usually referred as to a one-zone model. To take account of the low ionization fraction of the disk, the induction equation includes both ohmic dissipation and the Hall effect. We induce a transition from outburst to quiescence by an instantaneous decrease of the temperature. The evolution of the MRI during the transition is found to be very sensitive to the temperature of the quiescent disk. As long as the temperature is higher than a critical value of about 2000 K, MHD turbulence and angular momentum transport is sustained by the MRI. However, MHD turbulence dies away within an orbital time if the temperature falls below this critical value. In this case, the stress drops off by more than 2 orders of magnitude, and is dominated by the Reynolds stress associated with the remnant motions from the outburst. The critical temperature depends slightly on the distance from the central star and the local density of the disk.Comment: 20 pages, 2 tables, 6 figures, accepted for publication in Ap

Dissipation in Compressible MHD Turbulence

We report results of a three dimensional, high resolution (up to 512^3) numerical investigation of supersonic compressible magnetohydrodynamic turbulence. We consider both forced and decaying turbulence. The model parameters are appropriate to conditions found in Galactic molecular clouds. We find that the dissipation time of turbulence is of order the flow crossing time or smaller, even in the presence of strong magnetic fields. About half the dissipation occurs in shocks. Weak magnetic fields are amplified and tangled by the turbulence, while strong fields remain well ordered.Comment: 5 pages, 3 Postscript figures, LaTeX, accepted by Ap.J.Let