Session CO8: Magnetic Reconnection and Other Basic Plasma Physics

Parker Problem in Hall Magnetohydrodynamics

Parker problem in Hall magnetohydrodynamics (MHD) is considered [1]. Poloidal shear into the toroidal flow generated by the Hall effect is incorporated. This is found to lead to a {\it triple deck} structure for the Parker problem in Hall MHD, with the magnetic field falling off in the intermediate Hall- resistive region more steeply (like $1/x^3$) than that (like $1/x$) in the outer ideal MHD region.\\ \noindent [1] B. K. Shivamoggi: {\it Phys. Plasmas} {\bf 16}, 052111, (2009).   

Possible trigger mechanisms for the Double Tearing Mode nonlinear destabilization

Recently, advanced scenarios seems to be a promising key to continuous operation with fusion plasmas. However, those are limited by MHD instabilities such as the double tearing mode (DTM), which leads to the formation/interaction of magnetic islands along 2 rational surfaces. Calculations with intermediate DTM [1-2] show explosive dynamics from a quasi-steady nonlinear behavior. In the later stage, the energies increase much and the islands deform each other up to total reconnection. To understand the mechanisms leading to such phenomenon, we have conducted an instability analysis of the quasi-steady equilibrium resulting from the first nonlinear stage of the DTM [3] by numerically solving the linearized 2-field reduced MHD equations. In slab geometry, the new equilibrium with steady magnetic islands is found to be unstable. Further investigations near marginal stability (=no nonlinear destabilization), show that the growth rate of the resulting secondary instability is strongly dependent on the amplitude of the flux function, suggesting similar features as a modulational instability. This secondary instability evolution is discussed as a possible mechanism for the generation of strong flows arising in the nonlinear evolution of the DTM, leading to the explosive dynamics. [1] Y.Ishii \textit{et al.}, PRL, \textbf{89}, 205002 (2002) [2] Z.X.Wang \textit{et al.}, Phys. Plasmas, \textbf{15} 082109 (2008) [3] M.Janvier \textit{et al.}, \textit{to be pub.} JPFR (2010)   

Buneman Instability in a Magnetized Current-Carrying Plasma with Velocity Shear

Electron beams and current sheets associated with magnetic reconnection can often drive Buneman instabilities. Understanding how velocity shear in the beams driving Buneman instability affects the instability is relevant to turbulence, heating, and diffusion in magnetic reconnection. Using Mathieu-equation analysis\footnote{M. Goldman, et al, 31$^{st}$ EPS Conf. Plasma Phys., 1.072 ECA \textbf{28G}, (2004)} for weak cosine velocity shear together with Vlasov simulations and other methods for more general velocity profiles, we study relevant effects of shear on the kinetic Buneman instability in magnetized plasmas. In the linearly unstable phase, shear enhances the coupling between oblique waves and the sheared beam, resulting in lower growth rates and a wider range of unstable modes with common growth rates. In the nonlinear phase the modes trap electrons and form phase space holes.   

Jet deflection by very weak guide-fields during magnetic reconnection

Simulations of anti-parallel reconnection have shown that collimated electron jets emanate from the x-point along the x-axis and are associated with both ``inner'' and highly elongated ``outer'' electron ``diffusion'' regions. New implicit PIC simulations show that jets are deflected from propagation along the x-axis by an out-of-plane guide field, B$_{g}$ as small as 0.05 times the asymptotic reconnecting field, B$_{0}$ in plasmas with a \textit{physical} ion-to-electron mass ratio of 1836 as well as with lower mass ratios. The outer electron diffusion region is distorted and broken up, but the diffusion rate is unchanged. Electron dynamics offers some insight into the underlying physics. Interpretations of reconnection signatures for both existing and for anticipated future measurements by spacecraft in the magnetosphere are likely to be influenced by these new results.   

Magnetic Reconnection with Strong Radiative Cooling

Magnetic reconnection in many high-energy-density astrophysical and laboratory environments is significantly affected by radiation and so traditional nonradiative reconnection models are not applicable for them. As a step towards remedying this situation, a Sweet--Parker-like theory of resistive reconnection with strong radiative cooling is developed. General relationships between key reconnection parameters and the radiative cooling function are obtained. For the zero guide field case (in contrast to strong guide field), intense radiative cooling leads to strong plasma compression, yielding a higher reconnection rate. The compression ratio and the layer temperature are governed by the balance between ohmic heating and radiative cooling. In both zero and strong guide-field cases, the lower temperature in a radiatively-cooled layer leads to a higher resistivity and hence a higher reconnection rate. Several specific optically-thin radiative processes (bremsstrahlung, cyclotron, and inverse Compton) are considered and concrete expressions for the reconnection parameters are derived, along with their applicability conditions.   

Numerical and Analytical Studies of Plasmoid-Dominated Reconnection

Magnetic reconnection at large Lundquist numbers ($S>10^4$) is studied analytically and numerically. It is found that the Sweet-Parker (SP) theory is not valid in this regime: the current sheet is violently unstable to the formation of multiple plasmoids (secondary islands) that quickly grow wider than the original SP layer and start to dominate the reconnection process. Our resistive-MHD numerical simulations are carried out for sufficiently long times to achieve a statistical steady state. The steady- state SP layer gets replaced by a much broader, turbulent-like reconnection region with a fast (independent of~$S$) effective time-averaged reconnection rate. Secondary interplasmoid current sheets themselves become unstable to the same instability, giving rise to a truly hierarchical, multiscale (and multifrequency) structure. The plasmoid size distribution function is analyzed; in particular, we observe the occasional formation of unusually large plasmoids, which may have observational implications. A theoretical picture describing the main features of plasmoid-dominated reconnection is also presented.   

Fast magnetic reconnection induced by collisionless effects and flux pile-up in laser-produced plasma bubbles

Recent experiments have observed magnetic reconnection in high-energy-density, laser-produced plasma bubbles [1,2], with reconnection rates observed to be much higher than can be explained by classical theory. This is a novel regime for magnetic reconnection study, characterized by extremely high magnetic fields, high plasma beta and strong, supersonic plasma inflow. Furthermore, due to the high temperatures attained, this experimental technique may be the first to obtain the high-Lundquist number regimes of astrophysical relevance. Reconnection in this regime is investigated with particle-in-cell simulations. Work to-date with collisionless simulations identifies two key ingredients, simultaneously present for the first time: two-fluid reconnection mediated by collisionless effects (that is, the Hall current and electron pressure tensor), and strong flux-pileup of the inflowing magnetic field. The first is expected to be important since the ion skin-depth is of finite size in these plasmas. The latter results from the super-sonic and super-Alfvenic inflow. The combination of the two boosts the reconnection rate above the nominal 0.1 $V_A$ to values consistent with experiment.\\{} [1] P. M. Nilson, et al, \textit{PRL} \textbf{97}, 255001 (2006).\\{} [2] C. K. Li, et al, \textit{PRL} \textbf{99}, 055001 (2007).   

Stochastic Flux-Freezing and Fast Magnetic Reconnection

The convention that magnetic field-lines are ``frozen-in'' to an MHD plasma is based on the Alfven Theorem, which holds only for smooth, laminar solutions of ideal MHD. In the presence of resistivity, the motion of field-lines is instead naturally regarded as stochastic and can be represented by a path-integral formula. When the plasma is also turbulent and the velocity is ``rough''---with a Kolmogorov-type energy spectrum---then the line-motion remains stochastic even for a tiny resistivity. This ``spontaneous stochasticity'' is due to the properties of turbulent 2-particle (Richardson) diffusion. Two plasma elements starting on the same magnetic field-line will lie on two distinct field lines, very far apart, a short time later. Charged particles spiral along magnetic field lines, but electric fields arise in the rest frame of the plasma to balance drag forces from collisions. These electric fields cause slight shifts in field-line attachments that are explosively amplified by turbulent 2-particle diffusion. Flux-freezing thus fails in the standard sense and only holds stochastically in a turbulent plasma at high magnetic and kinetic Reynolds numbers. As an application, we consider the problem of large-scale magnetic reconnection. Replacing resistive diffusion in the laminar Sweet-Parker solution by turbulent Richardson diffusion recovers the Lazarian-Vishniac theory of fast magnetic reconnection   

Application of Virtual Reality Technology to Research of Plasma Physics and Fusion Plasmas

Virtual Reality (VR) technology is a very powerful tool in analysis of simulation data and development of experimental devices, because it is possible to analyze the complex structures in three-dimensional space with a deep absorption into the VR world by scientific visualization technology. National Institute for Fusion Science (NIFS), Japan, installed VR System ``CompleXcope'' based on CAVE system in 1997, it has been developed continuously. In this paper, we introduce software for analysis of time-dependent simulation data and approach for contribution of simulation to experiment by VR technology. In the software with an animation function, we can visualize the objects of time-dependent fields and the particle trajectories in the time-dependent electromagnetic field in the VR space. By using this software, we analyze the relationship between the kinetic effects and the mechanism of magnetic reconnection. In the approach for contribution of simulation to experiment, both of simulation results and experimental device data are visualized simultaneously by the VR system to analyze directly the simulation results in the device. We show a pressure isosurface, magnetic field line and particle trajectory in the virtual Large Helical Device.   

Ion velocity measurements in expanding argon plasmas

Using a 7 Watt continuous wave argon ion laser at 488 nm to probe plasmas created by an Nd:YAG pulsed laser, we can make the ions in the argon plasma fluoresce at 422 nm. With phase coherence velocimety, we make direct measurements of ion velocities in the expanding plasma along with direct measurements of the local turbulent strength from the strength of the velocity fluctuations. Since high irradiance laser fields could couple with local turbulent parameters, we determine the influence of a 0-1 Kilowatt cw laser on the turbulent velocity signatures. We discuss these results in the context of new turbulent astrophysics and fusion science.   

Instability generated by Maxwell's Demon wire array

Previous experiments have shown that in a low pressure, low temperature plasma, positively biasing an array of thin wires can increase electron temperature by creating an angular momentum trap to absorb cold electrons. In this experiment, such a Maxwell Demon device was reproduced by looping 0.0025mm tungsten filaments around a stainless steel shaft covered with ceramic. Such device used to raise electron temperatures from 1eV to 2eV in a multi-dipole chamber operating in the sub-mTorr regime. Geometry of the device is not found to be essential to its functioning. However, at higher positive voltage, an instability in the kHz range prevents acquisition of meaningful temperature data. This instability is measured by a cylindrical probe, and its frequencies are extracted by means of Fast Fourier Transform. The conditions of this instability are investigated by varying gas composition, neutral pressure, plasma density and applied voltage.   

Electron Temperature and Ion Beam Scaling with RF Input Power in an Argon Helicon Plasma

A flowing argon helicon plasma is formed in a 10 cm diameter, 1.5 m long Pyrex chamber with an axial magnetic field in nozzle or flat configuration, variable up to 1 kG in the source region. A new expansion chamber has been constructed and initial laser induced fluorescence (LIF) results including ion velocities and temperatures are presented. Microwave interferometry (105 GHz), collisional radiative spectroscopic codes and diamagnetic loops are used to measure electron density and temperature during pulsed (5 ms) RF operation. Calculated variation of the RF frequency (from 12 MHz to 15 MHz) during the pulse allows for low ($<$5\%) reflected powers during the gas breakdown and the approach to and formation of the steady state plasma. The scaling of electron temperature with RF power is also examined for high ($>$3 kW) RF powers. The effect of different flow rates, magnetic field expansion variation and pressures are measured to observe the variation of the ion distribution function via LIF and the axial variation of acceleration due to neutral depletion. Possible double layer creation and sustainment in the downstream (relative to the RF antenna) transition to the expansion chamber is also examined at low flow rates and high RF powers.   

Integrity of the Plasma Magnetic Nozzle

The efficient conversion of plasma thermal energy into directed thrust using a magnetic nozzle is an oft-stated assumption in high power interplanetary mission concepts. This paper analyzes certain aspects of plasma flow through a magnetic nozzle, in particular the integrity of the plasma-magnetic field interface for various operational parameters. An expression is derived for the initial thickness of the plasma-field interface. A comparison is made between classical resistivity and gradient-driven anomalous resistivity, from which interface thickening is derived as function of time. An expression for the plasma temperature, density, and velocity dependencies is derived and found to agree with classical resistivity at local plasma temperatures of around 200-eV. Macroscopic flute instabilities within the interface in regions of adverse magnetic curvature are discussed, and a growth rate formula for magnetic nozzle design is derived. Preliminary analysis indicates that only one to two e-foldings of the most unstable Rayleigh-Taylor mode will occur; a more complete treatment of the Rayleigh-Taylor effect will include the Hall effect and ion magnetoviscosity. The Hall effect is incorporated into Ohm's law, and a critical nozzle length expression is derived below which the interface thickness is limited to approximately one ion gyroradius.   

Measuring and analyzing the magnetic field in (SERAJ system) theta pinch device using the Magnetic probes

Using the internal and the external magnetic probes in different positions inside and outside plasma discharge chamber of Seraj's Theta-pinch system (L = 150cm, D = 8.4cm), the generated magnetic field on the coil of Seraj's system has been calculated. When the plasma is discharged (by discharging the four capacitors bank connected on parallel, the capacity of each is (12.5$\mu$F) in the system coil (12nH)) the characteristics of plasma can be defined as how much magnetic field is affected. Comparing the magnetic field in the absence or presence of plasma, trapped magnetic field and Diamagnetic effect could be determined in the Theta pinch system. In the present study, one could determine the appropriate operating circumstances to produce suitable plasma with specific features (density \& electron temperature) as an advantage for different application.