Barrett N. Rogers

Alfvénic collisionless magnetic reconnection and the Hall term

The Geospace Environment Modeling (GEM) Challenge Harris current sheet problem is simulated in 2 1/2 dimensions using full particle, hybrid, and Hall MHD simulations. The same gross reconnection rate is found in all of the simulations independent of the type of code used, as long as the Hall term is included. In addition, the reconnection rate is independent of the mechanism which breaks the frozen-in flux condition, whether it is electron inertia or grid scale diffusion. The insensitivity to the mechanism which breaks the frozen-in condition is a consequence of whistler waves, which control the plasma dynamics at the small scales where the ions become unmagnetized. The dispersive character of whistlers, in which the phase velocity increases with decreasing scale size, allows the flux of electrons flowing away from the dissipation region to remain finite even as the strength of the dissipation approaches zero. As a consequence, the throttling of the reconnection process as a result of the small scale size of the dissipation region, which occurs in the magnetohydrodynamic model, iio longer takes place. The important consequence is that the minimum physical model necessary to produce physically correct reconnection rates is a Hall MHD description which includes the Hall term in Ohms law. A density depletion layer, which lies just downstream from the magnetic separatrix, is identified and linked to the strong in-plane Hall currents which characterize kinetic models of magnetic reconnection.

Fast reconnection in high-Lundquist-number plasmas due to the plasmoid Instability

Thin current sheets in systems of large size that exceed a critical value of the Lundquist number are unstable to a super-Alfvenic tearing instability, referred to hereafter as the plasmoid instability. The scaling of the growth rate of the most rapidly growing plasmoid instability with respect to the Lundquist number is shown to follow from the classical dispersion relation for tearing modes. As a result of this instability, the system realizes a nonlinear reconnection rate that appears to be weakly dependent on the Lundquist number, and larger than the Sweet–Parker rate by nearly an order of magnitude (for the range of Lundquist numbers considered). This regime of fast reconnection is realizable in a dynamic and highly unstable thin current sheet, without requiring the current sheet to be turbulent.

Edge turbulence imaging in the Alcator C-Mod tokamak

The two-dimensional (2D) radial vs poloidal structure of edge turbulence in the Alcator C-Mod tokamak [I. H. Hutchinson, R. Boivin, P. T. Bonoli et al., Nucl. Fusion 41, 1391 (2001)] was measured using fast cameras and compared with three-dimensional numerical simulations of edge plasma turbulence. The main diagnostic is gas puff imaging, in which the visible Dα emission from a localized D2 gas puff is viewed along a local magnetic field line. The observed Dα fluctuations have a typical radial and poloidal scale of ≈1 cm, and often have strong local maxima (“blobs”) in the scrape-off layer. The motion of this 2D structure motion has also been measured using an ultrafast framing camera with 12 frames taken at 250 000 frames/s. Numerical simulations produce turbulent structures with roughly similar spatial and temporal scales and transport levels as that observed in the experiment; however, some differences are also noted, perhaps requiring diagnostic improvement and/or additional physics in the numerical m...

Linearized model Fokker–Planck collision operators for gyrokinetic simulations. II. Numerical implementation and tests

A set of key properties for an ideal dissipation scheme in gyrokinetic simulations is proposed, and implementation of a model collision operator satisfying these properties is described. This operator is based on the exact linearized test-particle collision operator, with approximations to the field-particle terms that preserve conservation laws and an H-theorem. It includes energy diffusion, pitch-angle scattering, and finite Larmor radius effects corresponding to classical real-space diffusion. The numerical implementation in the continuum gyrokinetic code GS2 Kotschenreuther et al., Comput. Phys. Comm. 88, 128 1995 is fully implicit and guarantees exact satisfaction of conservation properties. Numerical results are presented showing that the correct physics is captured over the entire range of collisionalities, from the collisionless to the strongly collisional regimes, without recourse to artificial dissipation.

Plasma turbulence in the scrape-off layer of tokamak devices

Plasma turbulence is explored in the scrape-off layer of tokamak devices using three-dimensional global two-fluid simulations. Two transport regimes are discussed: one in which the turbulent fluctuations saturate nonlinearly due to the Kelvin-Helmholtz instability, and another in which the fluctuations saturate due to a local flattening of the plasma gradients and associated removal of the linear instability drive. Focusing on the latter regime, analytical estimates of the cross-field transport and plasma profile gradients are obtained that display Bohm-scaling diffusion properties.

Noncurvature-driven modes in a transport barrier

Transport barriers that form in both the edge and interior regions of high temperature magnetically confined discharges are characterized by steep plasma gradients, strong E×B and diamagnetic flows, and varying levels of magnetic shear. This study addresses the linear stability of such configurations in the context of a simple slab model using both analytic calculations as well as numerical simulations from the gyrokinetic GS2 code. Three linear modes of potential importance are found: the Kelvin–Helmholtz instability, the tertiary mode, and a nonlocal drift wave instability. Each mode is unstable only in the presence of nontrivial spatial variations in either the E×B flow and∕or the plasma gradients. The strongest conclusion of this study is that the drift wave mode may be an important driver of anomalous transport in the edge region of magnetic confinement devices. Two other weaker conclusions that warrant further study are as follows: (1) the Kelvin–Helmholtz instability may be associated with edge-loc...

Transport scaling in interchange-driven toroidal plasmas

Two-dimensional fluid simulations of a simple magnetized torus are presented, in which the vertical and toroidal components of the magnetic field create helicoidal field lines that terminate on the upper and lower walls of the plasma chamber. The simulations self-consistently evolve the full radial profiles of the electric potential, density, and electron temperature in the presence of three competing effects: the cross-field turbulent transport driven by the interchange instability, parallel losses to the upper and lower walls, and the input of particles and heat by external plasma sources. Considering parameter regimes in which equilibrium E×B shear flow effects are weak, we study the dependence of the plasma profiles—in particular the pressure profile scale length—on the parameters of the system. Analytical scalings are obtained that show remarkable agreement with the simulations.

Gyrokinetic simulations of collisionless magnetic reconnection

Linear and nonlinear gyrokinetic simulations of collisionless magnetic reconnection in the presence of a strong guide field are presented. A periodic slab system is considered with a sinusoidally varying reconnecting magnetic field component. The linear growth rates of the tearing mode in both the large and small regimes are compared to kinetic and fluid theory calculations. In the nonlinear regime, focusing on the limit of large , the nonlinear reconnection rates in the gyrokinetic simulations are found to be comparable to those obtained from a two-fluid model. In contrast to the fluid system, however, for TiTe and very small initial perturbation amplitudes, the reconnection in the gyrokinetic system saturates in the early nonlinear phase. This saturation can be overcome if the simulation is seeded initially with sufficient random noise.

Gyrokinetic linear theory of the entropy mode in a Z pinch

The linear gyrokinetic theory of the entropy mode is presented in a Z-pinch configuration in the regime of plasma β⪡1, focusing primarily on the parameter regime in which the ideal interchange mode is stable. The entropy mode is a small-scale, nonmagnetohydrodynamic mode that typically has peak growth rates at kρs∼1[ρs2=(T0e+T0i)∕(miΩci2)], with magnitudes comparable to those of ideal modes. The properties of this mode are studied as a function of the density and temperature gradients, plasma collisionality, and electron to ion temperature ratio.

A study of asymmetric reconnection scaling in the Lyon-Fedder-Mobarry code

Using a three-dimensional magnetospheric simulation code we have studied the properties of magnetic reconnection at the subsolar point on solar wind parameters for southward interplanetary magnetic field conditions and compared the results with the predictions of the Cassak-Shay theory. We find that this theory predicts reconnection rates on the order of our observations and produces reasonable predictions of the reconnection outflow speed. We have quantified the contributions that differences between the assumed and measured mass, energy, and outflow density scalings make to predictions of the reconnection rate and outflow speed. In general, the theory makes reasonable assumptions about the mass and energy flux into the reconnection layer, but their outflowing counterparts are overestimated due to the narrowness of the reconnection outflow jet. Lastly, we find that newly reconnected flux tubes exit the merging region before their mass density can equilibrate, requiring a correction to the predicted outflow density.