P. N. Guzdar’s research while affiliated with University of Maryland, College Park and other places

The stability of the ideal magnetohydrodynamic (MHD) interchange mode at marginal conditions is studied. A sufficiently strong constant magnetic field component transverse to the direction of mode symmetry provides the marginality conditions. A systematic perturbation analysis in the smallness parameter, $|b_2/B_c|^{1/2}$, is carried out, where $B_c$ is the critical transverse magnetic field for the zero-frequency ideal mode, and $b_2$ is the deviation from $B_c$. The calculation is carried out to third order including nonlinear terms. It is shown that the system is nonlinearly unstable in the short wavelength limit, i.e., a large enough perturbation results in instability even if $b_2/B_c>0$ (linearly stable). The normalized amplitude for instability is shown to scale as $|b_2/B_c|^{1/2}$. A nonlinear, compressible, MHD simulation is done to check the analytic result. Good agreement is found, including the critical amplitude scaling.

In this report, a novel new mode, named the electron geodesic acoustic mode, is presented. This mode can occur in toroidal plasmas like the conventional geodesic acoustic mode (GAM). The frequency of this new mode is much larger than that of the conventional GAM by a factor equal to the square root of the ion to electron mass ratio.

Groups of animals often tend to arrange themselves in flocks that have characteristic spatial attributes and temporal dynamics. Using a dynamic continuum model for a flock of individuals, we find equilibria of finite spatial extent where the density goes continuously to zero at a well-defined flock edge, and we discuss conditions on the model that allow for such solutions. We also demonstrate conditions under which, as the flock size increases, the interior density in our equilibria tends to an approximately uniform value. Motivated by observations of starling flocks that are relatively thin in a direction transverse to the direction of flight, we investigate the stability of infinite, planar-sheet flock equilibria. We find that long- wavelength perturbations along the sheet are unstable for the class of models that we investigate. This has the conjectured consequence that sheet-like flocks of arbitrarily large transverse extent relative to their thickness do not occur. However, we also show that our model admits approximately sheet-like, 'pancake-shaped', three-dimensional ellipsoidal equilibria with definite aspect ratios (transverse length- scale to flock thickness) determined by anisotropic perceptual/response characteristics of the flocking individuals, and we argue that these pancake-like equilibria are stable to the previously mentioned sheet instability.

The alternating-direction-implicit finite-difference time-domain method is used to analyze the Metal-Insulator- Semiconductor-Metal interconnects by solving Maxwell's equations in the time domain. The dielectric quasi-TEM mode, the slow wave mode, and the skin-effect mode are all analyzed. This analysis shows that the silicon substrate losses and the metal line losses can be modeled with high resolution. The analysis provides attenuation and phase constant values versus semiconductor doping and frequency. We find that semiconductors readily operate in the slow wave mode and skin effect mode for selected doping densities. Accurate prediction of interconnect losses is critical to high-frequency design with highly constrained timing requirements. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26085

The magnitude of the energy and particle fluxes in simulations of edge-localized modes (ELMs) is determined by the edge gradients in the pressure, density, ion temperature, and electron temperature. The total edge pressure gradient is the dominant influence on ELMs by far. An increase (decrease) of merely 2% in the pressure gradient results in an increase (decrease) of more than a factor of ten in the size of the ELM bursts. At a fixed pressure gradient, the size of the ELM bursts decreases as the density gradient increases, while the size of the bursts increases as the electron temperature gradient or, especially, the ion temperature gradient increases.

The stability of kink modes in a cylindrical plasma column under the influence of an oscillating magnetic field applied in the azimuthal direction is studied. The frequency of the applied field is chosen close to the ion-acoustic frequency. It is found that coupling to damped ion-acoustic waves induces a small real frequency to the kink modes which can significantly affect their growth rates. Conditions for parametrically suppressing (or exciting) these modified kink modes are specified.

Tokamak density fluctuation levels at which significant scattering of an electron cyclotron heating wave occurs are estimated. The analysis utilizes a wave kinetic equation to describe the scattering process in conjunction with a toroidal ray-tracing code. One result is that severe scattering occurs for a typical reactor plasma at a fluctuation level of one per cent.

The thermal equilibrium of tokamak plasmas is studied by solving the electron and ion energy balance equations numerically. Three basic types of thermal equilibria, depending on different boundary temperatures, are found. The presence of a fold or cusp catastrophe structure of the equilibrium curves for temperature versus density in the case of low-boundary temperature indicates the existence of a density limit beyond which no equilibrium or only a trivial equilibrium exists. Major disruptions are considered to occur because of the loss of thermal equilibrium of the tokamak plasma. The study shows that major disruptions can be parametrically controlled. Thermal equilibria with higher boundary temperature display a monotonic structure of the temperature versus density curve, indicating good confinement characteristics and hence the absence of disruptions.

A theoretical investigation of the thermal equilibria of detached plasmas has been performed. The transport model used is that for the ηe modes. The model explains the scaling of the plasma radius, central electron temperature, impurity radiation, electron energy confinement time with plasma current and central density as observed on TFTR. Furthermore, assuming that the plasma disrupts when the safety factor reaches two at the edge of the detached plasma, a density limit ncr is obtained. For the impurity density nI independent of the plasma density, whereas for nI = fn(0), where f is the fraction of impurities, ncr ∝ BT/R.

Several different techniques are used to study the stability of electrostatic drift wave eigenmodes in a resistive plasma with finite magnetic shear. It is found that in the slab approximation, where usual shear damping is operative, resistivity contributes to an enhancement of this damping and the enhancement factor increases with the electron-ion collision frequency νei. Thus no unstable eigenmodes result. If the shear damping is nullified, either by introducing a strong spatial variation of the density gradient or by working in toroidal geometry with strong toroidal coupling effects, then unstable eigenmodes with growth rates increasing with νei are recovered. A perturbation calculation shows that retention of electron temperature fluctuations associated with the mode and inclusion of temperature gradients do not alter these conclusions. Extensive numerical calculations are also presented.