Thermal behavior of radiation damage cascades via the binary collision approximation: Comparison with molecular dynamics results

Based on the profile of the energy deposition obtained using the binary collision model, we follow the diffusion of energy by solving a simplified version of the heat equation. An estimation of the molten zone compares very well with the molecular dynamics prediction for the same event. We discuss the reasons for this agreement and the relevance of such simplified procedure in terms of present-day computer limitations to simulate high energy cascades using molecular dynamic

Four-phase patterns in forced oscillatory systems

We investigate pattern formation in self-oscillating systems forced by an external periodic perturbation. Experimental observations and numerical studies of reaction-diffusion systems and an analysis of an amplitude equation are presented. The oscillations in each of these systems entrain to rational multiples of the perturbation frequency for certain values of the forcing frequency and amplitude. We focus on the subharmonic resonant case where the system locks at one fourth the driving frequency, and four-phase rotating spiral patterns are observed at low forcing amplitudes. The spiral patterns are studied using an amplitude equation for periodically forced oscillating systems. The analysis predicts a bifurcation (with increasing forcing) from rotating four-phase spirals to standing two-phase patterns. This bifurcation is also found in periodically forced reaction-diffusion equations, the FitzHugh-Nagumo and Brusselator models, even far from the onset of oscillations where the amplitude equation analysis is not strictly valid. In a Belousov-Zhabotinsky chemical system periodically forced with light we also observe four-phase rotating spiral wave patterns. However, we have not observed the transition to standing two-phase patterns, possibly because with increasing light intensity the reaction kinetics become excitable rather than oscillatory.Comment: 11 page

Global stability of plasmas with helical boundary deformation and net toroidal current against n = 1,2 external modes

CRPPSPCProc. Joint Varenna - Lausanne Int. Workshop on Theory of Fusion Plasmas, Varenna, Italy, August 1996, ISPP-17, 349 - 355 (1996

External kinks in plasmas with helical boundary deformation and net toroidal current

The global ideal magnetohydrodynamic (MHD) stability of plasmas with a prescribed helical boundary deformation and nonvanishing toroidal current is investigated with respect to the (m,n) external modes with n = 1,2,3, and m = n + 1. L = 2, 3 and mixtures (of both) configurations were studied by systematically varying parameters such as the aspect ratio, the number of equilibrium field periods, the toroidal current density, and the pressure profiles. The study focuses mainly on the q < 2 area. It is shown that a fixed helical boundary deformation can stabilize the (m,n) external modes with n = 1,2,3, and m = n + 1 at values of beta similar or equal to 1%-2%. These modes are unstable in the circular tokamak at the same value of beta. If delta is a measure of the plasma boundary deformation, then windows of stability [delta(min)delta(max)] may exist depending strongly on equilibrium parameters. The results are described in terms of stability zones in the (q(axis),q(edge)) plane. (C) 1997 American Institute of Physics

Global stability of n = 1 external modes for plasmas with helically boundary deformation and net toroidal current

CRPPSPCProc. 23rd EPS Conference on Controlled Fusion and Plasma Physics, Kiev, Ukraine, June 199

Global ideal MHD stability of plasmas with toroidal, helical and vertical field coils

A simple configuration that consists of a set of toroidal, helical and vertical field coils is used to calculate free boundary equilibria with non-zero plasma current and approximately helically symmetric plasma boundary shape. The amount of helical boundary deformation is controlled by the ratio of the current in the helical field coils to the current in the toroidal field coils. When this ratio is increased, the (m, n) = (2, 1) external kink is stabilized at beta similar or equal to 1% for inverse rotational transform profiles in the region q < 2 and an aspect ratio A similar or equal to 10