Distribution of poloidal (left) and toroidal (right) currents on the resistive wall during a downward moving VDE. Horizontal axis is the toroidal angle. Poloidal angle, measured from the outboard midplane, is on the vertical axis.
Research Objectives
The goal of this research is to investigate the currents generated on plasma-facing components during large scale, macroscopic displacements of the plasma column in elongated tokamaks, caused by disruptions or loss of positional control.
Computational Approach
Vertical stability of tokamaks has been studied extensively in the past, using sophisticated, but one- or two-dimensional, computational models. Our approach here is a self-consistent and three-dimensional reexamination of the problem, using our nonlinear magnetohydrodynamic code CTD that has been appropriately modified to handle nonlinear free-boundary problems without any built-in symmetry assumptions. The plasma region, described by the MHD equations, is surrounded by a wall of finite thickness and resistivity. Toroidal equilibrium and plasma shaping are provided by external currents assumed to be flowing in circuits of large inductance, so that the currents can be taken to be constant in time. To date, production runs have been done on the vector machines (C90), but CTD is gradually being rewritten in C++ and ported over to the T3E at NERSC.
Accomplishments
In double-null equilibria self-consistently generated with CTD, we are examining the nonlinear evolution of vertical displacement events (VDEs). We have qualitative observations of large poloidal and toroidal nonuniformities in the currents generated in the resistive wall. The electric field set up by the decaying plasma current, in conjunction with the eddy currents generated by the motion of the plasma column, leads to a very large poloidal nonuniformity in the toroidal component of the wall current, with a possible reversal of the current near the strike point. The poloidal halo currents in the wall, mainly due to coupling of the diamagnetic plasma currents to the conducting wall after the column comes in contact with it, are by their very nature poloidally localized; but they also exhibit toroidal nonuniformities when the VDE is coupled with an n=1 MHD mode. A more quantitative characterization of these processes and an extension of the work to single-null geometry are under way.
Significance
Loss of vertical stability, due to a disruption or a failure in the feedback system, is an event that probably cannot be completely avoided in the operation of a tokamak. Thus, a quantitative understanding of the currents generated by a VDE and the electromagnetic forces generated by them on the conducting structures around the plasma column is essential for the design and operation of the next generation of tokamaks.
Distribution of poloidal (left) and toroidal (right) currents on the resistive wall during a
downward moving VDE. Horizontal axis is the toroidal angle. Poloidal angle, measured
from the outboard midplane, is on the vertical axis.