Experimental observation of internal and external kinks in the resistive wall machine.

摘要

A cylindrical line tied screw pinch has been constructed at the University of Wisconsin-Madison with the goal of exploring the magnetohydrodynamic stability of current driven kinks in association with varying boundary conditions. The first plasma was created in the spring of 2004, and several milestones have since been achieved. Magnetohydrodynamic stability has been explored in relation to four different boundary conditions. In addition, a technique to precisely control the amount of current in the plasma throughout the pulse has been developed.;Internal kinks are observed to become unstable and grow when the safety factor q=4p2r2Bz m0Ipr L falls below 1 in the plasma. One internal mode has an m = 1 symmetric eigenmode peaked in the middle of the machine. The second internal mode is not symmetric and has an m = 1 eigenmode peaked at one end. The presence or absence of a conducting wall at the boundary does not substantially alter the growth or onset of these modes. The antisymmetric mode is noted to dominate the plasma when the current profile is undergoing fast relaxation events suggestive of reconnection. Experiments with two different wall conductivities have been carried out and the resistive wall mode has been identified. The resistive wall mode becomes unstable when the safety factor at the edge of the plasma is close to I and has a growth rate well correlated to predictions based on the wall conductivity and plasma parameters.