Passive Vertical Stability in the Next Generation Tokamaks

The next generation of reactor-size tokamaks will have substantially different vertical stability characteristics than existing large-scale experimental devices. Separation between plasma and stabilizing elements such as vacuum vessel and poloidal field coils is expected to be much greater than in present experiments, and high plasma elongation will be required for high plasma current and good plasma confinement. These features exacerbate the vertical stability control problems. The passive stability properties, growth rate, and stability margin of a plasma moving vertically in a set of toroidally continuous, resistive, current carrying elements is presented. An eigenvector analysis method is used to explore the vertical stability margin and growth rates of these equilibria. These methods are applied to quantify the passive stability properties of a large-scale fusion device, ITER (International Thermonuclear Experimental Reactor). Representative plasma equilibria with plasma elongations (κ9595, at 95% poloidal flux) varying from 1.6 to 2.4 are produced based on reactor-type scaling laws. The results show that a highly elongated plasma (κ95 ≤ 2.0) can be stabilized within acceptable limits with the proper placement of a toroidally continuous wall around the outer portion of the plasma. Use of a partial poloidal wall is almost as effective as enclosing the plasma in a complete shell.