Neutral Point Shows Promise of Reducing Disruption Effects
Tokamak plasmas are subject to disruptions, which cause a sudden, undesirable loss of plasma energy, followed by termination of the discharge. Disruptions generally lead to rapid vertical motion of the plasma and generation of electromagnetic forces in structures near to where the plasma hits, with the potential to damage internal hardware. This problem is expected to be particularly severe in future reactor-scale devices. On the Japanese tokamak JT-60U, it has been found that by positioning the plasma center at a particular height in the tokamak vacuum vessel, called the "neutral point", the initial vertical drift of the disrupting plasma is significantly slower than usual, and sometimes can even be arrested, which could reduce or eliminate damage to internal components.
In an ongoing collaboration between MIT and the Japan Atomic Energy Research Institute, the neutral point concept is being subjected to a more challenging test on MIT's Alcator C-Mod tokamak, which has a plasma shape prototypical of future reactor plasmas. The existence of a neutral point on C-Mod has now been experimentally confirmed at a height of z=+2.7 cm (plasma minor radius=22 cm), although the physics of the neutral point stability on C-Mod appears to be somewhat different than that on JT-60U. The plasma has remained vertically stable for up to 9 ms after the disruption, which in principle, is long enough for the tokamak control system to respond, if programmed appropriately. Further research to test and optimize the control system response on C-Mod is being planned, with the ultimate goal of eliminating disruption vertical motion entirely. (Paper JO1.006; contact Robert Granetz, Y. Nakamura)