Alcator C-Mod

Sarah Angelini

Updated: 07/14/06

When disruptions occur in tokamaks, there is a loss of confinement and large amounts of current and energy are forced into the first wall causing substantial melting and erosion. In a large scale fusion reactor, the disruptions would cause crippling damage and this would halt electricity production. The International Tokamak Physics Activity (ITPA) disruptions database was created to collect information from experiments worldwide to be applied toward understanding disruption mitigation in ITER. By studying trends in disruption data, it would be possible to develop proper mitigation methods which would increase stability and prevent damage from occurring.

Over the past year, I have worked to populate Alcator's MDSplus trees with data from EFIT and now, at General Atomics, I am analyzing and comparing it to data entered into DIII-D's tree. Certain major disruptions have been observed in DIII-D's data that seem to have a faster current decay time than would be expected. Major disruptions are characterized by a prompt loss of stored energy prior to any loss of position, and followed by a rapid current quench (CQ). The CQ typically produces an uncontrollable loss of vertical position, leading to plasma-wall contact. By the time the plasma has moved enough to make contact with a wall, virtually all of the stored energy and much of the current density has been dissipated, minimizing many potentially damaging effects to the machine. However, extremely rapid CQ's can induce large destructive eddy currents in surrounding conducting structures. In the absence of motion, the CQ time is proportional to the pre-disruption plasma area, among other dependencies. The ITPA Disruptions Database seeks to normalize CQ times by plasma area for cross-machine comparisons, although this ignores the role of vertical motion in accelerating the CQ rate. The faster major disruptions in DIII-D display unusually rapid vertical motion and a normalized current decay time less than 1ms/m^2, which would present a challenge to ITER's engineering design. These plasmas share a low squareness configuration not accessible to many devices. I seek to discover the shared characteristics of these disruptions, to compare them to disruptions observed on Alcator, and to determine whether such disruptions could occur in ITER.