Runaway electron generation during plasma shutdown by killer pellet injection
Tokamak discharges are sometimes terminated by disruptions that may cause large mechanical and thermal loads on the vessel. To mitigate disruption-induced problems it has been proposed that ‘killer’ pellets could be injected into the plasma in order to safely terminate the discharge. Killer pellets enhance radiative energy loss and thereby lead to rapid cooling and shutdown of the discharge. But pellets may also cause runaway electron generation, as has been observed in experiments in several tokamaks. In this work, runaway dynamics in connection with deuterium or carbon pellet-induced fast plasma shutdown is considered. A pellet code, which calculates the material deposition and initial cooling caused by the pellet is coupled to a runaway code, which determines the subsequent temperature evolution and runaway generation. In this way, a tool has been created to test the suitability of different pellet injection scenarios for disruption mitigation. If runaway generation is avoided, the resulting current quench times are too long to safely avoid large forces on the vessel due to halo currents.
A Fast Shutdown Technique for Large Tokamaks
A practical method is proposed for the fast shutdown of a large ignited tokamak. The method consists of injecting a rapid series of 30-50 deuterium pellets doped with a small ( 0.0005%) concentration of Krypton impurity, and simultaneously ramping the plasma current and shaping fields down over a period of several seconds using the poloidal field system. Detailed modeling with the Tokamak Simulation Code using a newly developed pellet mass deposition model shows that this method should terminate the discharge in a controlled and stable way without producing significant numbers of runaway electrons. A partial prototyping of this technique was accomplished in TFTR.
Effect of antioxidants, on deterioration by partial discharge in LDPE and XLPE [insulation materials]
The effect of antioxidants in low-density polyethylene (LDPE) and cross-linked polyethylene (XLPE) on deterioration by partial discharge (PD) was investigated. Experiments were conducted using LDPE and XLPE samples containing phenolic antioxidants. Samples subjected to the PD were analyzed using atomic force microscopy and Fourier transform infrared analysis (AFM and FTIR). AFM images demonstrated that the magnitude of pits created in the sample containing antioxidants was smaller than that in samples containing no antioxidant. FTIR showed that oxidative degradation occurred on LDPE and XLPE surfaces when they were subjected to PD. These results confirmed that the amount of oxidized substances was smaller in samples containing phenolic antioxidants than in those containing no antioxidant. This report elucidates the mechanism of antioxidant action on deterioration by PD.