Pellet driven disruptions in tokamaks

Pellet injection can trigger ballooning like instability in tokamaks, driven by the large pressure perturbation of the pellet cloud. The instability is easier to excite when pellets are injected on the low magnetic field side of the tokamak, which is the usual case. This is because the pellet pressure perturbation tends to add to the background pressure drive. When pellets are injected on the high field side, the instability is harder to excite, because the pellet pressure perturbation opposes the background pressure drive. Nonlinearly, the instability causes fragmentation of the pellet cloud, and can give anomalous pellet penetration. This effect might explain recent experimental observations.