Speaker
Description
The effect of pellet injection on the m/n=2/1 tearing mode (TM) is investigated with the three-dimensional nonlinear magneto-hydro-dynamics (MHD) code CLT. Based on typical pellet parameters and ablation theory, the pellet ablation module has been developed within the CLT code. And the nonlinear evolution of mode is studied when the pellet is injected into the plasma during the saturation phase of the TM. It is found that the dominant 2/1 tearing mode does not change significantly, whereas non-dominant harmonics are excited, magnetic flux surfaces are destroyed and the field stochasticity appears when the pellet reaches the q=2 rational surface. These changes further destabilize the tearing mode and degrade confinement, which facilitates magnetic energy release and helps mitigate disruption-induced damage. Meanwhile, simulations with different toroidal injection locations, corresponding to different initial island phases, show that the tearing mode structure at the pellet injection cross-section remains unchanged. Regardless of whether the pellet is injected near the X-point or the O-point, the island rotates and eventually reorients its O-point toward the pellet, indicating that pellet injection has good phase robustness for Shattered Pellet Injection (SPI) applications. And the phase of the 2/1 mode starts to evolve before the pellet reaches the q=2 rational surface, indicating that a phase-adjustment mechanism faster than the pellet motion is involved. Detailed analysis indicates that due to the 3D effect of the pellet, the density increase will generate a drift motion instability and induce a convective magnetic field, making the island rotate before the pellet arrives. This provides a new mechanism in the process of disruption mitigation by pellet injection.