The triggering effect of SMBI on MHD instabilities on HL-2A tokamak

In the 2018 experiment campaign in HL-2A, the investigation of triggering MHD instabilities by SMBI has been intensively conducted. Two SMBI systems have been applied in these experiments with gas pressure 1.25 MPa. As show in discharge 32386, these two system inject almost the same quantity of D₂, indicating by the increase of the density. It has been found that the system without a skimmer could trigger the MHD instability, while the system with a skimmer could not. Further investigations show that the triggering of MHD instability often appears with a massive gas injection. As shown in the figure, the low velocity component in the injected gas contributes little to the total density, but it reduces the plasma temperature and shrinks the plasma, thus triggering the MHD instability. Similar to the observations of pellet injection, the characteristics of experiment of triggering of MHD instability by SMBI can be identified as: the obvious increase of plasma density, along with a 3 kHz MHD oscillation on the Mirnov Signal, and the MHD instability is usually the tearing mode with the mode number m/n = 2/l. (authors)