Influence of circulating energetic ions on MHD instabilities in negative magnetic shear plasma

The influence of circulating energetic ions (CEIs) on double tearing mode (DTM) is investigated by a global kinetic-MHD hybrid simulation. It is shown that the co-CEIs with a small orbit width have a destabilizing effect on the DTM. As the orbit width increases, this destabilizing effect is reduced. If the orbit width is large enough, the DTM is transformed to single tearing modes due to the co-CEIs, so the mode becomes more stable than the DTM without CEIs. The effect of the counter-CEIs with small orbit width is similar to that of the co-CEIs. For a large orbit width, the counter-CEIs also have a stabilizing effect. If the energetic ion beta is larger than a threshold, an energetic particle driven mode (EPM) is excited due to the resonance between counter-CEIs and the bulk plasma. The effects of energetic ion beta, orbit width and resistivity on this EPM are studied systematically. The onset threshold of EPM depends on the orbit width and resistivity. It decreases with orbit width increasing, while it increases with resistivity increasing. The mode transition from DTM to EPM with energetic ion beta increasing is also found for large orbit width.