Mitigation of long-lived modes by lower hybrid wave on the HL-2A tokamak

Mitigation of long-lived mode (LLM) by lower hybrid wave (LHW) is recently achieved on the HL-2A tokamak. The LLM changes from a typical steady-state to a fishbone-like frequency-chirping characteristic, and its higher poloidal harmonics disappear when LHW is injected into toroidal plasma. It is found that density fluctuation declines during this process while the total neutron count increases gradually. Those evidences indicate there is a mitigation effect of LHW on LLM, and the underlying mechanism can be explained as follows. On one hand, high power LHW firstly causes a drop in toroidal rotation and then results in decline of E x B shear, which is unfavorable for the maintenance of the internal transport barrier. The resulting relaxation of ion temperature gradient enhances thermal transport and leads to a drop of ion temperature at the core region. Thus, the plasma pressure becomes flattened and finally contributes to the mitigation of highly saturated internal mode. On the other hand, hybrid simulation by M3D-K suggests that the off-axis LHW can also reduce the grower rate of LLM via changing safety factor and magnetic shear.