In an experiment conducted in 2016, a saturated long-lived helical mode (LLMs) has been observed for the first time in EAST hybrid plasmas achieved with neutral beam injection (NBI). The long-lived helical mode is found to play an important role in achieving steady state high performance plasma.
In a recent research, scientists have new findings about LLMS. On one hand, the presence of LLM would cause many deleterious effects such as confinement degradation, fast ion losses. Notably, in EAST case, LLM caused severe toroidal rotation damping in the core so that the rotation profile inside the q=2 surface is flattened. The flattened part of the rotation profile extents up to half of the minor radius.
On the other hand, it is found that the long-lived mode could contribute to maintaining steady state plasma by a self-regulating magnetic flux pumping mechanism. A three-dimensional resistive nonlinear simulation with realistic tokamak configuration has been applied to investigate the evolution of long-lived helical instability. It is found that in the saturated phase, the initial weak shear safety factor profile in the core becomes completely flat. This change of safety factor profile indicates that the plasma current is further prevented from penetrating into the plasma core.
For future work, the mechanism of flux pumping effects will be further investigated.
Fig.1 Experimental observation of the long-lived mode. (Image by YUAN Yi)
Contact:
ZHAO Weiwei
Hefei Institutes of Physical Science (http://english.hf.cas.cn/)
Email: annyzhao@ipp.ac.cn