Runaway electron behavior in plasma density modulation experiments in HT-7 Tokamak

The generation mechanism of runaway electrons in non-quasi-stable state plasmas and the influence of massive gas injection on the confinement of plasmas are investigated in Ohmic and low hybrid current drive (LHCD) plasma density modulation experiments in HT-7 Tokamak. It is found that massive gas injection during the discharge made the plasma from quasi-stable state turn into non-quasi-stable state in which the quasi-stable state assumption condition of Dreicer mechanism and secondary mechanism (avalanche) of runaway electron generation does not exist. Therefore, it is necessary to use the normalized critical velocity to explain the runaway electron behavior in non-quasi-stable state plasmas. It is also found that the plasma confinement became poor during massive gas injection in the plasma density modulation experiments in HT-7 Tokamak.