Effect of gas puffing on current ramp-down ohmic discharges in HT-7 tokamak

A new behaviour of current ramp-down ohmic deuterium discharges has been observed by additional deuterium fueling/gas puffing in HT-7 tokamak. It has been investigated, that the additional gas puffing converts the fast pitch angle scattering (FPAS) event to normal pitch angle scattering (NPAS) event characterized by higher electron energies and shifting the trigger time of the event, which is dependent on the pulse injection time. The attempt has also been made to investigate the scaling of critical values for runaway electrons generation and determination of their related energies along with trigger parameters of the instability and conversion relation between NPAS and FPAS events by ramping-down plasma current and changing the toroidal magnetic field in low-density ohmic discharges. The direct and inverse relationship has been observed for critical trigger value ωpe/ωce of runaways generation with the plasma current and the toroidal magnetic field strength respectively. The runaway instability regime is characterized by relaxations in the electron cyclotron emission due to relativistic anomalous Doppler resonance (ADR) effect, which transfers energy from parallel to perpendicular motion.