Characterization of the plasma current quench during disruptions in ADITYA tokamak.

The rate of plasma current-quench during tokamak plasma disruptions determines the electromagnetic forces on the in-vessel components/vacuum vessel. Also halo currents and rapid changes of poloidal field due to the plasma vertical displacement contribute to loads on vessel and in-vessel components and hence needs to be studied thoroughly to safeguard these tokamak peripherals. The plasma current quench occurrence during the spontaneous major disruption has been investigated for a set of ADITYA tokamak disrupted discharges and average plasma current quench and instantaneous current quench rates have been estimated. The fastest area-normalized plasma current (I_P) quench time is observed to be ∼5 ms m⁻². The estimated post disruption plasma electron temperatures (PDET) are observed to be ∼15–35 eV and proportional to area-normalized plasma current quench time. Further analysis of several disruptive discharges of ADITYA tokamak reveal that the current quench time is inversely proportional to the pre-disruptive values of edge safety factor, q_a, and the current quench properties are strongly correlated with the prevailing pre-disruptive plasma magnetohydrodynamic (MHD) activities. For larger values of pre-disruptive q_a, the larger island widths of m= 2 and m = 3 MHD modes leads to a significant overlap of these islands. Such an overlap along with the deeper locations of the islands inside the plasma column, as compared to discharges having smaller values of pre-disruptive q_a, seems to facilitate the faster current quench. [ABSTRACT FROM AUTHOR]