Equilibrium Magnetic Field Requirements during Plasma Initiation and Current Ramp-up Phase in ADITYA/ADITYA-U Tokamak Discharges

Abstract

Plasma equilibrium in ADITYA/ADITYA-U is provided by two pairs of vertical field coils (BV1 & BV2) placed outside the vessel. A peak loop voltage of ∼20 V is required for successful breakdown and start-up in ADITYA, which leads to a higher I_P ramp-rate ∼6 - 8 MA/Sec during the first ∼7 ms of discharge. To hold the plasma column in equilibrium, the vertical field should also be ramped-up at the same rate. Series connections of vertical field (BV) coils do not provide the required ramp rate due to the high L/R time-constant of the coils and 12 pulse converter firing. Therefore, additional arrangements are made to achieve it. The addition of a pre-charged capacitor of 500 μF/3 kV with VF converter based power supply allows successful start-up but causes concern about a slight dip that is observed in the plasma current. To obtain proper stabilization, two techniques are used. One is the paralleling of BV coils, and second is using the combination of another capacitor bank of 19.5 mF/1.2 kV and IGBT based power supply have improved the plasma performance and raised the I_P ∼ 150 kA with dI_P/dt ∼ 3.0 - 3.5 MA/s in ADITYA. In this paper, the effect of the equilibrium field in accordance with plasma performance is discussed in detail.