Characterization of Ohmic breakdown phase for the ADITYA discharges

ADITYA is a mid-size air core tokamak, with major radius 0.75 m and minor radius 0.25 m. Breakdown is achieved via inductive start-up with assistance of filament pre-ionization. The nature of pre-breakdown and its effect on quality of plasma formed later is studied with parameters influencing it, like toroidal electric field E, operating pressure p, toroidal magnetic field B_T, error magnetic field B_error and initial electron density n_e. Breakdown failure due to various causes are identified, from which boundary limit and general condition for set of primary operating parameters is derived to achieve successful discharge in ADITYA tokamak. It is learned that breakdown is achieved for operating pressure ranging between 0.5^-1 X 10^-4 torr and ratio of toroidal electric field E (V/m) to error magnetic field produced by the Ohmic coil B_err (G) above 0.32 V/G^-m. Breakdown model is developed to estimate breakdown time by considering electron production and loss mechanism for a given discharge conditions, which agrees well with filament assisted plasma discharges. The condition for successful breakdown E_𝜙 B_T/B_⊥ > 10³ V/m is found to be valid and experimentally confirmed in many tokamaks including ADITYA tokamak. (author)