Overview of operation and experiments in the ADITYA-U tokamak

The first Indian tokamak ADITYA, operated over 2 decades with circular poloidal limiter has been upgraded to a tokamak named ADITYA Upgrade (ADITYA-U) for attaining shaped-plasma operations with open divertor in single and double-null configurations. The experimental research in ADITYA-U has made significant progress, since last FEC 2016. After installation of PFC and standard tokamak diagnostics in ADITYA-U, the Phase-I plasma operations has been initiated from December 2016 with graphite toroidal belt limiter. Ohmically heated circular plasmas supported by filament pre-ionization with plasma parameters, Ip ~ 80 – 95 kA, duration ~ 80 – 180 ms with maximum toroidal field ~ 1T and chord-averaged electron density ~ 2.5 x 10^19 m^-3 has been successfully obtained. The runaway electron (RE) generation, transport and mitigation experiments along with MHD activities and density enhancement with H2 gas puffing experiments have carried out in Phase-I, which is completed in March 2017. The Phase-II operation preparation in ADITYA-U includes calibration of magnetic diagnostics followed by commissioning of major diagnostics and installation of baking system. After repeated cycles of baking the vacuum vessel up to ~ 135°C, the Phase-II operations resumed from February 2018 and are continuing to achieve plasma parameters close to the design parameters of circular limiter plasmas using real time plasma position control. The plasma current has been raised to ~ 135 kA in phase-II with maximum chord averaged electron density of ~ 4 x 10^19 x m^-3. Hydrogen gas breakdown has been observed in more than ~ 2000 discharges including Phase-I and Phase-II operation without a single failure. Several experiments, including the control of REs with fuelling of Supersonic Molecular Beam Injection as well as sonic H2 gas puffing during current flat-top, MHD modes studies using multiple periodic gas puffs and radiative improved modes using Neon gas puff have been carried out. The experimental results from Phase-I and Phase-II operations of ADITYA-U tokamak are discussed in this paper.