Start-Up Studies of GLAST-III Spherical Tokamak in the Presence of Poloidal Field

GLAST-III (glass spherical tokamak) is a small device having an aspect ratio of two (R/a = 2) with major radius R = 20 cm and minor radius a = 10 cm. Experiments are performed to examine the effect of increasing the poloidal field (PF) produced by poloidal coils as well as to study the effect of the inductance of PF coils on the plasma current formation. To investigate the GLAST-III plasma, several diagnostic tools such as Rogowski coil, loop voltage, photodiode, Optical emission spectroscopy, high-speed camera, and Langmuir probe are used. After optimizing the working pressure, the first experiment is performed using toroidal field coils and central solenoid along with radio frequency (RF) pre-ionization source. A plasma current of 1 kA is achieved for 1.2 ms. In later experiments, the effect of the poloidal magnetic field is also included. It is observed that with increasing the PF, the plasma current increases, attain a maximum value (5 kA) and then after a critical value of the applied poloidal magnetic field, the plasma current starts decreasing. Similarly, with the inclusion of inductor in series with the PF coils, the plasma current value is reduced and its pulse duration is increased. Both optical emission and Langmuir probe diagnostics results show a similar trend of the plasma current.