Effect of equilibrium radial position on horizontal instability of middle single null divertor configuration in the iron core tokamak

The unique high-field side middle single null divertor configuration in the Joint-Texas Experimental Tokamak is attractive in studying detachment, impurity control, L-H transition, and negative triangularity. Its equilibrium configuration with a horizontally elongated (or flattened) poloidal cross section causes position instability and difficulty in equilibrium control. This work disassembles the vertical magnetic field of each group of poloidal field windings and calculates the field decay index (n-index) at different radial positions. It is found that the instability growth time τg of horizontal displacement increases as the equilibrium radial position of plasma moves outward, which is beneficial to the position control. The simulation and experiments verified the effect of equilibrium position on horizontal instability. The equilibrium position slightly outward from the geometric center of the vacuum chamber effectively decreases the horizontal displacement oscillation amplitude and guarantees the controllable horizontal displacement.