An H_α line-emission detection system was developed on the joint texas experimentaltokamak(J-TEXT),which is used to determine the H_α emission level during the gas breakdownand hereafter to control the startup of the plasma current.The detector consists of an H_α in-terference filter,a focusing lens,a photodiode and a preamplifier.In the J-TEXT operation,theH_α emission is taken as a monitor signal which is highly sensitive to the generation of a plasma.Furthermore,the power supply control system using the above signal as an input is capable of de-termining whether and when to fire the Ohmic heating capacitor banks,which are applied to drivethe plasma current ramp-up.The experimental results confirm that the H_α emission criterion isacceptable for controlling the plasma current promotion in the J-TEXT tokamak.
