Reduced effective ionic charge and enhanced plasma performance in the HT-7 tokamak

Many technical improvements were achieved recently in the HT-7 superconducting tokamak, such as ICRF boronization, new water cooling graphite limiters which is instead of molybdenum limiter, and application of ferritic steel for reducing the magnetic ripple, and so on. The global impurity content and impurity components were measured and studied by using of spectroscopy diagnostics in the HT-7 tokamak. Experimental results of the effective ionic charge Z_eff and impurity behaviors before and after technical improvements mentioned above are compared in this paper. It is observed that the integrated plasma performance is enhanced while the global impurity content and Z_eff drops with these technical improvements. Steady state plasma with higher performance and 5-min long pulse plasma were obtained.