Technical Issues With Steady-State Operation in KSTAR

Abstract

Achieving and sustaining long pulse and ultimately steady-state operation of a fusion reactor in terms of physics and engineering parameters beyond transient period is essentially one of key requirements in present non Deuterium Tritium (DT) discharge tokamaks. Since the successful long pulse operation for 25 s at the plasma current of 0.5 MA in 2013, the duration of the H-mode pulse length has been extended to over 70 s, which corresponds to a few multiples of the current diffusion time. In addition to long pulse operation, the plasma performance is enhanced further to high poloidal beta discharge. This characterizes a fully noninductive discharge longer than 12 s, and also achieves both ion and electron temperatures of over 5 keV simultaneously for a line-integrated electron density of $5 {\times} 10^{19}$ /m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and the plasma current of 0.8 MA. Recently, KSTAR reported the long pulse operation beyond 1 min at the injected power of about 5 MW and the plasma current of ~0.5 MA. The normalized beta was about 1.5 and the total injected energy to the plasma reached to about 300 MJ. It is also shown that the bootstrap current fraction is about 40% so that sufficient conditions are met for 100-s operation in terms of physics requirements of fully noninductive state. The plasma-facing components overheating issues due to prompt ion loss in the low plasma current operation was overcome by enlarging the outer gap between the plasma and outer poloidal limiter. An advanced tokamak scenario via profile control would be achieved with the new 6-MW neutral beam injection-2, and it is expected that 100-s operation can be possible in a fully noninductive manner at KSTAR. However, a reliable and innovative steady-state heating and current drive source is strongly recommended for achieving the final goal of the KSTAR.