Investigation of the effects of krypton seeding under various experimental conditions in KSTAR H-mode plasmas

Controlled krypton gas seeding experiments under various experimental conditions were performed in KSTAR H-mode discharges. ELM mitigation/suppression, divertor heat flux reduction, and ion internal transport barrier (ITB) formation were observed with increasing krypton levels. ELM was mitigated and suppressed in an intermediate level of krypton seeding (c_{Kr, local maximum} ~0.1%). When the krypton amount was increased to a high level (c_{Kr, local maximum} ~0.25%), ELM was suppressed and H-L back transition occurred. In an even higher level of krypton seeding (c_{Kr, local maximum} > ~0.5%), an ion ITB discharge with T_i,core > 10 keV and a significant reduction in particle flux onto both inner and outer divertor targets were achieved. To investigate the effects of krypton on the core plasma, linear gyrokinetic simulations for the ion ITB case have been conducted using the GKW code. The maximum growth rate (γ_lin,max) of instability was significantly reduced in the region around the peak of krypton concentration n_kr/n_e. To further explore the effect of krypton seeding on heat flux reduction, SOLPS-ITER modeling of divertor plasmas was conducted. As the concentration of krypton increased, radiated power fraction of the core region increased while that of the divertor region decreased.