Nuclear fusion is considered to be a clean and sustainable source for energy production for the future. A tokamak is a promising device for achieving nuclear fusion. In a tokamak, toroidal and poloidal magnetic fields are created by external coils and a plasma current, which confines hot fusion plasmas. The high-confinement mode (H-mode), which is characterized by a particle and energy transport barrier in the plasma edge, is the baseline operation scenario of future tokamaks due to its high plasma temperature and energy confinement. However, the H-mode has its characteristic magnetohydrodynamic (MHD) instabilities called edge localized modes (ELMs) due to the high-pressure gradient at the edge transport barrier. ELM periodically ejects large heat and particle fluxes toward the inner wall of a tokamak and can cause significant damage to plasma facing components (PFCs). For ITER, the heat flux by large ELMs is expected to be above the tolerable range of the tungsten divertor. Therefore, various methods for mitigating or completely suppressing large ELMs have been widely studied in many tokamaks. Recently, it has been reported that divertor gas impurity seeding can significantly mitigate or suppress ELMs. Divertor gas impurity seeding is considered to be the primary technique for the mitigation of excessive heat loads on PFCs.
In this study, the results of ELM mitigation and suppression by krypton (Kr) gas seeding are presented. Kr gas was seeded with various levels into KSTAR H-mode plasma. For mid-level Kr seeding, ELM was briefly suppressed, and the ELM mitigation phase followed, yet there was no change for low-level Kr seeding. For high-level Kr seeding, the shorter ELM suppression phase appeared, which was followed by H-L back transition. Using the two-dimensional (2D) radiation images reconstructed from an infrared imaging video bolometer, the distributions of Kr ion density were calculated, and core Kr concentrations in ELM mitigation and suppression phases were also presented. The changes in edge instability growth rate and eigenmode structures during Kr seeding were analyzed using the edge MHD stability code. For the ELM mitigation case, there was no significant change in the edge stability diagram, which suggests that ELM mitigation is due to changes in pedestal shape rather than the ELM type. For a high Kr level, the heating power was reduced by the excessively high center emission light, and the H-L back transition occurred. On the other hand, the ELM mitigation phase was sustained until the end of discharge after Kr outflow in the mid-level seeding shot, which shows there is an appropriate amount of Kr to significantly mitigate ELM while maintaining plasma performance. High-level Kr seeding with on-axis electron cyclotron heating (ECH) shows an extended ELM suppression phase while plasma performances are maintained at a similar level. This suggests that the total Kr amount and Kr distribution both affect ELM suppression.
In addition to ensuring ELM mitigation and suppression, it was also observed in KSTAR that Kr seeding can create an internal transport barrier (ITB). ITB is characterized by an energy transport barrier inside the plasma bulk rather than the edge pedestal. ITB is regarded as one of the advanced scenarios for future fusion devices, as it can greatly enhance core plasma temperature and, consequently, the fusion reaction rates. However, its formation and maintenance conditions still remain unclear. Therefore, Kr-induced ITB formation in this study is encouraging results, in that impurity injection can be another recipe for ITB formation and core confinement enhancement. In this study, strong core peaking of toroidal rotation, ion, and electron temperature was observed after Kr seeding. Ion and electron heat diffusivity profiles after Kr seeding suggest the suppression of a turbulent energy transport. Using Kr density profiles calculated from radiation, Kr behavior during ITB formation is also discussed. For understanding the role of Kr in ITB formation, further studies including gyrokinetic simulation and Kr transport analysis are planned.