ELM Suppression and Formation of Internal Transport Barrier by Krypton Gas Seeding in KSTAR Plasmas*

ELMs were successfully mitigated and suppressed by krypton (Kr) seeding in the KSTAR divertor. After Kr seeding with 1.7x10¹⁹ particles, ELM was mitigated with reduction in electron density and temperature, and stored energy. When Kr amount was increased (~3.5x10¹⁹ particles), ELM was suppressed and H-L back transition occurred. Energy confinement time remained almost unchanged during both Kr seeding cases while normalized beta decreased. A peeling-ballooning stability analysis showed that the decrease of pedestal plasma pressure gradient due to Kr plays a dominant role in ELM mitigation or suppression. Tangentially-reconstructed 2D radiation images suggest that pedestal pressure was reduced by radiative cooling and heating power loss by Kr. At a higher level of Kr seeding (~5.0x10¹⁹ particles), an internal transport barrier (ITB) was formed. At the plasma core (0 < r/a < 0.4), electron and ion temperatures and toroidal rotation increased distinctly. TRANSP calculation showed that both ion and electron heat diffusivities decreased significantly inside the ITB.