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.