Recent progress on developing a W-mitigated H-mode scenario in KSTAR

The recent upgrade of the tungsten (W) divertor in KSTAR introduces challenges to achieving high-performance H-mode due to core radiation loss by core accumulation of high-Z impurity. To address this, we developed a W-mitigated H-mode scenario for KSTAR, leveraging the previous reports [1-3] from other devices with metallic walls. The key elements include: (1) a fast beta ramp using neutral beam and electron cyclotron heating to achieve H-mode before significant W accumulation, (2) divertor gas puffing to reduce tungsten sputtering, and (3) modification of ELM characteristics to enhance W flushing [4]. These integrated strategies enabled a substantial performance recovery, lowering core radiated power fraction from f_rad >0.4 to 0.1 and improving energy confinement from H₉₈ ~0.75 to 1.0. Interestingly, a strong ion pedestal is shown to be beneficial for reducing core-impurity accumulation, suggesting the possible role of Ti pedestal screening [5]. The future work on further improving the H-mode scenario will also be discussed. [1] R. Neu et al., JNM 2013, 34 [2] A. Kallenbach NF 57, 102015 [3] F. Romanelli et al., NF 55, 104001 [4] R. Dux et al., JNM 2009, 858, [5] A. R. Field et al., NF 63, 016028

*This work is supported by the US DOE Contract DE-SC0020372, DEAC02-09CH11466, and Korean Ministry of Science and ICT under KFE and International R&D Programs (KFE-EN 2503-01).