Overview of initial experimental results from the KSTAR operation with tungsten divertor*

In KSTAR, the installation of a new lower tungsten diverter has been completed to enable high-performance plasma operation by the high heat load resistance of tungsten. The new divertor comprises 64 toroidal cassettes made of tungsten monoblocks. The first plasma operation aiming to verify the stable operation of the newly installed divertor and its impact on plasma performance has been conducted. Due to the new divertor geometry, the plasma equilibrium shaping is altered to place the divertor strike point into the designed target region covered by tungsten monoblocks. The plasma start-up scheme is modified by adjusting the pre-magnetization and subsequent feed-forward PF coil waveforms and gas injection. These adjustments successfully reproduced plasmas with key parameters, including ion temperature, normalized beta, and H-mode pulse duration, comparable to those from earlier experiments with a full carbon wall. The tungsten impurities from the divertor accumulate in the low-field side midplane region inside the plasma as measured by visible spectroscopy and bolometry. The plasma performance is largely affected by the higher radiation losses due to tungsten impurities and by the strike point locations. The radiation losses are mitigated by boronization, and nitrogen seeding moves the radiation front position toward the lower X-point. The results of the experimental studies conducted during the initial tungsten divertor operation will be reported.