Initial Results from Neoclassical Tearing Mode Stabilization Experiment in KSTAR High Normalized Beta Plasmas

Active stabilization of neoclassical tearing modes (NTMs) is critical for high beta plasmaoperation in the KSTAR tokamak. In recent device operation, an experiment was conducted todevelop am/n=2/1 NTM stabilization in high normalized beta (beta N) plasmas having beta(N )> 3 byusing electron cyclotron current drive (ECCD). The experiment is designed to first demonstratethe direct mode stabilization effect of the device EC system by varying the ECCD depositionlocation around the mode rational surface to prepare for future NTM stabilization in KSTARusing feedback schemes. In the experiment, the toroidal magnetic field strength,BT, is reducedto 1.5-1.6 T to create a highly localized ECCD profile on a large island width expected toproduce a high stabilization effect. To align the ECCD with the mode,BTis varied betweendischarges by keeping the EC wave injection angles fixed to move the current depositionlocation across theq=2 mode rational surface in similar to 1 cm steps along the plasma midplane. Theresult shows a prompt reduction of the mode amplitude by similar to 60% with a partial recovery of theloss of stored energy when the ECCD with 0.7 MW power is applied promptly after the modeonset atBT=1.54 T. This abrupt reduction of the mode amplitude is significantly weaker ordisappears in other discharges having slightly differentBTin which the ECCD is deposited a fewcentimeters away. This result indicates a direct interaction between the driven EC current andthe radially localized island structure first observed in the device. The EC ray-tracing analysisusing the TORAY code shows that the applied EC-driven current aligns with the mode rationalsurface when the mode amplitude is observed to decrease. The stability of the observed 2/1NTM is examined by constructing the modified Rutherford equation (MRE). The calculated ECpower requirement for complete mode stabilization by assuming perfect alignment of ECCD onthe mode is 0.8-1.7 MW by considering the uncertainties in the equation. This MRE-computedmode stability agrees with the experiment. The result projects that the present KSTAR ECsystem can stabilize the 2/1 NTM disrupting high beta Nplasmas, and provides the requirement ofthe mode-ECCD alignment for complete mode stabilization in future experiments in which anaccurate alignment is planned to be made by feedback schemes being developed.