Measurements of n=1 MHD stability in high beta discharges using multi-modal active MHD spectroscopy*

The multi-modal active MHD spectroscopy [1] is applied for the first time in high-performance plasmas to study their stability time evolution, revealing an intriguing dependence on both q_min and β_N. A tailored applied 3D field provides a 3D plasma response to extract, both in real-time and offline, the complex growth rate of the least stable mode. The measured growth rate by the MHD spectroscopy shows its reliability by exhibiting a correlation with the programmed rises in plasma beta for multiple DIII-D discharges and the appearance of instabilities as measured by the magnetic sensors. Furthermore, the estimated growth rate finds a decrease in stability when the minimum in the safety factor (q) passes through 2.0, despite nearly constant β_N at this time. The stability improves as the minimum q continues to decrease below 2.0. The MHD spectroscopy thus reveals the danger of instability at integer q_min even above the usual q=1 sawtooth condition. This has important implications for the optimal high-q_min profiles for high beta scenario developments. Additional stability analysis and measured imaginary part of the growth rates will be compared with this experiment to understand the underlying physics of these evolutions, which will help identify an effective pathway to stable, high β_N scenarios.

[1] T. Liu et al., NF61, 056009