Multi-mode plasma response and experimental validation*

A multi-mode plasma response model, extended by including rotating three-dimension (3D) field perturbations, has been developed, which provides significant insight into the damping rates and spatial structures of magnetohydrodynamic (MHD) modes in stable fusion plasmas. The enhanced model is validated with dedicated experiments in DIII-D and EAST tokamaks, where, for the first time, the comprehensive capabilities of 3D actuators (magnetic coils) and sensors allow high-fidelity construction of multi-pole plasma transfer functions for stable plasmas, based on simultaneous scans of both the frequency and spatial structure of the applied 3D field. Comparison of MARS-F n=1 multi-mode response modeling with the extracted multiple eigenmodes in DIII-D experiments indicates the importance of the conducting wall for AC response of the plasma, as the rotation frequency of 3D fields exceeds 10 Hz. Based on measured data, reconstruction of multi-mode plasma response model, for the n=2 mode locking experiments in DIII-D L-mode plasmas, appears to reveal a strong correlation between the secondary stable mode and the field penetration threshold. These encouraging results thus demonstrate that this enhancement of the MHD spectroscopy approach not only helps to improve the physics understanding of general stable eigenmodes, but can also offer great potential for understanding and controlling of, e.g. edge localized modes by resonant magnetic perturbations, or for real-time monitoring of the plasma stability, with the latter serving as a key element of an integrated approach for disruption prediction and avoidance in future reactors such as ITER.