Automated calculation of time-evolving magnetic island widths in DECAF and implicit analysis of neoclassical tearing mode stability

Neoclassical tearing modes (NTMs) in tokamaks involve the growth of magnetic islands, which limits performance and often leads to disruptions. Accurate identification of these modes and time-evolved measurement of the widths of the islands is critical for understanding the physics of NTMs and their relation to disruption event chains. DECAF [1] can now automatically detect and calculate the width of magnetic islands over time using electron cyclotron emission (ECE) data. We plan to enhance this capability with equilibrium reconstructions and other diagnostics, including magnetics, ECE imaging (ECEI), and Thomson scattering. These widths can then be used to guide an implicit analysis of the modified Rutherford equation (MRE), the governing equation for island width evolution, by constraining the possible values of parameters in the MRE and enabling the evaluation of partial derivatives of the MRE with respect to adjustable parameters like safety factor gradient scale length, pressure gradient scale length, and poloidal plasma beta. These dependences could inform NTM control algorithms by dictating the direction in parameter space that is expected to lead to stabilization, and they may enable the discovery of high-performance regions in state space that are stable to large seed islands.

[1] S.A. Sabbagh, et al., Phys. Plasmas 30, 032506 (2023). https://doi.org/10.1063/5.0133825