Lijuan Liu, Yuming Wang, Quanhao Zhang, Jingnan Guo, Yutian Chi
In May 2024, NOAA active region (AR) complex 13664/8 appeared as one of the most productive regions of the current solar cycle. It produced 12 X-class flares and over 20 coronal mass ejections (CMEs), triggering the strongest geomagnetic storm since 2003. We investigated why the AR complex is so productive, particularly in CMEs. Primarily using observations from the Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager aboard the Solar Dynamics Observatory, we analyzed the photospheric magnetic evolution, eruption sources, and eruption waiting times and compared the magnetic parameters with five other ARs. The region initially comprised only AR 13664 and exhibited limited flare activity until AR 13668 emerged on May 4, after which clustered major flares and CMEs occurred. Rapid, complex flux emergence substantially increased the region's area, magnetic flux, complexity, and non-potentiality. A comparison with five other ARs suggests that both flare-rich and CME-rich ARs exhibit elevated overall non-potentiality, as indicated by their high total magnetic free energy density and total current helicity, while CME-rich ARs exhibit particularly large mean current helicity, possibly implying stronger localized non-potentiality. The increased complexity manifests through at least 12 emerging bipoles and six collisional polarity inversion lines (cPILs) formed between nonconjugated polarities. All six cPILs exhibit sustained collision and shearing, serving as eruption sources. Decay index distributions show systematically lower critical heights ($<45$ Mm) for a torus instability above the CME sources. The CME waiting time distribution exhibits two peaks, suggesting that multiple cPILs enhance CME productivity by increasing both recurrent CMEs from the same location and disturbance-triggered CMEs from nearby locations. The results highlight that in addition to sufficient non-potentiality and rapid background field decay, a high degree of magnetic complexity accompanied by dynamical collisional shearing at multiple cPILs is crucial for the region's extreme CME productivity.