Greenwald-Integrated Safe-Instability Envelope (SIE): A Density-Aware Control Update for Reactor-Scale Fusion Plasmas

This technical update extends the Safe-Instability Envelope (SIE) framework by integrating the Greenwald density limit as a first-class control variable. The envelope is conditioned on real-time density margin, enabling automatic tightening near density-driven disruption thresholds and relaxed, low-power operation during benign phases. The result is a unified control layer that simultaneously reduces parasitic control power, enables stable operation at higher plasma density, and improves hardware protection in ITER-, STEP-, and DEMO-class fusion reactors.