Plasma Position Control and its Complexity: A Conceptual Systems Engineering Approach
As nuclear fusion advances from experimental science to commercial power generation, the complexity of managing plasma behavior in tokamaks necessitates a shift from algorithm-centric control to systems-centric thinking. In this paper, we reframe plasma vertical and radial position control as a structural and behavioral coordination problem embedded within a broader network of sensing, estimation, actuation, and constraint management. Rather than proposing new control laws, we adopt a model-based systems engineering (MBSE) perspective to articulate the architectural dependencies, functional roles, and interface assumptions that shape control capability. We present a subsystem-level decomposition of the control loop, developed using Capella and grounded in mission-level objectives, functional hierarchies, and real-time feedback constraints. This conceptual framework reveals how architectural mismatches—often buried in legacy design practices—can hinder scalability, maintainability, and operational resilience. Our goal is to elevate control design from reactive tuning to proactive architectural reasoning, supporting integration, fault tolerance, and sustainability in next-generation fusion plants. The methodology is particularly relevant for early-phase design programs such as STEP, where structural clarity and traceability are critical enablers of long-term reliability.