“Helios” is Thea Energy, Inc.’s quasi-axisymmetric (“QA”) stellarator fusion power plant designed for steady-state, disruption-free power generation. This paper presents the preconceptual design of the Control, Diagnostics, and Instrumentation stack that enables safe, reliable, and efficient operation. Helios adopts a plant-wide control architecture built around two major systems. First, a two-level hierarchical Plant Control System supervises all operational functions: at the top level, the Main Control Unit (“MCU”) serves as the master plant controller and global state manager, hosting the Plasma Control System (“PCS”) and orchestrating plant-wide sequences, supervisory control actions, and data aggregation; at the lower level, dedicated plant subsystem Instrumentation and Control (“I&C”) units provide localized automation for magnets, heating, fueling, cryogenics, and balance-of-plant. Second, a functionally independent Safety Control System (“SCS”) implements interlocks, access control, and protection functions. Diagnostics and machine instrumentation form the essential sensing spine for this architecture, driving protection, control, and performance accounting. Key plasma diagnostics include magnetic measurements for boundary reconstruction, polarimetric-interferometric systems for high-speed density signals, and Electron Cyclotron Emission (“ECE”) for thermal supervision and trip limits. Beyond plasma diagnostics, a comprehensive plant-wide instrumentation system monitors thermal, flow, radiation, and structural channels. Notably, this includes embedded fiber Bragg gratings for high-speed quench detection in the superconducting coils. The design described is preconceptual: it establishes architectural decisions, functional allocations, and preliminary performance targets. An upcoming conceptual design phase will expand on functional and performance requirements, hardware and software interfaces, introduce hardware-in-the-loop testing, consolidate diagnostic sight lines and error budgets, and prototype front ends for harsh environments. • Two-level Plant Control: MCU supervises dedicated subsystem controllers. • Plasma Control: Classical and AI/ML in multi-rate CPU/GPU real-time loops. • Independent Safety System: SIL PLCs manage interlocks, access, and protection. • Fast FPGA and slow PLCs manage magnets, heating, fueling, cryoplant, and BoP. • Integrated diagnostics provide inputs for control, protection, and monitoring.
Slepchenkov et al. (2026) studied this question.