Exploring the fusion power plant design space: comparative analysis of positive and negative triangularity tokamaks through optimization

The optimal configuration choice between positive triangularity (PT) and negative triangularity (NT) tokamaks for fusion power plants hinges on navigating different operational constraints rather than achieving specific plasma performance metrics. This study presents a systematic comparison using constrained multi-objective optimization with the integrated FUsion Synthesis Engine (FUSE) framework. Over 200,000 integrated design evaluations were performed exploring the trade-offs between capital cost minimization and operational reliability (maximizing $q_{95}$) while satisfying engineering constraints including 250 $\pm$ 50 MW net electric power, tritium breeding ratio $>$1.1, power exhaust limits and an hour flattop time. Both configurations achieve similar cost-performance Pareto fronts through contrasting design philosophies. PT, while demonstrating resilience to pedestal degradation (compensating for up to 40% reduction), are constrained to larger machines ($R_0$ $>$ 6.5 m) by the narrow operational window between L-H threshold requirements and the research-established power exhaust limit ($P_{sol}/R$ $<$ 15 MW/m). This forces optimization through comparatively reduced magnetic field ($\sim$8T). NT configurations exploit their freedom from these constraints to access compact, high-field designs ($R_0 \sim 5.5$ m, $B_0$ $>$ 12 T), creating natural synergy with advancing HTS technology. Sensitivity analyses reveal that PT’s economic viability depends critically on uncertainties in L-H threshold scaling and power handling limits. Notably, a 50% variation in either could eliminate viable designs or enable access to the compact design space. These results suggest configuration selection should be risk-informed: PT offers the lowest-cost path when operational constraints can be confidently predicted, while NT is robust to large variations in constraints and physics uncertainties.

💡 Research Summary

This paper presents a comprehensive, integrated design‑space exploration of positive‑triangularity (PT) and negative‑triangularity (NT) tokamak concepts for future fusion power plants, using the Fusion Synthesis Engine (FUSE) as a unified multi‑objective optimization platform. Over 200,000 whole‑plant configurations were evaluated, simultaneously minimizing capital cost and maximizing operational reliability (expressed as the edge safety factor q₉₅), while enforcing a suite of engineering constraints: net electric output of 250 ± 50 MW, tritium breeding ratio (TBR) > 1.1, power‑exhaust limits (SOL‑R < 15 MW m⁻¹), a one‑hour flat‑top, and a minimum L‑H transition power for PT designs.

FUSE couples plasma physics (core transport, edge pedestal, neutron transport), engineering subsystems (magnet stress, structural build, cooling), and economic models into a single data flow. For PT plasmas, the well‑established EPED model predicts a steep H‑mode pedestal, linking the L‑H power threshold to pedestal stability. NT plasmas, lacking a strong pedestal, are modeled with a newly developed Weak‑Pedestal (WPED) model. WPED is data‑driven: it enforces a constant edge‑to‑core thermal‑energy ratio (c_WPED ≈ 0.30) observed across >300 DIII‑D discharges, and constructs smooth exponential temperature and density profiles that satisfy energy conservation and gradient continuity. The WPED inner‑loop optimizes exponential decay parameters, while an outer loop adjusts the edge temperature to meet the prescribed energy ratio.

Design variables include major radius R₀, on‑axis magnetic field B₀, plasma current, heating and current‑drive powers, blanket thickness, and coil material (copper vs. high‑temperature superconductor). Constraints enforce power balance (≥ 200 MW), TBR > 1.1, SOL‑R limit, L‑H power threshold (for PT), and q₉₅ ≥ 3. A genetic algorithm explores the high‑dimensional space, generating Pareto fronts for each configuration type.

Key findings:

1. Pareto similarity but divergent design pathways – Both PT and NT achieve comparable cost‑performance trade‑offs, yet the optimal design philosophies differ markedly.
2. PT designs are forced into a large‑radius, low‑field regime (R₀ > 6.5 m, B₀ ≈ 8 T) because the allowable window between the L‑H power threshold and the SOL‑R limit is narrow. PT configurations remain viable even if pedestal pressure degrades by up to 40 %, but they rely heavily on accurate predictions of L‑H scaling.
3. NT designs exploit the absence of an L‑H constraint and a relaxed SOL‑R limit, enabling compact, high‑field machines (R₀ ≈ 5.5 m, B₀ > 12 T). This aligns naturally with emerging HTS magnet technology and provides intrinsic ELM‑free operation and superior power‑handling.
4. Sensitivity analysis shows that a ±50 % variation in either the L‑H power threshold or the SOL‑R limit can either eliminate all feasible PT designs or open a pathway to the compact NT‑like region. NT designs are far less sensitive to these variations, indicating greater robustness to physics and engineering uncertainties.
5. Risk‑informed selection – PT offers the lowest capital cost when operational constraints are well‑known and predictable; NT offers a more resilient, albeit slightly higher‑cost, alternative when uncertainties dominate.

The study demonstrates that integrated, multi‑objective optimization is essential for realistic tokamak plant design, as it captures the complex interplay between plasma physics, engineering limits, and economics. It also highlights that NT plasmas, despite being less explored experimentally, can provide competitive economic performance when paired with high‑field HTS magnets. Future work should incorporate vertical stability control, detailed HTS coil design, and broader operational scenarios to further refine the design space and validate the WPED model against next‑generation experiments.