The Troyon Beta Limit
The Troyon limit caps normalized beta before instability. It is the binding constraint on the Kronos design point at β_N = 4.96.
- What
- Empirical ceiling on normalized beta β_N
- Kronos operating β_N
- 4.96 (Mode D, wall-stabilized)
- Computed limit
- β_N ≤ 5.0 (binding)
- Mode-C-era point
- 4.33 under a 4.5 clamp (labeled history)
The Troyon limit is the empirical ceiling on how much normalized beta (β_N) a tokamak can hold before pressure-driven instabilities set in. It is one of the handful of hard limits that bound any reactor design, and for Kronos MetroVolt it is the binding constraint: the ratified Mode-D point sits at β_N = 4.96 against the computed wall-stabilized limit of 5.0, which the control system enforces as the not-to-exceed clamp (the Mode-C-era 4.33/4.5 posture is labeled history).
With active resistive-wall-mode stabilization the achievable limit rises to the wall-stabilized β_N = 5.0 — an envelope the robustness analysis (S79) priced and the Mode-D re-basis then adopted as the operating regime. Troyon stays the parameter to watch: the operating point sits 1% under the computed limit, and it is where the plasma would first become unstable.