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 = 4.33.

What

Empirical ceiling on normalized beta β_N

Kronos operating β_N

4.33 (Mode C)

Not-to-exceed

β_N ≤ 4.5 clamp

Wall-stabilized

β_N = 5.0 upside (S22/S25)

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 design point sits at β_N = 4.33, with the control system enforcing β_N ≤ 4.5 as a not-to-exceed clamp.

The design's forward-looking studies show that with active resistive-wall-mode stabilization the achievable limit rises toward β_N = 5.0 — which the robustness analysis (S79) translates into roughly +24% fusion power at unchanged confinement. Troyon stays the parameter to watch: it is where the extra performance is found, and where the plasma would first become unstable.