MetroVolt drives 42.5 MA of plasma current, among the largest ever specified. What keeps that ambition honest is a self-imposed floor: the edge safety factor q95 stays at or above 5, verified by free-boundary equilibrium, not estimated from formulas.
The safety factor counts how many times a field line winds the long way around for each short-way turn; low q95 invites the kink instabilities and disruptions that end discharges violently. Formula estimates flatter shaped plasmas, so MetroVolt computes q95 from the actual FreeGS equilibrium at δ = −0.30: 4.94 at the 43 MA benchmark, with the operating point set at 42.5 MA to respect the ≥5.0 floor.
The robustness study (S79) then asks the impolite question: what if current falls short? Answer, published: a 10% Ip shortfall is not recoverable by density trim — plasma current is the least forgiving parameter in the design. That finding shapes the control philosophy and the disruption engineering budget.
A disruption at 42.5 MA stores real energy (10.3 GJ; ~590 MN vertical load — quantified in the deposit), so the q95 discipline is the difference between a power plant and a research risk. Buyers of firm power are really buying operating margin; ours is computed, floored, and stress-tested against its own worst parameter.
| Plasma current Ip | 42.5 MA |
| Safety-factor floor | q95(eq) ≥ 5.0 |
| Benchmark | q95 4.94 at 43 MA (FreeGS) |
| Least forgiving parameter | Ip (S79, adverse, published) |
| Disruption ledger | 10.3 GJ · ~590 MN (mitigated by SPI) |