The Greenwald Density Limit
The Greenwald limit is an empirical ceiling on tokamak plasma density. Negative-triangularity operation lets Kronos run above the conventional fraction.
- Definition
- n_GW = I_p / (π a²)
- MetroVolt Greenwald fraction
- f_GW = 1.16 (1.33 in Mode B)
- Density
- n̄_e = 1.9 × 10²⁰ m⁻³
- Anchor
- non-disruptive NT operation to 1.8 × n_GW
The Greenwald density limit is an empirical rule for how much plasma density a tokamak can hold before disruptions become likely. It scales with plasma current and inversely with the square of the minor radius: nGW = Ip / (π a²). Fusion power rises with the square of density, so operating close to — or above — this limit is valuable, but risky in conventional plasmas.
Kronos MetroVolt runs at a Greenwald fraction of fGW = 1.16 (line-averaged density n̄e = 1.9 × 10²⁰ m⁻³), i.e. slightly above the nominal limit. This is anchored on the demonstrated record that negative-triangularity plasmas sustain non-disruptive operation up to 1.8× the Greenwald density — headroom that a positive-triangularity H-mode plasma does not reliably have.
Running with density headroom rather than at the ragged edge of a disruption is part of what makes the negative-triangularity choice a stability decision as much as a confinement one.