Physical mechanism of the tokamak density limit
In plain words
Tokamaks usually disrupt or lose good confinement when the fuel density passes an empirical ceiling set by the plasma current, the Greenwald limit. Fusion power grows with density squared, so knowing what really sets this ceiling matters for reactors.
Precise statement
Discharges disrupt or return to L-mode when line-averaged density approaches the Greenwald density $n_G = 1e14\,\mathrm{cm}^{-3} x (I_p / 1\,\mathrm{MA}) / (\pi (a / 100\,\mathrm{cm})^2)$, with $I_p$ the plasma current and a the minor radius. Identify the mechanism (edge turbulence transition at critical collisionality or adiabaticity, radiative collapse and MARFE formation, tearing-island growth, plasma-wall self-organization) and derive whether the maximum density scales with heating power and wall conditions. An answer is a predictive criterion validated on several devices, including cases with $n/n_G$ above 1.
What would settle it
A criterion that predicts the density limit, including its power and wall dependence, across at least three tokamaks.
Status in the literature
Unverified note
In 2025 DIII-D negative-triangularity plasmas reached Greenwald fractions near 2 with power-dependent limits (arXiv:2503.10854), and EAST reached $1.3\ \text{to}\ 1.65\,n_G$ with ECRH-assisted start-up (arXiv:2505.02710).