Physical mechanism of the tokamak density limit · revision history

Revision 2

Accepted · core change · record

Changes against revision 1:

Title
Physical mechanism of the tokamak density limit
Plain
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 · changed · core
Revision 1: 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.
Revision 2: Discharges disrupt or return to L-mode when line-averaged density approaches the Greenwald density $n_G = 10^{14}\,\mathrm{cm}^{-3} x (I_p / 1\,\mathrm{MA}) / (\pi (a / 100\,\mathrm{cm})^2)$, 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.
Settled by
A criterion that predicts the density limit, including its power and wall dependence, across at least three tokamaks.
Kind
phenomenon
Answer type
mechanism
References
(empty)
Literature status
open
Status 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).
Posed since
1988

Decisions