Description
The empirical Greenwald scaling for the density limit (DL) implies an unfavorable dependence of the maximum achievable density on plasma size, which is a tokamak-specific major constraint on the operational point of a reactor. However, recent evidence [1] points to a dependence of the DL on power, with different theoretical DL models [2,3] agreeing on a scaling as $P_\mathrm{sep}^{0.3-0.5}$ but not on the physics behind it, nor on the additional dependence on field, size and current beyond the Greenwald scaling. Still, a power-dependent DL would have profound implications for a reactor, since $P_\mathrm{sep} \sim$ 100 MW could allow operation at much larger densities than suggested by the Greenwald scaling.
A variety of power-dependent theoretical density limit models are evaluated against an existing database [4] of ASDEX Upgrade (AUG) plasmas and dedicated experiments in TCV with different heating powers. Emphasis is placed on the main dependences of the deviation of the DL from the Greenwald scaling on engineering parameters (power, current, field, size), aiming to discriminate between predictions from theory. The potential benefits of operating a reactor at high densities depend critically on how the confinement scales with density [5], which is investigated for this set of plasmas both experimentally and with state-of-the-art core-edge integrated workflows [6-8]. A selected AUG high-density plasma is modelled with the recently-developed SICAS workflow [6], which couples ASTRA to SOLPS-ITER and includes realistic neutrals and atomic physics, allowing a more complete characterization of the plasma state as the DL is approached. The implications for a reactor are discussed.
[1] P. Manz et al 2023 NF 63 076026
[2] M. Giacomin et al 2022 PRL 128 185003
[3] P. Zanca et al 2019 NF 59 126011
[4] B. Sieglin et al 2024 PPCF 66 025004
[5] C. Angioni et al 2026 NF 66 026006
[6] A. Welsh et al 2025 NF 65 044002
[7] D. Fajardo et al 2024 NF 64 046021
[8] T. Luda et al 2021 NF 61 126048