Description
The high confinement mode (H-mode) is foreseen for deuterium-tritium (DT) stationary operation in tokamak fusion reactors. A major challenge is the low-to-high confinement (L–H) transition, which is not fully understood and is difficult to predict. Recent DT experiments in JET-ILW (ITER-Like Wall) have provided new ITER-relevant observations.
In previously dedicated JET-ILW experiments$^1$, the heating power required to reach H-mode ($P_{LH}$) was found to be proportional to the effective conductivity $\chi_{eff}$ of the plasma and inversely proportional to the effective mass $A_{eff}$ of the hydrogenic species used. Experiments were conducted at fixed magnetic configuration ($B_{tor}^{axis}=1.91 T$, $1.65 MA$, $q_{95}=3.65$) with a favourable lower single-null geometry for H-mode access.
These experiments showed that a pure deuterium plasma requires less power to reach H-mode than a 50% hydrogen + 50% tritium plasma ($P_{LH}^D=1.68 MW<P_{LH}^{H+T}=2.98 MW$), despite having a similar effective mass $A_{eff}=2$. A similar observation is made at $A_{eff}=2.5$ where a 25% hydrogen + 75% tritium plasma requires more power than a 50% deuterium + 50% tritium plasma to reach H-mode ($P_{LH}^{D+T}=1.66 MW<P_{LH}^{H+T}=2.42 MW$).
Via high-fidelity local gyrokinetic simulations (GENE$^{2}$) at $\rho_{tor}=0.95$ using JET-ILW experimental conditions$^1$, we observed similar heat flux levels at the same effective mass, close to experimental levels (2-3 MW)$^3$. Turbulence is found to be of electron drift-wave nature, regardless of the isotope species.
Including a non-negligible radial electric field shear allows different heat flux levels at the same effective mass between isotopic mixtures (H+T, D+T) and singular isotopes$^3$ (D and synthetic $A_{eff}=2.5$). The heat flux in simulations is higher for the H+T cases compared to their respective effective mass equivalents of either singular isotopes or D+T mixture. This difference increases with the shear amplitude.
This difference correlates with a different response of the zonal flow energy, higher in the D+T and singular isotope cases than in the corresponding H+T cases. This suggests stronger zonal-flow–turbulence coupling in certain isotopic configurations, favouring D+T operation.
$[1]$ G. Birkenmeier et al., Nuclear Fusion, 2022
$[2]$ F. Jenko et al., Physics of plasmas, 2000
$[3]$ G. Lo-Cascio et al., Nuclear Fusion, 2025