Fusion reactor heat model works—until it doesn't, JET study finds
Scientists testing predictions for how heat moves in fusion reactors found their model nails one measurement but misses another by a factor of two. The gap suggests different physics rules apply in different regions of the reactor, a finding that could reshape how engineers design the next generation of fusion plants targeting commercial power generation.
Originaltitel: Comparison of turbulent electron heat flux model predictions of the H-mode electron pedestal temperature profile across isotope mix and gas fuelling rate scans in JET with the Be/W wall
<p>Predictions of the pedestal temperature profile calculated using a model for electron-temperature-gradient (ETG) turbulent electron heat transport Field et al. (2023 Philos. Trans. R. Soc. A, vol. 381, p. 20210228) are compared with the pedestal structure of H-mode plasmas in JET-Be/W (with Be wall and W divertor) over scans of the deuterium–tritium (D:T) isotope mix and hydrogenic gas fuelling rate Frassinetti et al. (2023 Nucl. Fusion, vol. 63, p. 112009). Predictions for the electron temperature at the location of the density pedestal top Te(ψne,topN ) (where ψN, is the normalised poloidal flux) are found to agree well with measured values over both scans across the full range of D:T ratio. However, the pedestal top temperature Te,ped, typically located somewhat inside the density pedestal top, is under-predicted by as much as a factor ∼2. This implies that the ETG heat flux scaling appropriate for the steep-density gradient region, on which the model is based, is not applicable where the density gradient is weak. This difference might be attributed to a difference between the physics of the ETG turbulence in regimes where the density gradient is either strong or weak, which are thought to be dominated by either the ‘slab’ or ‘toroidal’ branches of ETG turbulence. Other branches of turbulence might also play a role in the electron heat transport, particularly in regions of weak-density gradient. As in the experiment, the predicted Te across the pedestal decreases with the ratio of separatrix to pedestal density ne,sep/ne, ped , which increases with the gas fuelling rate. Results from three models combining the ETG heat flux model with the EPED1 pedestal (EPED) model (Snyder et al., Phys. Plasmas, 2009, vol. 16, p. 056118) are also presented, including one which also incorporates the density pedestal prediction mode of Saarelma et al. (Nucl. Fusion, 2023, vol. 63, p. 052002), this model providing a complete prediction of the pedestal profiles.</p>