Leonard-Petru Turica

This study makes use of plasma-profile data from the EUROfusion pedestal database (Frassinetti et al. 2020 Nucl. Fusion vol. 61, p. 016001), focusing on the electron-temperature and electron-density profiles in the edge region of H-mode ELMy JET ITER-Like-Wall (ILW) pulses. We make systematic predictions of the electron-temperature pedestal, taking engineering parameters of the plasma pulses and the density profiles as inputs. We first present a machine-learning (ML) algorithm which, given more inputs than theory-based modelling, is able to reconstruct unseen temperature profiles within 20 % of the experimental values. We find a hierarchy of the most consequential engineering parameters for such predictions. This result confirms the conceptual possibility of accurate data-driven prediction. Next, taking a simple theoretical approach that assumes a definite local relationship between the electron-density (𝑅/𝐿𝑛𝑒) and electron-temperature (𝑅/𝐿𝑇𝑒) gradients, we find that a range of power-law scalings 𝑅/𝐿𝑇𝑒 =𝐴⁢(𝑅/𝐿𝑛𝑒)𝛼 with 𝛼 ≈0.4 correctly capture the behaviour of the electron-temperature in the steep-gradient region. Fitting 𝐴 and 𝛼 independently for each pedestal reveals a clear one-to-one correlation, suggesting an underlying constraint in pedestal physics. The measured 𝜂𝑒 =𝐿𝑛𝑒/𝐿𝑇𝑒 values across the pedestal exhibit a wide distribution, significantly exceeding the slab-ETG linear stability threshold, implying either a non-linear threshold shift or a measurably supercritical saturated turbulent state. Finally, we fit parameters for scalings that relate the turbulent heat flux to the gradients 𝑅/𝐿𝑇𝑒 and 𝑅/𝐿𝑛𝑒, similarly to models extracted from gyrokinetic simulations. The inclusion of more experimental parameters is necessary for such models to match the accuracy of our ML results.

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,𝑡⁢𝑜⁢𝑝
𝑁) (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,𝑝⁢𝑒⁢𝑑, 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,𝑠⁢𝑒⁢𝑝/𝑛𝑒,𝑝⁢𝑒⁢𝑑, 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.