Theoretical astrophysics and plasma physics at RPC

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.