Speaker
Description
Avoiding tungsten (W) accumulation in the core plasma will be vital for successful operations of next generation tokamaks. For the development of high-performance scenarios, which target steady state operation, it is essential that core W transport and the plasma conditions required for impurity screening at the confined plasma periphery are understood. Spontaneous formation of internal transport barriers (ITBs) may also occur in burning plasma scenarios, motivating the need to understand ITB physics to maintain control of the barrier’s strength or prevent it entirely. Studies of core impurity transport in ITB plasmas in JET-C found evidence of core impurity accumulation due to neoclassical transport, which dominates inside the ITB where the turbulent transport is stabilised [1, 2]. Previous analysis of JET-ILW (Be/W wall) hybrid scenarios demonstrated peripheral tungsten screening in D and DT plasmas [3, 4]. This analysis builds upon previous work by investigating core W transport in ITB scenarios in JET-ILW.
Strong ITBs have been produced in low density $n_\mathrm{e}$ JET-ILW plasmas with high NBI heating that generates high core ion temperature $T_\mathrm{i}$, high toroidal Mach number $\Omega_\mathrm{\phi}$ and a peaked $n_\mathrm{e}$ profile [5]. The resulting steep $T_\mathrm{i}$ and $n_\mathrm{e}$ gradients have substantial effects on the neoclassical transport that dominates core W transport. Due to the sizeable concentrations of Be and Ni in these pulses, impurity-impurity collisions must be retained for accurate core W transport predictions. The drift-kinetic code NEO [6] used in this modelling captures this important physics, predicting core W accumulation after the formation of the strong ITB, which weakens outward convection of W. An integrated impurity analysis tool [7] enables interpretative analysis of the plasma impurity composition, using experimental measurements from multiple diagnostics. This tool provides input Be and Ni density profiles, which are the dominant contributors to $Z_\mathrm{eff}$, as well as W density profiles. NEO predictions are consistent with the experimental measurements, indicating core W accumulation, which codes using simplified approximations of neoclassical transport are unable to reproduce. Sensitivity scans on the gradients of key driving parameters $\nabla T_\mathrm{i}$, $\nabla n_\mathrm{e}$ are performed within experimental uncertainties to enhance confidence in the model predictions.