Self-consistent pedestal prediction for JET

In H-mode plasmas the energy confinement is strongly linked to pressure pedestal height (p_ped). At the same time, the core pressure has a stabilising effect on the peeling-ballooning modes (PBM) that limit p_ped. Therefore, in a truly predictive model the core and pedestal have to be resolved self-consistently.

The most widely used model for pedestal predictions, EPED requires advance knowledge of the density pedestal (n_ped) and the global beta which are not available when predicting future experiments. We have further developed the EPED model to incorporate
self-consistent modelling of core transport and n_ped along with p_ped.

For the prediction of n_ped, we have used a model based on neutral penetration. While this model lacks the important contribution of inter-ELM pedestal transport, it is able to predict n_ped in a large JET-ILW database with an average of 22% error. For the self-consistent core and pedestal prediction we use heating power as input and couple pedestal prediction with BohmgyroBohm transport model. For experimental JET plasmas found at the PBM stability limit this method accurately predicts both the pedestal and core as long as fast particle pressure is included in the prediction. When the experimental pedestals are not limited by PBMs, the pedestal height is overpredicted.

We also investigated the effect of the mass of the main ion isotope (m_i) on the pedestal prediction. The isotope affects directly the diamagnetic stabilisation of the PBMs. This alone does not explain the higher pedestals of deuterium plasmas compared to hydrogen. However, the experimental density pedestal increases with mi (both n_sep and n_ped increase from H to D). With these changes
imposed to the density pedestal p_ped is well predicted suggesting that the main isotope effect in JET-ILW H-mode is through the modification of n_ped that in turn affects p_ped prediction.