Effect of resistivity on the pedestal MHD stability in JET

Abstract

The ELM triggering mechanism in tokamaks is not yet fully understood. For example, in the JET tokamak with ITER-like wall (commonly called JET-ILW), the ELMs are sometimes triggered before the ideal peeling-ballooning boundary is reached. This typically occurs for shots with high input power and high gas rate. The discrepancy between model and experiment has in previous works been clearly correlated with the relative shift between the electron temperature and density pedestals. The discrepancy has also been correlated with the resistivity in the middle-bottom of the pedestal. The present work shows that resistive MHD can have a significant impact on the peeling-ballooning stability of JET pedestals. The inclusion of resistivity removes the correlation between the discrepancy from the peeling-balloning stability and the relative shift (the difference between the position of the electron temperature and density pedestals) and significantly improves the agreement between peeling-balloning model and experimental results. The work also shows that the key parameter is the resistivity at the pedestal bottom, near the separatrix, while the resistivity near the middle/top of the pedestal has a negligible effect on the peeling-balloning stability of JET plasmas.

Effect of the isotope mass on pedestal structure, transport and stability in D, D/T and T plasmas at similar β N and gas rate in JET-ILW type I ELMy H-modes

“…A sensitivity test has been performed for the key simulations to assess the difference between this approach and the standard assumption T sep e = 100 eV (the value typically used in JET-ILW PB stability analysis). The result of the sensitivity test is that the two approaches do not lead to any significant difference, as already shown for many other datasets in JET-ILW [36,81]. Moreover, the following results have assumed T i = T e .…”

“…In figure 26(b) the growth rates have been determined using the j edge,max /⟨j⟩ of the T pulse. This type of diagram has been discussed in details in [81]. Here, we simply note that two behaviours can be observed.…”

“…This disagreement has been systematically investigated in several studies [33][34][35][36]51] and it has been empirically correlated with the relative shift between density and temperature position (which is, in turn, correlated to n sep e /n ped e ). The key idea is that increase of n sep e /n ped e leads to an increase in the pedestal turbulent transport that reduces the temperature gradient and makes resistive effects non-negligible [36,81]. Indeed, the use of resistive MHD has led to a much better agreement with the experimental data [81], even if further physics effects (such as finite Larmor radius effects and a proper treatment of the diamagnetic stabilization [10,58,59]) are likely necessary to reach a perfect agreement with experiment.…”

The JET hybrid scenario in Deuterium, Tritium and Deuterium-Tritium

“…But also the higher necessary gas injection, described later, could be related to this. The increased stability in T could be driven by similar physics (resistive MHD) as presented in [81,82], even though the pedestal top temperatures here are higher, because mostly the near separatrix part of the profile is important. A more detailed analysis has not been attempted yet for the first Tritium ELM presented here.…”

Probing non-linear MHD stability of the EDA H-mode in ASDEX Upgrade

“…However, the MISHKA simulations neglect all non-ideal effects, notably the destabilizing resistivity and the stabilizing ExB and diamagnetic flows. Nonlinear [28,29] and linear [30] resistive MHD simulations have shown to move the ballooning boundary to lower pressure gradients, such that the operational point could easily be unstable to resistive (peeling-)ballooning modes. Indeed, the JOREK simulations that will be presented in the following sections find that the equilibrium is in fact unstable to resistive peelingballooning modes at realistic plasma resistivity.…”