Towards understanding reactor relevant tokamak pedestals

Abstract

The physics of the tokamak pedestal is still not fully understood, for example there is no fully predictive model for the pedestal height and width. However, the pedestal is key in determining the fusion power for a given scenario. If we can improve our understanding of reactor relevant pedestals we will improve our confidence in designing potential fusion power plants. Work has been carried out as part of a collaboration on reactor relevant pedestal physics. We report some of the results in detail here and review some of the wider work which will be reported in full elsewhere. First, we attempt to use a gyrokinetic-based calculation to eliminate the pedestal top density as a model input for Europed/EPED pedestal predictions. We assume power balance at the top of the pedestal, that is, the heat flux crossing the separatrix must be equal to the heat source at the top of the pedestal and investigate the consequences of this assumption. Unfortunately, this method was not successful. Second, we investigate the effects of non flux surface density on the bootstrap current. Third, type I ELMs will not be tolerable for a reactor relevant regime due to the damage that they are expected to cause to plasma facing components. In recent years various methods of running tokamak plasmas without large ELMs have been developed. These include small and no ELM regimes, the use of resonant magnetic perturbations and the use of vertical kicks. We discuss the quiescent H-mode here. Finally we give a summary and directions for future work.

Simulating the effect of poloidal particle source on the bootstrap current and benchmarking to analytic estimates in tokamak edge pedestal

“…15 is in qualitative agreement with the analytic theory in Ref. [15] which derived the coefficient for the poloidal density perturbation as a function injection angle. This coefficient has its maximum for inboard injection.…”

“…In Ref. [15] the contribution arising from poloidal asymmetry in density was called "incremental bootstrap current".…”

“…In Ref. [15], an expression for the effect of point source causing incremental bootstrap current is given as: Ref. [26] Figure 18: Effect of magnetic field to the incremental bootstrap current.…”

Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient

“…These results would appear to be particularly relevant in the context of the edge regions of a tokamak, where both the safety factor and the temperature gradients are large (see, e.g., Ham et al. 2021 and references therein).…”

“…It is not a forgone conclusion, however, that the limit of can be achieved in experimentally relevant conditions. Though an emerging paradigm for JET-ILW (ITER-like wall) pedestal transport appears to be that the ILW conditions modify the pedestal density in ways that preferentially decrease its gradient (Hatch et al 2019; Ham et al 2021) – thereby increasing both and – other recent studies have found that the average value of in the pedestal appears to saturate at – during the inter-edge-localised-mode period (Field et al 2020; Guttenfelder et al 2021). Within the latter context, the stiff heat-flux scalings derived in § 6 can be viewed as an argument against ETGs much above the critical linear threshold being achievable.…”