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

Abstract

The work describes the pedestal structure, transport and stability in an effective mass (A eff) scan from pure deuterium to pure tritium plasmas using a type I ELMy H-mode dataset in which key parameters that affect the pedestal behaviour (normalized pressure, ratio of the separatrix density to the pedestal density, pedestal ion Larmor radius, pedestal collisionality and rotation) are kept as constant as possible. Experimental results show a significant increase of the density at the pedestal top with increasing A eff, a modest reduction in the temperature and an increase in the pressure. The variations in the pedestal heights are mainly due to a change in the pedestal gradients while only small differences are observed in the pedestal width. A clear increase in the pedestal density and pressure gradients are observed from deuterium to tritium. The experimental results suggest a reduction of the pedestal inter-edge localized mode (inter-ELM) transport from deuterium to tritium. The reduction is likely in the pedestal inter-ELM particle transport, as suggested by the clear increase of the pedestal density gradients. The experimental results suggest also a possible reduction of the pedestal inter-ELM heat transport, however, the large experimental uncertainties do not allow conclusive claims on the heat diffusivity. The clear experimental reduction of η e (the ratio between density and temperature gradient lengths) in the middle/top of the pedestal with increasing A eff suggests that there may be a link between increasing A eff and the reduction of electron scale turbulent transport. From the modelling point of view, an initial characterization of the behaviour of pedestal microinstabilities shows that the tritium plasma is characterized by growth rates lower than the deuterium plasmas. The pedestal stability of peeling-ballooning modes is assessed with both ideal and resistive magnetohydrodynamics (MHD). No significant effect of the isotope mass on the pedestal stability is observed using ideal MHD. Instead, resistive MHD shows a clear increase of the stability with increasing isotope mass. The resistive MHD results are in reasonable agreement with the experimental results of the normalized pedestal pressure gradient. The experimental and modelling results suggest that the main candidates to explain the change in the pedestal are a reduction in the inter-ELM transport and an improvement of the pedestal stability from deuterium to tritium.

Divertor power load investigations with deuterium and tritium in type-I ELMy H-mode plasmas in JET with the ITER-like wall

“…For the energy fluence, the predictions from a model using energy balance considerations [15] are in line with the absence of an explicit mass dependence of the outer target energy fluence. The pedestal top pressure in tritium plasmas is slightly higher than in deuterium plasmas with similar engineering parameters, in agreement with the analysis in [26,27]. The measured values in both, deuterium and tritium plasmas are within the 1:1 and 3:1 boundaries from the model, similar to the original experimental study in deuterium [15].…”

“…The pedestal top pressure is slightly higher in tritium due to a higher density. The general trend of higher pedestal top density in tritium plasmas is consistent with the behaviour observed throughout the tritium campaign [26,27].…”

“…Since the experiment was not done at constant engineering parameters, no conclusion on how the isotope mass affects the pedestal can be extracted. For a detailed analysis of the isotope mass impact on the pedestal see [26,27]. It is therefore noted that no detailed comparison between individual pulses is performed here, the analysis for the ELM energy fluence and power fall-off length is done in a data base approach.…”

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

“…The observation, that the Tritium pulse crosses the stability boundary is not robust enough to make a strong statement here. But clear from the relative movement is, that the pedestal stability in Tritium is improved compared to Deuterium, as reported in detailed type-I ELMy H-mode experiments in D, T and D-T [80,81]. One consequence is the longer ELM free period in Tritium as described before.…”

“…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.…”