The role of ETG modes in JET–ILW pedestals with varying levels of power and fuelling

Abstract

We present the results of GENE gyrokinetic calculations based on a series of JET-ILW type I ELMy H-mode discharges operating with similar experimental inputs but at different levels of power and gas fuelling. We show that turbulence due to electron-temperature-gradient modes (ETGs) produces a significant amount of heat flux in four JET-ILW discharges, and, when combined with Neoclassical simulations, is able to reproduce the experimental heat flux for the two low gas pulses. The simulations plausibly reproduce the high-gas heat fluxes as well, although power balance analysis is complicated by short ELM cycles. By independently varying the normalised temperature gradients ωTe and normalised density gradients ωne around their experimental values, we demonstrate that it is the ratio of these two quantities ηe = ωTe / ωne that determines the location of the peak in the ETG growth rate and heat flux spectra. The heat flux increases rapidly as ηe increases above the experimental point, suggesting that ETGs limit the temperature gradient in these pulses. When quantities are normalised using the minor radius, only increases in ωTe produce appreciable increases in the ETG growth rates, as well as the largest increases in turbulent heat flux which follow scalings similar to that of critical balance theory. However, when the heat flux is normalised to the electron Gyro-Bohm heat flux using the temperature gradient scale length LTe , it follows a linear trend in correspondence with previous work by different authors.

Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient

“… 7 Chapman-Oplopoiou et al (2022) found such a scaling of the heat flux with in their investigations of nonlinear pedestal turbulence driven by ETG modes [see their equation (1), and the following discussion] suggesting, perhaps, that this scaling may hold in more realistic – and complex – plasma systems than that considered here.…”

Scale invariance and critical balance in electrostatic drift-kinetic turbulence

“…For example, Chapman-Oplopoiou et al. (2022) performed nonlinear, electromagnetic simulations of JET–ILW pedestals for , and observed the same scaling of the heat flux with as (2.7) at gradients sufficiently far above the linear threshold. However, if the outer scale lies on scales larger than the flux-freezing scale, i.e.…”

“…2022, Chapman-Oplopoiou et al. 2022 and Field et al. 2023, where a strong dependence of the heat flux on the equilibrium density gradient was identified).…”

Linear stability of the JET H-mode pedestal

“…In H-mode [1,2] tokamak plasma, electron fluxes can be driven by a rich variety of waves and instabilities covering a large range of temporal and spatial scales making its modeling (in terms of its width and height) challenging. Effectively, several studies on the origin of the electron heat transport in the pedestal region [3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21] show that drift-wave, such as the electron-temperature-gradient (ETG) instability, the trapped-electron mode (TEM), and electromagnetic instabilities, such as kinetic-ballooning mode (KBM) and microtearing (MT) modes of interest here, can explain the electron heat transport observed experimentally. These instabilities can develop in the pedestal region, leading to turbulence, which affects the transport and the confinement of heat and particles.…”