Direct gyrokinetic comparison of pedestal transport in JET with carbon and ITER-like walls

Abstract

This paper compares the gyrokinetic instabilities and transport in two representative JET pedestals, one (pulse 78697) from the JET configuration with a carbon wall (C) and another (pulse 92432) from after the installation of JET's ITERlike Wall (ILW). The discharges were selected for a comparison of JET-ILW and JET-C discharges with good confinement at high current (3 MA, corresponding also to low ρ * ) and retain the distinguishing features of JET-C and JET-ILW, notably, decreased pedestal top temperature for JET-ILW. A comparison of the profiles and heating power reveals a stark qualitative difference between the discharges: the JET-ILW pulse (92432) requires twice the heating power, at a gas rate of 1.9×10 22 e/s, to sustain roughly half the temperature gradient of the JET-C pulse (78697), operated at zero gas rate. This points to heat transport as a central component of the dynamics limiting the JET-ILW pedestal and reinforces the following emerging JET-ILW pedestal transport paradigm, which is proposed for further examination by both theory and experiment. ILW conditions modify the density pedestal in ways that decrease the normalized pedestal density gradient a/L n , often via an outward shift of the density pedestal. This is attributable to some combination of direct metal wall effects and the need for increased fueling to mitigate tungsten contamination. The modification to the density profile increases η = L n /L T , thereby producing more robust ion temperature gradient (ITG) and electron temperature gradient driven instability. The decreased pedestal gradients for JET-ILW (92432) also result in a strongly reduced E × B shear rate, further enhancing the ion scale turbulence. Collectively, these effects limit the pedestal temperature and demand more heating power to achieve good pedestal performance. Our simulations, consistent with basic theoretical arguments, find higher ITG turbulence, stronger stiffness, and higher pedestal transport in the ILW plasma at lower ρ * .

The dependence of exhaust power components on edge gradients in JET-C and JET-ILW H-mode plasmas

“…The strong W radiation from the mantle region cools the electrons, thereby increasing the collisional, ion-electron exchange and hence the fraction of heat transported through the electron channel. This is consistent with strong temperature gradient driven turbulence driven by the high values of η e ∼ 5 − 10 at the pedestal top [12] and with a high fraction of electron thermal transport through the pedestal [11]. We also estimate that the direct heat loss by radiation from within the ETB represents only a small fraction (∼ 10%) of the total power conducted through the pedestal.…”

“…8 of Ref. [11] for various cases. While neo-classical transport carries only ∼ 15% of the total conducted power, depending on the assumed impurity dilution, ITG turbulence can account for between ∼ 10% (dominant Be impurity) and ∼ 40% (dominant Ni).…”

“…as discussed in Ref. [11], the term 'stiffness' is used to refer to the condition where the turbulent heat flux q turb , increases faster than linearly with the excess normalised temperature gradient above the critical threshold ∆(R/L T ) = R/L T − R/L T,cr , which in turn implies that the turbulent diffusivity χ turb increases with ∆(R/L T )). Usually, this is investigated in simulations by varying R/L T with other equilibrium parameters held constant.…”

“…The most recent of such non-linear simulations using GENE, of both ion and electron-scale turbulence, reported in Ref. [11] are for the same matched JET-ILW and JET-C pulses discussed here in §3.2 and use the ion temperature profile measured by the CXRS system. These simulations for the JET-ILW pulse #92432 predict a total transport power across the ETB in the JET-ILW pulses which compares well with the inter-ELM loss powers reported here (which account for ELM losses as well as radiation), i.e.…”

“…[8][9][10][11], and has been attributed to an increase in the turbulent pedestal heat transport due to the altered pedestal structure. In particular, the reduced normalised density gradient R/L ne (≡ −R (dn e /dr)/n e ) across JET-ILW pedestals increases the parameter η = L n /L T for both the ions and electrons, which is known to increase the growth rates of temperature gradient driven micro-instabilities (ETG, ITG and MTMs) that cause turbulent transport [7,11,12]. One of the aims of this paper is to quantify the inter-ELM loss power through the pedestal for comparison with non-linear, gyrokinetic simulations of the pedestal heat flux, e.g.…”