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 ITER-like 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 ${\rho }_{\ast }$) 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 $1.9\times 10^{22}e/s$, 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 $\eta =L_n/L_T$ , 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 $E\times B$ 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.

References in corpus (6)

• Gyrokinetic projection of the divertor heat-flux width from present tokamaks to ITER
• Kinetic instabilities that limit β in the edge of a tokamak plasma: a picture of an H-mode pedestal
• Impact of nitrogen seeding on confinement and power load control of a high-triangularity JET ELMy H-mode plasma with a metal wall
• MHD and Gyro-kinetic Stability of JET Pedestals
• Local and global Fokker-Planck neoclassical calculations showing flow and bootstrap current modification in a pedestal
• Flow Shear Suppression of Pedestal Turbulence--A First Principles Theoretical Framework

Cited by in corpus (7)

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• Reduced models for ETG transport in the pedestal
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• Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient
• Gyrokinetic Simulations Compared with Magnetic Fluctuations Diagnosed with a Faraday-Effect Radial Interferometer-Polarimeter in the DIII-D pedestal
• Scale invariance and critical balance in electrostatic drift-kinetic turbulence
• Toroidal and slab ETG instability dominance in the linear spectrum of JET-ILW pedestals