Gyrokinetic analysis and simulation of pedestals, to identify the culprits for energy losses using fingerprints

TL;DR

This paper introduces the concept of fingerprints to identify the instabilities responsible for energy losses in tokamak edge transport barriers, using gyrokinetic analysis and simulations to distinguish between different mode types.

Contribution

It develops a novel fingerprinting method based on transport ratios and fluctuation frequencies to identify dominant instabilities in tokamak pedestals, supported by gyrokinetic theory and simulations.

Findings

MTM and ETG modes are primary energy transport agents in studied cases.

MHD-like modes mainly cause electron particle losses.

Fluctuation frequency aids in mode identification.

Abstract

Fusion performance in tokamaks hinges critically on the efficacy of the Edge Transport Barrier (ETB) at suppressing energy losses. The new concept of fingerprints is introduced to identify the instabilities that cause the transport losses in the ETB of many of today's experiments, from widely posited candidates. Analysis of the Gyrokinetic-Maxwell equations, and gyrokinetic simulations of experiments, find that each mode type produces characteristic ratios of transport in the various channels: density, heat and impurities. This, together with experimental observations of transport in some channel, or, of the relative size of the driving sources of channels, can identify or determine the dominant modes causing energy transport. In multiple ELMy H-mode cases that are examined, these fingerprints indicate that MHD-like modes are apparently not the dominant agent of energy transport; rather, this role is played by Micro-Tearing Modes (MTM) and Electron Temperature Gradient (ETG) modes, and in addition, possibly Ion Temperature Gradient (ITG)/Trapped Electron Modes (ITG/TEM) on JET. MHD-like modes may dominate the electron particle losses. Fluctuation frequency can also be an important means of identification, and is often closely related to the transport fingerprint. The analytical arguments unify and explain previously disparate experimental observations on multiple devices, including DIII-D, JET and ASDEX-U, and detailed simulations of two DIII-D ETBs also demonstrate and corroborate this.