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

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.

Pedestal structure, stability and scalings in JET-ILW: the EUROfusion JET-ILW pedestal database

“…The increase of ne sep and of ne pos lead to the increase of ηe, due a simple geometrical effect [39]. In turn, this destabilizes micro-instabilities, hence increasing the turbulent transport [73,75,76] and leading to a reduction of the gradient. The reduction of the gradient (in particular of Te) leads to the increase of resistivity, making resistive MHD effects non-neglibible, see figure 27(a).…”

Multi-scale interactions of microtearing turbulence in the tokamak pedestal

“…One common such means is through Hmode operation [1], where an edge transport barrier called the pedestal is established. While this configuration tends to be unstable to edge-localized modes -which, however, may be mitigated by, e.g., resonant magnetic perturbations (RMPs) [2,3] -that are associated with the peeling-ballooning instability threshold [4], the overall evolution of the pedestal requires essential contributions from electron microinstabilities [5,6].…”

Direct Observation of Nonlinear Coupling between Pedestal Modes Leading to the Onset of Edge Localized Modes

“…I ELMy H-modes on the DIII-D tokamak, This letter focused on long inter-ELM phases during which the pedestal parameters are pinned to a metastable state that cannot be explained by the PB theory. Besides possibly regulating the pedestal transport [11,26], these pedestal modes play a key role prior to the onset of ELM events. Three dominant pedestal modes, exhibiting both magnetic and density fluctuations, have been identified, and their amplitudes, frequencies, and radial profiles have been tracked during these long ELM cycle.…”