The residual turbulent transport in an edge transport barrier, in combination with the corresponding heat and particle sources, determines the heating power necessary to achieve a given pedestal temperature; the inter-ELM evolution of pedestal density and temperature profiles, which ultimately determines the operating point at which an ELM is triggered; and the accessibility and properties of ELM-free regimes. Here we report on the 2019 DOE-FES theory performance target (TPT), whose aim was to develop a deep understanding of the turbulent transport mechanisms, along with the corresponding heat and particle sources, that govern pedestal dynamics. This was accomplished via two sets of complementary computational tools: (1) gyrokinetic codes (GENE and CGYRO), which can analyze the instabilities and transport that arise in the pedestal, and (2) edge codes (SOLPS and UEDGE---assuming saturated inter-ELM profiles), which can provide the best possible estimate of particle and heat sources. This study was carried out from the perspective of the so-called transport 'fingerprint' conceptual framework, which relies on basic physical signatures of the prospective pedestal instabilities to compare with the breadth of available experimental data, including frequency spectra, fluctuation scales and amplitudes, transport ratios, and inter-ELM profile evolution.
This combined gyrokinetic and edge modeling analysis has been carried out for discharges spanning multiple devices (DIII-D, JET, C-Mod) and exploring a wide range of parameters, modes of operation (H-mode, I-mode, QH-mode), and wall materials (metal, carbon). Major results include:
- Edge modeling demonstrates that edge transport barriers typically lie in a regime in which heat diffusivity far exceeds particle diffusivity: $D_e / \chi_e \ll 1$, as shown in Fig. 1. This conclusion was robust to extensive sensitivity tests carried out with SOLPS to scope the major uncertainties in input parameters. The only exception among the discharges studied was a DIII-D discharge in which pellet fueling was employed to intentionally increase the edge particle source.
ETG and MTM are important mechanisms for electron heat transport and one or both are generally major contributors to $\chi_e$ in standard ELMy H-mode scenarios.
In addition to its transport signature, which is consistent with $D_e / \chi_e \ll 1$, MTM activity is further established by the close quantitative agreement between distinctive bands in magnetic spectrograms and corresponding bands of unstable MTM in gyrokinetic simulations. This is now established over several discharges on multiple machines. An example is shown in Fig. 2, where a magnetic spectrogram is shown alongside growth rates and frequencies from linear GENE simulations; the three distinctive frequency bands correspond very closely with the spectrogram. %If KBM is present it is not observed (or is not as prominent) in the magnetic spectrograms.
Although there is substantial uncertainty in ion temperature profiles, neoclassical transport in the ion heat channel is significant and plausibly accounts for the experimental transport levels in most scenarios. A possible exception is JET-ILW, where ITG appears to play a non-negligible role in the ion heat flux in the discharge examined.
Neoclassical electron particle transport is also generally not negligible but remains somewhat smaller than edge modeling predictions.
MHD modes like KBM produce diffusivities that are comparable in both the heat and particle transport channels. Due to the observed disparity between $D_e$ and $\chi$, KBM cannot simultaneously account for all transport in both channels, but, rather, must be limited largely to the particle channel, while still being free to fix the pressure gradient as suggested by EPED.
ITG and ETG are observed in some cases to produce pinch velocities of up to $\sim 0.2 m/s$ (attributing all flux to pinch and no diffusion).
Neoclassical, MTM, ETG, and ITG can all produce relevant electron particle transport levels (the latter two in either positive or negative directions). Within uncertainties, they can plausibly account for all particle transport. Generally, however, these mechanisms are found to collectively produce particle transport somewhat below the edge modeling predictions. This suggests that an additional particle transport mechanism, like KBM, may be necessary.
This unprecedented study of linear stability, turbulent transport, and interpretive analysis of edge pedestal transport leads to a compelling explanation for multiple experimental observations. The clear identification of multiple active mechanisms for particle and heat transport, implicating the importance of ETG and MTM, in addition to ITG, KBM and neoclassical, forms an important foundation of knowledge for prediction and optimization of edge transport barriers in future burning plasma devices.
Acknowledgements---This work was supported by U.S. DOE Contract No. DE- FG02-04ER54742; U.S. DOE Office of Fusion Energy Sciences Scientific Discovery through Advanced Computing (SciDAC) program under Award Numbers DE-SC0018429 and DE- SC0018148; and General Atomics award number 4500076923. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility, under Award DE-FC02-04ER54698. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
|Affiliation||Institute for Fusion Studies, University of Texas at Austin|
|Country or International Organization||United States|