Reduced models for ETG transport in the pedestal

TL;DR

This paper develops reduced models for electron temperature gradient (ETG) transport in the pedestal, based on extensive gyrokinetic simulations, achieving accurate predictions with a simple algebraic form.

Contribution

It introduces a generalized quasilinear model for ETG transport using a new simulation database and symbolic regression, with a key parameter eta for saturation.

Findings

Model achieves 15% RMS error with a single fitting coefficient.

A new gyrokinetic simulation database (MGKDB) was created.

Simple algebraic expressions for ETG transport are provided.

Abstract

This paper reports on the development of reduced models for electron temperature gradient (ETG) driven transport in the pedestal. Model development is enabled by a set of 61 nonlinear gyrokinetic simulations with input parameters taken from the pedestals in a broad range of experimental scenarios. The simulation data has been consolidated in a new database for gyrokinetic simulation data, the Multiscale Gyrokinetic Database (MGKDB), facilitating the analysis. The modeling approach may be considered a generalization of the standard quasilinear mixing length procedure. The parameter eta, the ratio of the density to temperature gradient scale length, emerges as the key parameter for formulating an effective saturation rule. With a single order-unity fitting coefficient, the model achieves an RMS error of 15%. A similar model for ETG particle flux is also described. We also present simple algebraic expressions for the transport informed by an algorithm for symbolic regression.