Probing Transport via Modulated Electron Cyclotron Deposition in Gyrokinetic Turbulence Simulations

As magnetic-confinement nuclear fusion research advances toward reactor-stage operation, dynamic control of plasma behaviour has become an urgent topic. In this context, heat transport models that are both accurate and computationally efficient will be essential for designing control strategies in future fusion devices. In magnetically confined plasmas, several mechanisms contribute to radial heat transport, but turbulence driven by microinstabilities — such as trapped-electron modes (TEM), ion-temperature-gradient (ITG) modes, and electron-temperature-gradient (ETG) modes — plays the dominant role. Because the underlying physics is highly complex, reduced advection–diffusion (AD) models with effective transport coefficients are widely employed, often calibrated against experiments. However, these closures rely on strong assumptions (e.g., linearity, quasi-stationarity, and locality) that may, under certain conditions, be significantly violated by the intrinsically nonlinear and nonlocal nature of turbulent transport. This work investigates heat transport in a temperature-gradient–driven TEM regime and examines under which conditions an AD closure ceases to be valid. We perform high-fidelity gyrokinetic simulations using a new Electron Cyclotron Resonance Heating (ECRH) deposition module for the Gyrokinetic Electromagnetic Numerical Experiment (GENE) code. The power deposition is modulated over time scales comparable to or longer than characteristic turbulence time scales, and the resulting radial temperature response is analysed in the frequency domain to better understand its interaction with turbulence. We then employ a linear heat transport model to estimate effective diffusion and advection coefficients, and to assess the limits of their validity. The results show that decreasing the modulation frequency increases the response amplitude and broadens the spatial window of high coherence between the temperature signal and the ECRH drive, which is conducive to the applicability of AD models. Yet the spatial asymmetry of the frequency response function reveals limitations of advection–diffusion closures at these scales, and the inferred transport coefficients remain sensitive to the deposition frequency. Moreover, reducing the ECRH modulation amplitude to mitigate nonlinear effects is constrained by lower signal-to-noise ratios, requiring a balance between weak perturbation and measurable response. Overall, this study combines perturbative transport estimation with gyrokinetic turbulence modelling, establishing a methodological framework for more precise inference of heat-transport coefficients and paving the way for improved modelling and control strategies in fusion plasmas.