2025 – Cross-scale interaction between high-k and low-k turbulence in high Te gradient regime

Cross-scale interaction between high-k and low-k turbulence in high Te gradient regime

2025 Research Campaign, Turbulence and Transport

Purpose of Experiment

This study investigates the cross-scale interaction between high-k and low-k turbulence in high electron temperature (Te) gradient regimes on the DIII-D tokamak. Previous multiscale simulations have revealed significant interactions between electron temperature gradient (ETG) and trapped electron mode (TEM) turbulence [1. S. Maeyama et al. Nature comm. (2022) 13:3166], while experimental studies on the Large Helical Device (LHD) demonstrated a negative correlation between high-k and low-k turbulence intensities [2. Nasu T. et al 2023 Electron-scale turbulence characteristics in LHD plasma Proc. 29th IAEA Fusion Energy Conf. (London 16–21 October 2023)]. Building upon these findings, we employ multiple Doppler backscattering systems to simultaneously measure high-k and low-k turbulence at identical spatial locations in DIII-D plasmas. Our experimental approach maintains constant total heating power while systematically varying the ECH injection between inner and outer radii, enabling a comprehensive scan of Te gradients. This methodology allows for direct observation of cross-scale turbulence interactions under controlled conditions, providing valuable experimental validation of theoretical predictions in fusion plasma turbulence.

Experimental Approach

The simulation research described above [1] utilized DIII-D plasma of normal Bt and normal Ip. #199663 is preferable reference shot. The experimental conditions are normal Bt = 2.0 T, normal Ip = 1.0 MA, and upper single null divertor configuration, which has unfavorable ion grad-B drift direction to achieve as high heating power in L-mode as possible. NB is used only for diagnostics i.e. CER, MSE, BES. Main heating source is ECH aiming to ρ=0.5 (inner) and ρ=0.7 (outer). Total power is maintained to constant while varying power balance between inner and outer heating shot-by-shot for wide scan of Te gradient referring to Ref. [3. J. C. DeBoo et al. Phys. of Plasma. (2012) 19 082518]. One ECH is modulated by 28 Hz for heat pulse propagation analysis. DBS240 and DBS240u are adjusted to observe ρ=0.6 by using GENRAY calculation to obtained discharge. Line averaged density is maintained to constant 2e19 m-3 using feedback control to fix the observation location of DBSs.

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