Description
The interaction of fast-ion-driven Alfven Eigenmodes with axisymmetric Zonal shear flows in toroidal plasmas has recently received significant attention due to the potential stabilization of microturbulence and reduction of radial transport [1,2,3]. Here, we present clear experimental evidence that multiple simultaneous Toroidicity-induced Alfven Eigenmodes [4] drive mesoscale ($\rho_i$ < 1/$k_r$ <$L_n, L_T$), radially localized Zonal shear flows and suppress ITG turbulence in DIII-D. An upper single null diverted target plasma ($B_t=1.25 T, I_p \approx0.61 MA, q_{95}\approx5.4$) is heated via neutral beam injection ($P_{NBI} \approx4 MW$), intermittently destabilizing TAE modes. Multi-channel Doppler Backscattering (DBS [5]) probes the local ExB flow, flow shear, and turbulence (density fluctuation) levels at normalized radii $0.2 ≤ \rho ≤ 0.8$. Using two multi-channel DBS systems located 180º apart toroidally, the axisymmetric nature of the measured Zonal shear flows has been confirmed. Near mid-radius, the local ExB shearing rate increases by roughly an order of magnitude (from $\omega_{ExB}\approx2-3$x$10^4 rad/s-$2x$10^5 rad/s$) and exceeds the plasma frame turbulence decorrelation rate intermittently when multiple TAE modes are present. Simultaneously, density fluctuation levels ($k_\theta \rho_s\approx0.4-0.8$) are reduced by 35-50%, with almost perfect anti-correlation between fluctuation level and local shearing rate. The observed shear flows are not localized to low order rational surfaces. DBS measurements confirm causality (the TAE amplitude leads the maximum ZF shearing rate); in turn the local ExB shearing rate leads ITG fluctuation suppression (by ~70$\mu s$) near mid-radius ($\rho\approx0.55$). In contrast, the shearing rate lags fluctuation suppression in the inner core ($\rho\approx0.35$), potentially indicating shear flow drive from the turbulence spectrum at this radius. Further work will be presented to elucidate the coupling and energy exchange between TAE modes, Zonal Flows and drift wave turbulence via bispectral/transfer entropy analysis.
*This work supported by US DoE under DE-FC02-04ER54698, DE-SC0020287, DE-SC0020337, DE-SC0019352 and DE-FG02-08ER54999.
[1] L. Chen and F. Zonca, Phys. Rev. Lett. 109, 145002 (2012).
[2] G.J. Choi et al., Nucl. Fusion 64, 016028 (2024).
[3] X.D. Du et al., Phys. Rev. Lett. 135, 265101 (2025).
[4] M.A. Van Zeeland et al., Phys. Rev. Lett. 97, 135001 (2006).
[5] W.A. Peebles et al., Rev. Sci. Instr. 81, 10D902 (2010).