Geodesic Acoustic Mode Structure in DIII-D

Geodesic Acoustic Modes (GAMs) are coherent flows induced by plasma turbulence that in turn affect the turbulence and turbulent transport. Recently, in a neutral beam and electron cyclotron heated L-mode plasma in the DIII-D tokamak, strong GAM oscillations have been observed in electron temperature fluctuations $\tilde{T}_e$ in addition to the often-observed GAM density fluctuations. The mode frequency is constant over a radial range ($\delta \rho \sim 0.2$), as expected of an eigenmode, with two different frequencies observed depending upon radius. Both modes exist at the location where one frequency transits to another as detected in $\tilde {T}_e$. GAM oscillations in density and $ExB$ flow peak at far edge (at $\rho\sim0.9$) and have similar profile shapes. In contrast, the GAM oscillations in $\tilde{T}_e$ peak much deeper into plasma (at $\rho\sim 0.7$). After the auxiliary heating power is turned off for $t\,\underline{>}$ 100 ms, the eigenmode feature evolves into a continuum. This observation of GAM properties may provide challenges for existing theories to understand GAMs and plasma turbulence.