Effects of Global Alfven Eigenmodes on Electron Thermal Transport in NSTX*

Very high levels of electron thermal transport (power balance $\chi _{e}\ge $10's m$^{2}$/s) are correlated with strong Global Alfven Eigenmode (GAE) activity in the deep core of beam heated NSTX plasmas [Stutman, et al., \textit{PRL} \textbf{102}, 115002 (2009)]. New measurements and recently proposed theoretical mechanisms offer possible explanations for this process. Overlapping large-amplitude GAE modes can induce stochastic particle trajectories in the bulk, trapped electron population resulting in rapid radial diffusion. However, numerical calculations using the ORBIT guiding center code and measured GAE amplitudes, obtained from a single line-integrated measurement of density fluctuations, have so far under-predicted the inferred $\chi _{e}$ by a factor of 5-10. Recent measurements of the GAE amplitude obtained from line-integrated density fluctuations at multiple tangential locations confirm the central location of the modes, in agreement with predictions of the initial value Hybrid and MHD (HYM) simulation code. The region of strong GAE amplitude also coincides with that of large $\chi _{e}$, further supporting a GAE induced transport mechanism. Furthermore, the dynamics of GAE activity (f$\sim $500-1000 kHz) demonstrate the bursting nature of these modes. Coupled with the strongly non-linear relation predicted between the mode amplitude and induced $\chi _{e}$, these results suggest that the \textit{peak} amplitude of the GAEs may be controlling the magnitude of stochastic electron transport, rather than their time-averaged amplitude, as previously used in computations. The ORBIT predictions would then be much closer to the experimentally inferred $\chi _{e}$'s. Finally, the experimental observations are examined from the perspective of a recent theory which claims that the central T$_{e}$ flattening occurs in NSTX through GAE mediated ``energy-channeling'' of the neutral beam power [Kolesnichenko, et al., \textit{PRL} \textbf{104}, 075001 (2010)].