Internal measurement of pedestal-localized broadband magnetic fluctuations in ELMy H-mode plasmas in DIII-D

In DIII-D ELMy H-mode plasmas, pedestal-localized broadband magnetic fluctuations have been directly observed internally, for the first time, using a new Faraday-effect polarimeter diagnostic to identify their role in pedestal transport. The broadband magnetic fluctuations have many characteristics indicative of micro-tearing-modes (MTM): (a) poloidal wave number $\sim $0.3/cm, frequencies ranging from f$=$100-500 kHz with peak at 250 kHz, and propagation in the electron diamagnetic direction in the plasma frame, as expected for unstable MTM from linear GYRO calculation at the pedestal; (b) radial magnetic field amplitude lower bound $\vert \delta $B$_{\mathrm{r}}\vert \sim $25 Gauss and $\vert \delta $B$_{\mathrm{r}}$/B$\vert \sim $0.12$\% $ (B$=$2 T is total magnetic field) over bandwidth 100-500 kHz, comparable to the saturated MTM amplitude predicted by non-linear theory ($\rho _{\mathrm{e}}$/L$_{\mathrm{Te}}>$0.1$\% $ in pedestal); (c) non-monotonic dependence of mode amplitude on collision frequency, peaking at $\nu_{\mathrm{ei}}$/f$\sim $0.4-2 ($\nu_{\mathrm{ei}}$ is pedestal top collision frequency), consistent with lowest order MTM theory; (d): poloidally asymmetric spatial distribution with minimum amplitude near mid-plane. Between ELMs, the broadband magnetic fluctuation amplitude correlates with saturation of the pedestal gradients of T$_{\mathrm{e}}$, n$_{\mathrm{e}}$ and p$_{\mathrm{e}}$, indicating a role in regulating the pedestal. Based on stochastic field theory, the measured $\vert \delta $B$_{\mathrm{r}}\vert $ can lead to experimentally-relevant electron thermal transport while mode growth has been observed to correlate with decreased pedestal pressure and global stored energy. The observations provide strong evidence that MTM exists in H-mode pedestal and play an important role in pedestal transport. These findings provide critical experimental input for model validation and development of predictive physics understanding of pedestal confinement.