In tokamaks, a high confinement operational mode (H-mode) is reached when the plasma self-organizes to generate a narrow edge transport barrier called the pedestal. Velocity shear in this region tears apart turbulent eddies, allowing for the growth of pedestal gradients in a quiescent metastable state until explosive global events are triggered, setting a hard limit on the maximum pressure and current gradients achievable in the edge region.

The question of what sets the temperature, density and pressure gradients in the pedestal has eluded plasma physics for decades. A few models have been proposed that are able to form semi-predictive estimates across this region, however, it seems increasingly likely that the height and width of the pedestal – which are responsible for improved fusion performance – are controlled by a host of various microinstabilities which induce transport across the pedestal despite the high levels of turbulent shear.

Over the past few decades, largely theoretical and computational work has uncovered five plasma instabilities that may contribute to inter-ELM transport through the H-mode pedestal. These include three electrostatic modes: the trapped electron mode (TEM), the electron temperature gradient (ETG) mode, and the ion temperature gradient (ITG) mode; and two electromagnetic modes: the kinetic ballooning mode (KBM) and the microtearing mode (MTM). High fidelity gyrokinetic modeling, which is generally limited to static time-slices of the plasma state, has shown that each of these modes could become unstable in the tokamak edge under certain conditions, but an experimental determination of which modes are active in the pedestal remains elusive due to the nebulous nature of the turbulence. Without empirical validation of individual modes, it is difficult to improve the physics basis of leading pedestal models.

In 2019, I led an explicit investigation into temperature-limiting transport mechanisms in the DIII-D plasma edge. Using a novel experimental technique (large, fast oscillations to perturb toroidal current profiles in the plasma edge) we were able to directly probe and measure a theorized class of microinstabilities in the plasma edge for the first time. Then, through careful combination of analytic theory and data analysis, I developed a simple and robust experimental workflow to combine high-resolution profile and fluctuation measurements with state-of-the-art modeling in order to determine the presence of these turbulent mechanisms in a large suite of different regimes. Since these microinstabilities play a large role in mitigating electron heat flux through the plasma edge, we can now incorporate their physics into predictive models of turbulent transport to refine confinement predictions in various machines.

Selected publications on this subject:

A survey of pedestal magnetic fluctuations using gyrokinetics and a global reduced model for microtearing stability
Curie, M., Larakers, J. L., Hatch, D., Nelson, A. O., Diallo, A., Hassan, E., Guttenfelder, W., Halfmoon, M., Kotschenreuther, M., Hazeltine, R. D., Mahajan, S. M., Groebner, R. J., Chen, J., Perez von Thun, C., Frassinetti, L., Saarelma, S., Giroud, C. & Tennery, M., Physics of Plasmas 29, 042503 (2022).

Time-dependent experimental identification of inter-ELM microtearing modes in the tokamak edge on DIII-D
Nelson, A. O., Laggner, F. M., Diallo, A., Smith, D. R., Xing, Z. A., Shousha, R. & Kolemen, E., Nuclear Fusion 61, 116083 (2021).