A close look at fast ion microturbulence in tokamaks builds predictive capability for fusion

The simultaneous fluid and electromagnetic properties of tokamak plasmas give rise to modes called Alfvén waves, the prediction of which earned their namesake the 1970 Nobel prize. The spectral form these waves take can indicate the likely transport of resonant fast ions to be diffusive or convective. Conventionally, research focuses on fast ion collisional effects on Alfvén waves to better understand what contributes to fast ion losses. Looking instead at the oft-overlooked microturbulence, as reported in Physics of Plasmas, gives rise to new experimentally confirmed predictive capabilities.

Alfvén waves can be prone to energetic particle-driven instabilities in tokamak plasmas, as fast ions exchange energy with them. Understanding what leads to their two typical oscillation forms, frequency chirped or fixed-frequency, provides behavior criteria for anticipating the expected scenario of given experimental conditions, developed in this article.

Authors looked closely at the fast ion microturbulence characteristics in both conventional and spherical tokamaks with theory, simulations, and experimental measurements acquired from three different tokamak facilities: NSTX, DIII-D and TFTR. Microturbulence can be essential in early-stage nonlinear mode dynamics, though it doesn’t produce appreciable macroscopic transport. In conventional tokamaks, where the developed Alfvén modes are commonly observed to exhibit constant frequencies, this form strongly correlated with high microturbulence levels. Conversely, spherical tokamaks, where chirping frequencies are ubiquitous, typically have lower microturbulence levels.

The ability to know early on how a system is likely to evolve can prove invaluable to addressing plasma confinement for fusion energy. These results also contribute to a better validation of reduced transport modeling needed for computationally efficient confinement predictions. For additional verification, these results motivated dedicated experiments on the DIII-D tokamak to stress-test the theory. They pushed the machine to operate in low microturbulence, observing that chirping was much more prevalent in this situation than in usual operation regimes, therefore experimentally strengthening the proposed interpretation.

Source: “Theory and observation of the onset of nonlinear structures due to eigenmode destabilization by fast ions in tokamaks,” by V. N. Duarte, H. L. Berk, N. N. Gorelenkov, W. W. Heidbrink, G. J. Kramer, R. Nazikian, D. C. Pace, M. Podestà, and M. A. Van Zeeland, Physics of Plasmas (2017). The article can be accessed at https://doi.org/10.1063/1.5007811 .

Described work was supported by FAPESP, Brazil under grants 2012/22830-2 and 2014/03289-4, and by US DOE under contracts DE-AC02-09CH11466 and DE-FC02-04ER54698.