Spectroscopic technique sheds insights on how confined plasmas flow

Plasmas confined in magnetic fields are known to flow like liquids, but exactly how they flow is still an open question. Characterizing the behavior of plasmas in space and time enriches our current understanding of this phenomenon, and in the future could enable large-scale fusion energy.

Craig et al. have reported the first direct measurements of plasma flows in a reversed field pinch, a device used to generate and confine plasmas. Their findings – which validate many common assumptions about plasma flow behavior and identify eddy currents as the dominant momentum loss mechanism during improved confinement – are published in Physics of Plasmas.

Co-author Darren Craig said the team used a technique called charge exchange recombination spectroscopy. The method works by first exciting impurity ion emission with a neutral atom beam and then measuring the Doppler shift to determine the velocity of the ions.

To make the measurements, the team had to overcome several technical challenges. “First, we built a custom high-throughput spectrometer to enable measurements with high time resolution,” Craig said. “Second, we had to develop techniques to deal with a high background signal in the MST plasmas we studied.” Developing a reliable absolute wavelength calibration was the final step that enabled the authors to measure how the plasma flows varied in space and time.

“One surprise was the strongly peaked flow profile that emerges in the center of these plasmas,” Craig said.

The technique is already being used to characterize plasmas under new and different conditions than the ones reported in this paper.

Source: “Intrinsic flow and tearing mode rotation in the RFP during improved confinement,” by D. Craig, E. H. Tan, B. Schott, J. K. Anderson, J. Boguski, D. J. Den Hartog, T. Nishizawa, M. D. Nornberg and Z. A. Xing, Physics of Plasmas (2019). The article can be accessed at https://doi.org/10.1063/1.5095620 .