Advances in the two-dimensional structural detail of plasma turbulence in a tokamak

Turbulence is the leading cause of energy leakage in magnetic confinement thermonuclear fusion devices, such as tokamaks. Plasma physicists at Princeton University sought to further elucidate the physics of plasma turbulence by examining two-dimensional structures at the plasma surface in NSTX – a medium sized, specialized tokamak. They report their findings in Physics of Plasmas.

The physicists used gas puff imaging (GPI) to capture turbulence over a camera’s full 64x80 pixel range, at ∼400,000 frames/sec. First author Stewart Zweben explained that although this isn’t a new technique, it’s the first time both dimensions have been examined simultaneously in a tokamak, and over such a large number of points, to measure spatial cross-correlation function in detail. Twenty different shots were analysed to produce a two-dimensional radial vs. poloidal cross-correlation function database.

The ellipticity and tilt angle of the correlation region and the size and position of the negative correlation regions were evaluated. Zweben says that looking for empirical trends in the data was the most interesting part of the study, and he spotlights the large minimum negative correlation of -0.3 they identified. In the paper, co-author James Myra provided a partial qualitative explanation for this negative correlation with “blob-hole” theory. “Blobs” are moving magnetized filaments of plasma, and “holes” are regions of reduced density. Significant variations were seen in correlation results within this database and one possible cause offered was the misalignment of the GPI view with the local B field.

To better quantify the viewing misalignment in future studies, direct measurement of the parallel extent of the gas cloud was suggested. Zweben says the study provides new input for plasma physics turbulence supercomputer simulations. But more quantitative data is necessary to improve the accuracy of what Zweben describes as “computer chess” to successfully design a tokamak reactor.

Source: “Two-dimensional turbulence cross-correlation functions in the edge of NSTX,” by S. J. Zweben, D. P. Stotler, F. Scotti, and J. R. Myra, Physics of Plasmas (2017). The article can be accessed at https://doi.org/10.1063/1.5002695 .