The Tokamak Density Limit : from Microscopics to Macroscopics

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It’s a virtual certainty that any burning plasma experiment will operate at high density, since fusion power gain increases as the square of the density. This observation has driven renewed interest in density limits, and has stimulated efforts to exceed the “conventional wisdom” of the subject (for edge fueling) — the Greenwald limit. In this talk, we’ll review the basics and history of the density limit; present the current understanding of the microphysics, including theory; and then discuss recent experiments at high power beam injection in Negative Triangularity DIII-D plasmas, and their implications for the future. Along the way, we’ll discuss two physics topics of broader interest. One is the trends in Kubo number 𝑘𝑢 and production ratio 𝑃𝑅 (a measure of turbulence drive by propagation vs. local drive) during a scan of Greenwald fraction. This yields some novel insights into the physics of 𝑘𝑢 > 1 turbulence regimes. In particular, we show that 𝑘𝑢 and 𝑃𝑅 are strongly correlated throughout the scan. We also discuss aspects of the radiative condensation instability in the presence of turbulence, which are relevant to the dynamics of MARFEs, associated with density limit phenomena.

 

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About the Speaker

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Patrick Diamond received his Ph.D. in Physics from MIT in 1979. His advisor was Thomas H. Dupree. He was a Visiting Member (postdoc) at the Institute for Advanced Study, Princeton, and later joined the Institute for Fusion Studies, Tx. He later moved to UC San Diego, where he is now Distinguished Professor of Astronomy & Astrophysics and Physics. Diamond was Director of the WCI Center for Fusion Theory in Rok, and has held visiting positions at Peking University, SWIP; Kyushu University; INI, Cambridge; KITP, UCSB; and CEA, France.

 

Diamond’s interests include turbulence, transport, MCF physics, self-organization in plasmas and fluids, complex fluids and biophysics. He has trained 36 Ph.D. students and a similar number of postdocs. He is active in the magnetic confinement fusion program.