April 4-6

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Author: Menglong Zhao
Requested Type: Poster
Submitted: 2022-03-11 15:09:18

Co-authors: T. Rognlien, B. Zhu, B. Meyer, B. Dudson, X. Xu, N. M. Li

Contact Info:
Lawrence Livermore National Lab
7000 East Ave., Livermore CA 9
Livermore, California   94550
United States

Abstract Text:
The plasma conditions for the existence of a sub-supersonic transition point in a 1D flux tube that mimics the scrape-off layer of a tokamak are studied using the UEDGE transport code that solves the set of Braginskii equations with a fluid neutral model. The steady state simulation results showed that without neutral recycling, the sub-supersonic transition happened around the X-point, similar to the study in [1]. However, with even a low recycling coefficient, the sub-supersonic transition will disappear [2] and ion flows stay subsonic in the entire flux tube and reach the sonic speed at the target. When increasing the upstream
collisionality to a certain level nu_f, a sub-supersonic point appears somewhere Te ~ 5 eV near the target, reaching the target at a supersonic speed. When decreasing the upstream collisionality from nu_f backwards, the solution doesn’t follow the same trajectory. A subsupersonic transition point near the target keeps being present until the upstream collisionality is decreased to nu_b . Therefore, for a certain upstream condition, two branches of steady state solutions are obtained in the simulations when the upstream collisionality is within the range between nu_b and nu_f . One branch (supersonic) of solutions is with the presence of a sub-supersonic transition point near the target while in the other branch (subsonic) ion flows are subsonic in the entire flux tube, reaching the sonic speed at the target. Compared to the subsonic branch, the supersonic branch tends to have a lower target Te and higher total ionization source, which thus leads to a higher collisional divertor condition.

Work supported by the US Department of Energy under DE-AC52-07NA27344 at LLNL.
[1] H. Bufferand et al., Plasma Phys. Control. Fusion 56 (2014) 122001
[2] R. Goswami et al., Phys. Plasmas 21 (2014) 072510