Two études on electromagnetic turbulence in idealised fusion plasmas

A seminar by Alex Shekochihin

Abstract: Schemes involving higher-beta fusion devices have led to intensified interest in electro-magnetic turbulence in fusion plasmas. I will discuss this topic in two radically simplified settings: ITG-driven turbulence in a Z-pinch and interchange-driven turbulence, again in a Z-pinch or in an open-field-line system. Both can be described by minimal models involving just a few “fluid” fields: density and temperature plus velocity and magnetic perturbations perpendicular to the mean magnetic field. Despite a drastic reduction in complexity that many seasoned fusion practitioners might consider excessive and indeed deeply immoral, these simple paradigms yield a wealth of interesting behaviour, some of which seems to capture key physical signatures of electromagnetic effects reported by more realistic numerical explorations — and might perhaps inform future such explorations (failing that, it is interesting enough to pique the curiosity of some theoreticians). Increasing beta in ITG-driven turbulence is shown to move the threshold for the low-to-high-transport (Dimits) transition towards lower temperature gradients. This is because in the struggle between the Reynolds and diamagnetic stresses that determines this transition in an electrostatic plasma [1], the Maxwell stress brought in by the Alfvenic activity associated with higher beta opposes the Reynolds stress and tips the system towards higher-transport states [2]. In the interchange-driven MHD turbulence, increasing beta (which in all such systems is equivalent to increasing the parallel size of the system, the temperature gradient, or the field-line curvature) also pushes the system from an electrostatic low-transport state to an electromagnetic (very)-high-transport one — this is perhaps the simplest conceivable system exhibiting the Dimits transition [3] (simpler even than [1]). It is shown however that when 2D interchange motions are forbidden, the low-beta electrostatic state can be replaced by an electromagnetic one with very high transport and some rather strange properties (while it is interesting theoretically, as well as perhaps in some analogous astrophysical systems, the relevance of this last result to fusion devices is left to post-seminar discussion). Overall, the takeaway message is that electromagnetic turbulence in fusion plasmas is a physics-rich subject that, while challenging practical modellers, should also excite theoreticians.

[1] P. Ivanov et al. JPP 86, 855860502 (2020) & JPP 88, 905880506 (2022)
[2] Y. Zhang et al., in preparation (2025)
[3] W. Clarke et al., in preparation (2025)

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