Speaker
Description
Plasma turbulence is a dominant factor limiting energy confinement time in toroidal fusion plasmas, making computationally cheap confinement proxies highly desirable for quickly identifying experimentally attractive scenarios. In this presentation, we investigate whether "Available Energy (AE)” - an upper limit on the thermal energy available to drive Trapped Electron Mode (TEM) turbulence in collisionless plasmas - can serve as such a proxy in the Heliotron J device [1], offering the first direct comparison between AE and experiment. Observations from Heliotron J show that energy confinement is significantly influenced by the toroidal mirror ripple (bumpiness), and the rotational transform [2,3]. We have verified if AE, calculated across the entire plasma, exhibits similar dependencies. We have demonstrated that AE varies significantly when altering the bumpiness and rotational transform, and, importantly, exhibits a clear negative correlation with the normalized stored energy ($W_p/V_p$) found in experiment. These findings suggest that AE can serve as an effective proxy of confinement by capturing the characteristics of the TEM-driven turbulence. Next, this AE-based proxy of confinement is applied to different configurations of the Varying Symmetry Torus (VAST), a next-generation advanced helical device at NIFS. We show that the quasi-axisymmetric (QA), quasi-isodynamic (QI), and symmetry-broken (SB1) magnetic fields of VAST can suppress TEM turbulence more effectively than Heliotron J, confirming the benefits of magnetic configuration optimization. Furthermore, to perform a comprehensive comparison between tokamak and helical systems, we will apply the AE simulation to tokamak geometries, aiming to further clarify the relationship between magnetic geometry and turbulence.
[1] R. Mackenbach, J. Proll, et al., Phys. Rev. Lett. 17 (2022) 175001
[2] T. Mizuuchi, et al., Fus. Sci. Technol. 50 (2006) 352
[3] K. Nagasaki, et al., 28th IAEA Fus. Energy Conf., EX/P6-34