Finite Ion Temperature Effects on the Merging of Current-Carrying ELM Filaments in the edge region of a tokamak

Abstract

Edge-localized-mode (ELM) filaments are crucial for cross-field transport at the tokamak edge; yet, their dynamics are often analyzed using the cold-ion approximation, despite experimental data indicating that Ti~Te . This study employs a normalized three-dimensional fluid model to investigate the influence of finite ion temperature on the dynamics of unidirectional current-carrying ELM-like filaments. We demonstrate that increasing ion temperature substantially alters filament propagation and interaction, resulting in a delay of filament merging despite an increase in total kinetic energy due to a stronger pressure-gradient drive. The examination of single-filament dynamics indicates that finite ion temperature generates asymmetric potential structures, strong poloidal flows, and persistent rotational motion, which channel kinetic energy from radial propagation into vortical dynamics. A comprehensive examination of the ion-to-electron temperature ratio reveals a distinct transition from radially dominated to rotation-dominated behavior as ion temperature increases. These results provide a unified physical explanation for reduced radial transport and delayed merging in the warm-ion domain, emphasizing the necessity of incorporating ion temperature effects in the modeling of ELM filament dynamics and edge plasma transport.

Figures (14)

• Figure 1: Time evolution of the density during the merging of two unidirectional current-carrying filaments for (a-d) the cold-ion case ($\tau=0$) and (e-h) the warm-ion case ($\tau=1.0$). Finite ion temperature leads to strong deformation and rotational motion of the density structures, delaying their direct merging.
• Figure 2: Time evolution of the electrostatic potential during filament merging for (a-d) the cold-ion case ($\tau=0$) and (e-h) the warm-ion case ($\tau=1.0$). Warm-ion effects generate asymmetric, dipolar potential structures that drive strong poloidal $E\times B$ flows and coherent rotation, in contrast to the predominantly radial flows in the cold-ion regime.
• Figure 3: Time evolution of the separation distance between the centers of two unidirectional current-carrying filaments for the cold-ion ($\tau=0$) and warm-ion ($\tau=1.0$) cases. Finite ion temperature leads to a significantly slower reduction of separation, indicating delayed merging due to enhanced poloidal and rotational dynamics.
• Figure 4: Time evolution of (left) peak vorticity $|\omega|_{\max}$, (middle) total circulation $\int |\omega|,dA$, and (right) shear rate $|\partial v_y/\partial x|$ for two interacting current-carrying filaments in the cold-ion ($\tau=0.0$) and warm-ion ($\tau=1.0$) regimes. The warm-ion case exhibits significantly enhanced vorticity generation, leading to a strong increase in circulation and shear at later times. This indicates a transition from translation-dominated dynamics in the cold-ion regime to rotation- and shear-dominated dynamics in the warm-ion regime, which redistributes kinetic energy into vortical motion and delays filament merging.
• Figure 5: Time evolution of the density for an isolated filament in (a-d) the cold-ion case ($\tau=0$) and (e-h) the warm-ion case ($\tau=1.0$). In the cold-ion regime, the filament remains compact and approximately axisymmetric while propagating predominantly in the radial direction. In contrast, the warm-ion filament develops deformation, elongation, and tilting of the density contours, indicating the rotational dynamics driven by finite ion temperature effects.