Abstract The Type-I edge localized mode (ELM) crash dynamics is investigated for the MAST Upgrade (MAST-U) spherical tokamak plasma through nonlinear two-fluid BOUT++ simulations. Contrary to conventional peeling-ballooning paradigms, in this work the ELM crashes in MAST-U are found to be triggered by resistive ballooning modes with growth rates scaling as γ∝η1/3, accompanied by formation of n=20–25 (n is the toroidal mode number) filamentary structures during nonlinear evolution. Parametric scans of the plasma resistivity (η) reveal an inverse proportionality between the ELM size and resistivity, aligning with prior MAST experimental observations but contrary to the classical resistive ballooning theory. Linear two-fluid simulations corroborate this anomalous scaling, while analysis of the numerical data identifies a critical stabilizing mechanism: electromagnetic-to-thermal energy exchange reduces the ELM size at high resistivity. Furthermore, nonlinear simulations demonstrate that the bootstrap current—though inactive in linear instability drives—amplifies pedestal collapse in the MAST-U scenario.&amp;#xD; 

Nonlinear Two-Fluid Simulations of Type-I Edge Localized Mode Crashes in MAST Upgrade

Abstract In the H-mode experiments conducted on the Experimental Advanced Superconducting Tokamak (EAST), fluctuations induced by the so-called edge localized modes (ELMs) are captured by a high-speed vacuum ultraviolet (VUV) imaging system. Clear field line-aligned filamentary structures are analyzed in this work. Ion transport induced by ELM filaments in the SOL under different discharge conditions is analyzed by comparing the VUV signals with the divertor probe signals. It is found that convective transport along open field lines towards the divertor target dominates the parallel ion particle transport mechanism during ELMs. The toroidal mode number of the filamentary structure derived from the VUV images increases with the electron density pedestal height. The analysis of the toroidal distribution characteristics during ELM bursts reveals toroidal asymmetry. The influence of resonance magnetic perturbation (RMP) on the ELM size is also analyzed using VUV imaging data. When the phase difference of the coil changes periodically, the widths of the filaments change as well. Additionally, the temporal evolution of the ELMs on the VUV signals provides rise time and decay time for each single ELM event, and the results indicate a negative correlation trend between these two times. 

VUV imaging of type-Ⅰ ELM filamentary structures and their temporal characteristics on EAST

Previous experimental results show that the poloidal mode spacing of the filamentary structures increases and the dominant toroidal mode number decreases in the edgelocalized mode (ELM) rising phase with increasing plasma current. In addition, the experimental results in this paper show that the energy loss ratio of the pedestal (ΔW/Wped) decreases as the edge safety factor (q95) increases. The BOUT++ three-field two-fluid model can reproduce the experimental results and provide a possible explanation mechanism. The pedestal density plays an important role in the characteristics of filamentary structures as the current ramps up. On the one hand, the resistivity related to the pedestal density drives the instability of the peeling–ballooning mode, and the resistive effect is stronger in the high current case, making the dominant toroidal mode number lower and the corresponding poloidal mode spacing wider in the high current case. A low q95 corresponds to a high pedestal collision rate and a high pedestal energy loss ratio. On the other hand, the ELM crash process is dominated by resistivity, so the ratio of pedestal energy loss caused by ELM is not inversely proportional to the pedestal collision rate.

Experimental and simulation studies on ELM characteristics under a plasma current ramping up discharge on EAST

A time-dependent nonlinear simulation was performed using the BOUT++ 3-field model to study the spatial characteristics of the filamentary structure observed in the EAST H-mode plasmas. The simulation results were qualitatively compared with the experimental images captured by the vacuum ultraviolet imaging system. During the nonlinear evolution period, the poloidal mode spacing increases and the dominant toroidal mode number of the edge localized mode (ELM) decreases with time. In addition, no significant change is observed in the pitch angle. Both the simulation results and experimental measurements show the same time dependence. In addition, it is found that the dominant toroidal mode number during the ELM crash does not decrease continuously with time in the experimental measurements, i.e., the dominant toroidal mode number is higher than that of the previous frame or is lower than that of the next frame during the ELM crash, which can be explained by the hypothesis of nonlinear mode coupling since it can be reproduced by using the output considering the nonlinear mode coupling effect.

Comparison study of nonlinear simulations and VUVI measurements of filamentary structures during an ELM crash

The poloidal width of the filaments induced by the type-I edge localized mode has power dependence in EAST. The poloidal widths of the filaments observed by the high-speed vacuum ultraviolet (VUV) imaging system are proportional to the heating power and the ELM size. To understand this power dependence, the BOUT++ nonlinear simulations have been performed with the reconstructed equilibriums from the experimental measurements in this paper. The synthetic filament structures from BOUT++ nonlinear simulation match the experimental observations by the VUV imaging system. The BOUT++ nonlinear simulations also reproduce the power dependence of the filament widths and the ELM size. The filament width and the ELM size are inversely proportional to the toroidal mode number. The low-n mode has a broader radial and poloidal structure, which causes the larger filament width and ELM size. In the high input power case, the mode spectrum shifts to low-n, a result of increasing peeling drive. Besides, we found the β p in a higher input power case leads to a broader pedestal, expanding the radial mode structure of the peeling-ballooning mode.

Study on filament width of type-I ELM in EAST using VUV imaging system and simulation

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Comparison study of nonlinear simulations and VUVI measurements of filamentary structures during an ELM crash

Experimental and simulation studies on ELM characteristics under a plasma current ramping up discharge on EAST

Study on filament width of type-I ELM in EAST using VUV imaging system and simulation