Nonlinear simulations of the peeling-ballooning instability of super H-modes in the HL-3 tokamak

As the newly built tokamak in China, HL-3 will explore high-performance operation scenarios, such as super H-mode. The energy confinement and core parameters in the super H-mode can be much larger than that in the normal H-mode. Based on the pedestal simulation code EPED, the operation space of the super H-mode is obtained in HL-3. Magnetic shear decreases with increasing triangularity; consequently, a super H-mode can be achieved. The threshold of triangularity for accessing a super H-mode in HL-3 is around 0.4. By using BOUT++, a nonlinear simulation study of the pedestal instabilities in the super H-mode equilibrium is executed for the first time. As expected, the low n peeling mode, which can cause much of the energy loss (17%) from the pedestal region, is dominant in the super H-mode. Such a large collapse in the pedestal region would lead to a transition from super H-mode to H-mode. It is crucial to expand the parameter space of the super H-mode or mitigate the edge-localized mode (ELM) size for sustaining the super H-mode operations. The E x B velocity shear is found to play an important role in controlling the ELMs in HL-3. The small E x B velocity shear leads to a large growth rate but results in a small ELM size around the peeling boundary. The ELM size is closely related to both the growth rate of peeling-ballooning mode and the duration time of the linear phase. In contrast, a large E x B velocity shear can stabilize the instabilities near the ballooning boundary. Next, the parameter space of the super H-mode can be enlarged.