Robust Avoidance of Edge-Localized Modes alongside Gradient Formation in the Negative Triangularity Tokamak Edge

In a series of high performance diverted discharges on DIII-D, we demonstrate that strong negative triangularity (NT) shaping robustly suppresses all edge-localized mode (ELM) activity over a wide range of plasma conditions: $⟨n⟩=0.1–1.5\times {10}^{20}{\mathrm{m}}^{-3}$, $P_{\mathrm{aux}}=0–15\mathrm{MW}$, and $|B_{\mathrm{t}}|=1–2.2\mathrm{T}$, corresponding to $P_{\mathrm{loss}}/P_{\mathrm{LH}08}\sim 8$. The full dataset is consistent with the theoretical prediction that magnetic shear in the NT edge inhibits access to ELMing $H$-mode regimes; all experimental pressure profiles are found to be at or below the infinite-$n$ ballooning stability limit. Our present dataset also features edge pressure gradients in strong NT that are closer to an $H$-mode than a typical $L$-mode plasma, supporting the consideration of NT for reactor design.

Figure 1

Calculation of the infinite-$n$ ballooning (dashed line) and peeling-ballooning (blue-red) limits for a typical high-performance NT plasma on DIII-D.

Figure 2

As the triangularity (a) is varied at constant input power (b), the plasma transitions smoothly from an ELM-free state (blue) to an ELMing $H$-mode regime (red) through an oscillatory transition (yellow). Measurements of the $D_{\alpha }$ line emission (c) and divertor heat flux (d) demonstrate that peak power incident on the machine walls is strongly reduced during operation at strong NT. (e) Inspection of high-frequency magnetic fluctuations during this time reveals enhanced turbulent activity during the ELM-suppressed periods.

Figure 3

Within experimental error bars, DIII-D discharges with strong NT ($\delta <-0.3$, blues) are limited by the first ballooning stability limit. For comparison, a selection of ELMy $H$-modes with weaker $\delta$ and access to the second stability region are shown in red. Inset: profiles (all with $P_{\mathrm{aux}}\sim 8\mathrm{MW}$) are compared for the representative NT ELM-free, weak NT $H$-mode and PT $L$ modes.

Figure 4

(a) For the entire NT dataset on DIII-D, the $H$-mode threshold power fraction $f_{\mathrm{LH}}$ is shown as a function of $\delta$. ELMy $H$-modes are colored in red, dithering periods in yellow, and ELM-free periods in blue. (b) The volume-averaged density $⟨n⟩$ and separatrix power $P_{\mathrm{loss}}$ reveal the breadth of the ELM-free space. (c) The edge pressure (at $\rho =0.8$) and (d) ${\beta }_{\mathrm{N}}$ show no degradation at strong $\delta <0$, despite the suppression of ELMs.

Figure 5

The volume-averaged pressure $⟨p⟩$ and pedestal pressure $p_{\mathrm{ped}}$ for ELM-free NT discharges (blue), PT RMP ELM-suppressed $H$-modes (magenta), and PT QH modes (black) on DIII-D are compared. ELMy $H$-mode (red) and dithering regimes (yellow) at weak NT are also shown.

Figure 6

For the entire NT dataset on DIII-D, (a) the edge pressure and (b) the volume-averaged pressure are plotted against auxiliary heating power.