Achievement of Reactor-Relevant Performance in Negative Triangularity Shape in the DIII-D Tokamak

Plasma discharges with a negative triangularity ($\delta =-0.4$) shape have been created in the DIII-D tokamak with a significant normalized beta (${\beta }_N=2.7$) and confinement characteristic of the high confinement mode ($H_{98y2}=1.2$) despite the absence of an edge pressure pedestal and no edge localized modes (ELMs). These inner-wall-limited plasmas have a similar global performance as a positive triangularity ($\delta =+0.4$) ELMing $H$-mode discharge with the same plasma current, elongation and cross sectional area. For cases both of dominant electron cyclotron heating with $T_e/T_i>1$ and dominant neutral beam injection heating with $T_e/T_i=1$, turbulent fluctuations over radii $0.5<\rho <0.9$ were reduced by 10–50% in the negative triangularity shape compared to the matching positive triangularity shape, depending on the radius and conditions.

Figure 1

Cross sections of the DIII-D vessel with flux plots of the (a) negative triangularity ($-\delta$) shape and (b) matching positive triangularity ($+\delta$) shape for the DIII-D experiments with $\delta =\pm 4$ and elongation $\kappa =1.3$.

Figure 2

Time history of a negative triangularity discharge with up to 9.2 MW of NBI and 2.3 MW ECH showing high beta and confinement in the high power phase.

Figure 3

Total stored energy vs coupled input power for negative triangularity discharges (triangles) and predicted stored energy from $H_{89P}$ scaling (pluses). All discharges had an $L$-mode edge.

Figure 4

Comparison of fluctuation levels between matched negative (blue) and positive (red) triangularity shots for (a) PCI diagnostic measuring the spectrum of ${\tilde{n}}_e$ and (b) CECE diagnostic measuring the $T_e$ fluctuations profile.

Figure 5

Time history of matched negative (blue) and positive (red) triangularity discharges with dominant NBI heating. The heating was identical for the two shots up until 2.2 s. The $+\delta$ discharge went into $H$ mode at 1.4 s; the $-\delta$ discharge retained an $L$-mode edge throughout.

Figure 6

Comparison of a local measurement of ${\tilde{n}}_e$ from the BES diagnostic at $\rho =0.7$ for a negative triangularity $L$-mode discharge (blue) and a positive triangularity $H$-mode discharge (red), both with 6 MW of NBI and 3 MW of ECH.

Figure 7

Profiles of $n_e$, $T_e$, $T_i$ and impurity rotation for negative (blue) and positive (red) triangularity discharges. The profiles are an average of 23 time slice fits in the phase 1.5–2.5 s, with constant heating power of 6 MW NBI and 3 MW ECH. Typical data points for one time slice in negative triangularity are shown (gray with error bars).