Session VI02: MFE: Negative Triangularity

Robust avoidance of edge localized modes alongside pedestal formation in negative triangularity plasmas

High performance plasmas without edge-localized modes (ELMs) are obtained over a wide range of magnetic field, current, pressure, and density in DIII-D plasmas featuring strong negative triangularity (NT) shaping. Detailed magnetohydrodynamic modeling of over 300 NT discharges is consistent with the edge pressure gradient being ultimately limited by high-n ballooning modes, confirming previous predictions [1]. As a result, plasmas with strong enough NT (triangularities δ < δ_crit, where δ_crit ~ -0.18 on DIII-D) remain completely ELM-free, even in high power diverted operation. The formation of an electron temperature pedestal in NT contributes to edge pressure gradients that exceed typical L-mode levels, enabling these ELM-free plasmas to routinely achieve higher normalized performance (simultaneous access to H_98y2 ≳ 1, β_N ≳ 2.5 and f_GW ≳ 1). Detailed fluctuation data from magnetics and turbulence diagnostics reveal a wealth of complex behavior in the NT edge that is associated with both ELM suppression and performance enhancement. NT plasmas on DIII-D are further compared to other established ELM-suppression strategies, showing that NT compares favorably in terms of access criteria, normalized performance and scrape-off-layer collisionality. Finally, since ELM suppression in NT results directly from geometry-induced changes of the magnetic shear, predictive calculations find that an ELM-free edge is sustained even in burning plasma regimes characteristic of reactor operation. This implies that it may be possible to eliminate critical integration issues raised by ELMs and exhaust power handling without compromising reactor performance by exploiting strong NT shaping in fusion power plant designs.

[1] A.O. NELSON, et. al., Nucl. Fusion 62 096020 (2022)   

Confinement Scaling and Rotation Dependence in DIII-D Negative Triangularity Plasmas

While DIII-D negative triangularity (NT) plasmas have energy confinement times (τ_E) similar to positive triangularity (PT) H-mode plasmas, first-of-their-kind experiments for NT have found different scalings with normalized gyroradius (ρ_*) and collisionality (ν_*) and similar scaling with toroidal rotation. NT tokamak plasmas are a potentially transformative reactor scenario as they combine high τ_E with a stable edge (NT-edge) that inhibits H-mode, lacks dangerous edge transients, and has naturally low impurity confinement. Recent experiments in DIII-D used a modified first wall to allow operation of high-power, diverted plasmas with the largest achievable NT. A key goal of this work was determining the NT τ_E scalings as these are needed to ascertain the viability of NT reactor scenarios. Dimensionless parameter scaling experiments holding β, q, etc. constant measured ρ_* scaling that is closer to Bohm than gyro-Bohm, with the thermal transport for ions being closer to Bohm than that for electrons. The scaling with ν_* was minimal. These scaling results, though subject to some limitation due to being in a relative infancy when compared to PT scaling investigations, more closely resemble the less favorable PT L-mode scalings than PT H-mode scalings. As in many PT scenarios, τ_E decreased as injected torque went from all co-current to nearly balanced (~reactor levels). The decrease was 20-40% in q₉₅~2.8 discharges but only 10-20% in q₉₅~4.2 discharges. Even at low rotation though, excellent MHD stability was observed. Also, the favorable, low impurity confinement was maintained at multiple rotation levels. For the NT scenario, these results present challenges to its scalability while also illustrating some of its naturally attractive properties.   

Scrape-Off Layer characterization and detachment integration in Negative Triangularity discharges in DIII-D

Negative triangularity (NT) experiments on DIII-D simultaneously achieved divertor detachment, an ELM-free edge, and tolerance to extrinsic impurities for divertor dissipation, demonstrating the potential for a core-edge integrated scenario. Detachment without impurity seeding was achieved via an increase in plasma density for the first time in NT discharges. Detachment onset conditions in the outer divertor were sustained with H-mode level confinement H_98-y2~1 and normalized pressure levels β_N~2. Energy confinement degradation (~30%) was observed with deeper detachment, which correlated with the loss of the edge electron temperature pedestal and the movement of the radiation front in contact with closed flux surfaces. Parametric dependence of access to detachment on plasma current and injected power was consistent with expectations from detachment scalings. However, densities higher than in positive triangularity (PT) (Greenwald fraction f_GW>0.9), were needed to achieve detachment, due to the shorter parallel connection length and narrowing of the scrape-off layer heat flux width at the strongest NT, approaching values typical of PT H-mode discharges. Impurity seeding with nitrogen and neon reduced the detachment density up to 30% at the expense of core dilution. Detachment onset density was 40% higher with ion B×▽B drift into the divertor compared to out of the divertor in NT (f_GW~1.3 vs. 0.9), while this difference is typically less than 15% in PT. The effect of cross-field drifts on divertor profiles was larger than observed in PT discharges due to the lower toroidal field at the X-point in NT shapes. Edge fluid simulations (UEDGE, SOLPS) quantitatively reproduce the observed drift dependences and high densities needed for detachment, increasing confidence in extrapolation to future NT divertor design.   

Scenario Development and MHD Stability of Negative Triangularity Plasmas in DIII-D

Novel negative triangularity (NT) L-mode scenarios have been developed to study stability and performance as functions of q₉₅, at q₉₅=3 and 4. Furthermore, novel access to a state with very small or no sawteeth and no fishbones was developed in NT L-mode, similar to hybrid scenarios in positive triangularity (PT). This state was previously observed transiently during high performance NT shots (H₉₈=1.15). Cases at both q₉₅s described here encounter m/n=2/1 tearing modes (TMs) at ~90% normalized pressure (b_N) of previously predicted ideal wall limits ~3.1. Establishing a stationary hybrid-similar scenario at q₉₅=4 to study stability required attempting early 3/2 TM onset, observed late in flattop at first. Experiments described here achieve good performance in NT L mode on flattops of up to 5 current relaxation times, τ_R≈0.6 s. This duration is sufficient to study MHD stability and scenario performance in stationary conditions, limited only by hardware availability at 7-10MW NBI, 2.2MW ECH. Confinement at both q₉₅ values reaches standard H-mode levels (H₉₈~0.95) without n=2 tearing modes. At q₉₅=4, n=2 TMs saturate at low amplitude and decrease confinement by ~13%, consistent with PT plasmas at comparable q₉₅. At q₉₅=3, n=2 TMs grow to large amplitude, causing b_N collapses. This behavior is comparable to the impact of n=2 TMs in the q₉₅=4 PT hybrid vs the q₉₅=3 ITER baseline scenario. Projections based on 0.5D rho* scaling shows both cases perform well with high-field reactor parameters akin to the ARC design (B_t=9 T, I_P=7.5-10 MA), yielding 400MW of fusion power, with fusion gain Q~10, b_N~1.5, and low input power P_aux~40 MW. These results mark significant progress toward development of NT fusion reactor scenarios and identify avenues for improvement. The latter include larger, optimized plasma shapes to improve performance, better n=1 error field correction, increased density to reduce fast ion losses, and tradeoffs between TMs and sawtoothing regimes.