The Negative Triangularity Tokamak Path for Fusion Pilot Plants: Experimental Progress and Future Prospects

Abstract: This paper reviews the experimental progress of negative triangularity (NT), a tokamak configuration where the poloidal cross-section is a reversed-D shape compared to the conventional positive triangularity (PT) shape. NT is a promising reactor scenario that addresses the fundamental tension between performance, exhaust, and robustness. NT studies have accelerated globally across these three pillars over the past several years. While tokamak pilot plants are typically designed for the standard PT H-mode regime, this approach faces significant challenges in balancing high core performance with manageable heat and particle exhaust as well as reliable robustness. In contrast, NT plasmas have achieved H-mode-level confinement while remaining robustly free of the deleterious edge localized mode (ELM) instability. Regarding exhaust, NT offers a larger divertor wetted area on the outboard side and demonstrates compatibility with detachment and operation at high core radiation fraction without the constraints of the L-H power threshold, while also exhibiting low core impurity retention. NT operates with high reproducibility over a wide operating space, demonstrated by robust discharge-to-discharge consistency, and has access to plasmas with very high Greenwald fractions and/or low edge safety factors compared to PT H-mode plasmas. Further research is required to answer outstanding questions related to reactor confinement extrapolation, the optimal triangularity for a reactor, and core-edge integration. NT studies in existing and planned tokamaks are increasing, as is interest in possible reactor concepts. The unique physics and engineering advantages of NT offer a robust and simplified foundation for a viable fusion power plant.

• The paper finds that strong negative triangularity can deliver H-mode-level confinement with H98y,2 ≥ 1, betaN above 2.5, and passive ELM suppression without relying on the L-H transition or active ELM control.
• The paper shows that negative triangularity supports high radiated-power fractions of up to 80%, lower impurity accumulation, and broader access to low-q95 and high-density regimes, although narrower heat-flux widths make divertor detachment more difficult.
• The paper identifies reactor-scale uncertainties involving confinement extrapolation under low-torque heating, the optimal balance between core performance and divertor loads, closed-divertor integration, and vertical stability.‘

The negative triangularity (NT) tokamak configuration, in which the poloidal cross-section is a reversed-D shape relative to the conventional positive-triangularity (PT) Dee shape, has re-emerged as a candidate operating regime for fusion pilot plants (FPPs). This review by Thome et al. surveys experimental progress on NT from its origins in the late 1970s through recent multi-machine campaigns, and argues that NT addresses a triad of requirements—performance, exhaust, and robustness—that standard PT H-mode scenarios satisfy only in tension with one another. The central claim is that sufficiently strong NT plasmas achieve H-mode-level confinement while remaining passively and reproducibly free of edge-localized modes (ELMs), without requiring the L-H power threshold, specialized wall conditioning, or active ELM control systems.

Motivation: the reactor scenario triad

A viable tokamak reactor must simultaneously deliver high plasma performance, manageable heat and particle exhaust, and reliable, reproducible operation at high capacity factor. The paper frames PT H-mode—the planned baseline for most FPP designs—as favoring performance at the expense of the other two pillars. H-mode raises the baseline exhaust burden because the L-H threshold power scales with density, field, and plasma surface area, all of which increase in a reactor; it introduces ELMs whose projected damage scales severely with device size; and its edge transport barrier lengthens impurity confinement times, promoting core impurity accumulation. On the robustness pillar, the authors document the "trophy discharge" phenomenon with a DIII-D example: among ten discharges from one half run day with equivalent applied power, only one reproduced a sustained $\beta_N \approx 4$ trajectory, with the population distribution showing a mean $\beta_N$ of 3.16 and a large standard deviation of 0.54. Moderate-to-high performance is reproducible; peak performance is not.

Against this backdrop, the paper's comparison table asserts that NT achieves sufficient confinement, sidesteps the L-H threshold entirely, is robustly ELM-free, has adequate impurity flushing, and offers a wider operational space than PT H-mode.

Foundations of NT physics

NT was first explored on PDX and Tokapole II around 1980, but abandoned because MHD theory predicted lower Mercier/ballooning stability limits for shapes lacking a deep magnetic well, and because H-mode was discovered concurrently in PT configurations. The revival came from TCV, whose 16 independently powered poloidal coils permit strong shaping. TCV found that electron energy confinement improves systematically as triangularity becomes more negative, particularly at low collisionality, and that this correlates with reduced density fluctuation amplitudes across $\rho = 0.5$–0.9. The mechanism identified is trapped electron mode (TEM) stabilization: in NT geometry, trapped electrons spend more time in the favorable-curvature inboard region, lowering TEM growth rates. Gyrokinetic work further suggests ion temperature gradient (ITG) modes may also be stabilized by NT shaping.

DIII-D then provided the decisive counterexample to early pessimism about MHD stability: inner-wall-limited plasmas at $\delta = -0.4$ with roughly 10 MW of auxiliary heating simultaneously achieved $\beta_N \approx 2.5$ and $H_{98y,2} = 1.2$ with no ELMs. Multi-machine experiments subsequently established that H-mode access requires exceeding a critical triangularity ($\delta > \delta_{crit}$); below that threshold, second-stability ballooning access is closed and the pedestal is clamped by the first-stability ideal-ballooning boundary, precluding ELMs.

Recent advances across the three pillars

Performance

Both DIII-D and TCV robustly achieve $H_{98y,2} \gtrsim 1$ in strong NT across a wide range of $q_{95}$, with DIII-D routinely accessing $\beta_N > 2.5$. Confinement improves monotonically with stronger NT on both devices; TCV scans show the effect is governed primarily by the triangularity of the non-X-point half of the plasma. AUG, MAST-U, and JET, constrained by their vacuum vessels to weaker NT ($\beta_N$0 to $\beta_N$1), mostly find $\beta_N$2, though with limited runtime. Notably, JET remained in L-mode despite up to 32 MW of injected heating.

Reactor extrapolation remains the weakest link. DIII-D data are broadly consistent with the $\beta_N$3 scaling but show stronger-than-expected dependence on plasma current and significant power degradation, plus a degradation with Greenwald fraction. DIII-D–TCV similarity experiments indicate confinement scaling between Bohm and gyro-Bohm, inconsistent with the gyro-Bohm basis of the $\beta_N$4 scaling—though over a narrow $\beta_N$5 range. Rotation studies show confinement loss at low torque at low $\beta_N$6 but not at higher $\beta_N$7. Fast-ion confinement appears comparable or better than PT, with modeling suggesting reduced Alfvén eigenmode drive.

Exhaust

ELM avoidance in NT is presented as geometrically guaranteed rather than tuned: across the entire 2023 DIII-D campaign, no ELM was triggered at $\beta_N$8 even with up to ten times the predicted L-H threshold power, despite deliberate attempts. The physical basis—closure of second-stability ballooning access—is consistent across TCV, DIII-D, AUG, and JET, though MAST-U shows small ELMs persist after second stability closes, indicating additional physics at low aspect ratio not captured by standard models.

Steady-state exhaust presents a genuine trade-off. The heat flux width $\beta_N$9 narrows with stronger NT on both TCV and DIII-D, approaching H-mode-like values rather than L-mode scalings—an unfavorable trend for divertor heat loads. Correspondingly, detachment requires higher upstream densities than in PT (typically above 90% of the Greenwald fraction on DIII-D), attributable to shorter parallel connection lengths and reduced radial transport. Fluid codes reproduce these detachment dynamics, supporting extrapolation confidence. Radiative mantle operation with neon, argon, and krypton seeding raised core radiated fractions from 0.2 to 0.45–0.55 and total fractions to 0.8 without an L-H threshold constraint—a capability unavailable in PT H-mode. Impurity behavior is favorable: DIII-D NT plasmas show $\rho = 0.5$0 and $\rho = 0.5$1 below PT H-mode values, indicating reduced accumulation risk.

Robustness

The strongest quantitative contrast in the paper comes from the matched half-run-day analysis. A DIII-D NT session targeting moderate $\rho = 0.5$2 yielded a mean $\rho = 0.5$3 of 2.79 with a standard deviation of only 0.24, versus 3.16 ± 0.54 for the PT session—half the scatter at comparable mean performance, with the NT probability density forming a tightly localized peak rather than the broad, flat PT distribution. The authors acknowledge caveats: the NT session had fewer usable discharges (4 vs. 10) because it lacked an established reference discharge, and the PT session started from a known high-performance shot. Beyond reproducibility, NT accesses $\rho = 0.5$4 and Greenwald fractions up to approximately 2 non-disruptively using gas puffing alone—regimes typically inaccessible or disruptive in PT H-mode. No major differences in disruptivity between NT and PT have been observed to date, though disruption characterization remains incomplete.

Limitations and open questions

The paper is explicit about three outstanding questions. First, reactor confinement extrapolation is under-constrained: existing data come predominantly from NBI-heated, rotating plasmas on carbon-walled machines, whereas reactors will be RF-heated, low-torque, metal-walled devices; the $\rho = 0.5$5 scaling itself is uncertain. Second, the optimal reactor triangularity is unresolved because core confinement improves with stronger NT while $\rho = 0.5$6 simultaneously narrows, worsening divertor conditions. Third, core-edge integration has been studied only in open, unpumped divertors; whether a closed pumped divertor can enable detachment at lower densities while preserving confinement is untested. Vertically, NT equilibria are intrinsically less stable than PT, though passive conducting plates are predicted to recover PT-comparable growth rates. The authors also note that most NT-capable machines were natively designed for PT, making high-power NT development an engineering challenge, and that DIII-D is currently the only facility able to explore NT at high power in the near term.

Path to a pilot plant

The engineering case rests on simplification: passive ELM freedom eliminates resonant magnetic perturbation coils and pellet-based ELM control; absence of an L-H threshold permits very high core radiation fractions without back-transition risk; and the outboard X-point provides a larger wetted area ($\rho = 0.5$7) and more accessible divertor. Design activity spans the DTT facility's NT option study, SPARC equilibrium assessments, and student-led concepts including MANTA (a pulsed, radiative, high-field design projecting 450 MW fusion power with only 23.5 MW to the scrape-off layer and 90 MW net electric) and CENTAUR, alongside private-sector interest from Startorus Fusion and Firefly Fusion.

Conclusion

This review consolidates a decade of accelerating NT experimentation into a coherent reactor argument: strong NT delivers H-mode-grade confinement and beta, geometrically guaranteed ELM suppression, high-radiation-fraction operation unconstrained by an L-H threshold, favorable impurity flushing, and demonstrably tighter discharge-to-discharge reproducibility than high-performance PT H-mode. The principal uncertainties—confinement extrapolation under reactor-relevant heating and wall conditions, the core-edge trade-off in optimal shaping, and closed-divertor integration—are well defined and motivate the planned upgrades on DIII-D, TCV, MAST-U, WEST, KSTAR, SMART, and AUG. Whether NT can close those gaps will determine if it displaces PT H-mode as the default pilot plant scenario.