The U.S. facility testing negative-triangularity plasma is General Atomics’ DIII-D National Fusion Facility, a flexible tokamak research machine in San Diego. In a four-week campaign during January and February 2023, researchers installed armor tiles that protected the vessel and formed a temporary divertor because the inverted, “backward-D” plasma sent heat exhaust outside DIII-D’s conventional divertor. The campaign produced 178 run hours with scientists from 15 institutions.
What DIII-D is testing
DIII-D is not a commercial power reactor; it is a U.S. magnetic-confinement research tokamak used to test plasma configurations, control systems and exhaust designs. Most tokamaks operate with positive triangularity, giving the plasma a familiar D-shaped cross-section. Negative triangularity mirrors that outline, producing a backward-D shape.
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The attraction is that changing the plasma cross-section can improve edge stability without giving up the high core performance needed for a future burning-plasma plant. The U.S. Department of Energy describes negative triangularity as a configuration that can limit edge-localized modes (ELMs), violent edge events that can damage reactor-facing materials.
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Positive triangularity and the installed divertor
In a conventional discharge, magnetic field lines guide particles and heat toward a divertor positioned for the machine’s positive-triangularity geometry. The divertor is a deliberately sacrificial, heavily armored region: it intercepts exhaust before that energy reaches the rest of the first wall.
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Negative triangularity moves the exhaust footprint
When the triangularity is made negative, the plasma boundary is inverted. That changes where the magnetic field lines carrying exhaust intersect the vessel. General Atomics reported that the resulting heat load can fall outside DIII-D’s permanent divertor. Simply changing the plasma shape without protecting the newly exposed wall would risk overheating components.
What “custom exhaust” means at DIII-D
For the 2023 campaign, researchers added specially arranged armor tiles. The tiles served two purposes: they shielded hardware from the displaced heat and created a temporary divertor region in the location required by the negative-triangularity configuration. This is a machine-specific experimental solution, not a commercially available add-on.
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What the 2023 campaign achieved
The campaign ran for four weeks in January and February 2023 and generated 178 run hours, according to General Atomics’ 2023 account. That was 22% of the 800 run hours DIII-D had planned for that calendar year, and the work involved researchers from 15 institutions.
The run demonstrated that DIII-D could operate negative-triangularity plasmas while testing how the relocated exhaust interacted with the temporary armored region. It did not establish a finished divertor design for a power plant; its purpose was to obtain operating data that can guide one.
Does negative triangularity reduce ELMs and heat loads?
Evidence from DIII-D indicates that negative triangularity can suppress the edge instabilities associated with ELMs. A peer-reviewed DIII-D study published in Physical Review Letters in 2019 reported discharges with triangularity delta = -0.4, normalized beta beta_N = 2.7 and H98y2 = 1.2. Those specific discharges had no edge pressure pedestal and no ELMs.
That result is encouraging but should not be read as a guarantee that every negative-triangularity discharge will be ELM-free or that wall heat loads disappear. The configuration changes where exhaust lands; it does not remove the need to intercept and spread that power. The engineering problem becomes designing the divertor and first wall around the new footprint.
DOE summarized the physics result as follows: “This research showed that negative triangularity plasmas are free of potentially damaging instabilities in the edge region of the plasma without sacrificing fusion performance.” The statement describes the reported experimental regime, not a certification of an integrated power-plant design.
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Positive versus negative triangularity
| Characteristic | Positive triangularity | Negative triangularity |
|---|---|---|
| Plasma cross-section | Conventional D-like outline | Inverted or backward-D outline |
| Edge behavior | Can exhibit ELMs depending on operating conditions | Reported DIII-D discharges had no ELMs and no edge pressure pedestal |
| Core performance | Established operating basis for conventional tokamak studies | 2019 DIII-D results reported beta_N = 2.7 and H98y2 = 1.2 at delta = -0.4 |
| Heat-exhaust location | Aligned with the standard installed divertor | Can move outside a conventional divertor |
| Divertor and first-wall engineering | Uses hardware designed for the standard geometry | Requires armor and divertor geometry designed for the displaced footprint |
| Control requirements | Based on established DIII-D shape-control arrangements | Requires expanded shape-control algorithms, hardware, power-supply pairings and grounding studies |
| Pilot-plant readiness | Benefits from a mature conventional design path | Still requires integrated control, turbulence and detached-divertor validation |
What DIII-D is doing next
DIII-D has investigated negative triangularity since 2016. Its 2024 control study sought to enlarge the usable operating space with improved algorithms, additional hardware, new power-supply pairings and grounding changes.
The FY2024 work included scans at higher plasma current, different X-point locations and faster shape-control response. Researchers also ran matched positive- and negative-triangularity turbulence experiments, helping separate effects caused by the shape from effects caused by other operating parameters.
Another part of the program measured divertor detachment. Detachment reduces the power reaching material surfaces by radiating and redistributing energy before it strikes the target. Those measurements are intended to inform a dedicated negative-triangularity divertor rather than rely indefinitely on DIII-D’s temporary armor arrangement.
What this means for a future fusion plant
Negative triangularity offers a potentially valuable combination: strong core confinement and performance with fewer damaging edge events. Its cost is architectural. A plant designed around the configuration would need the plasma shape, magnetic coils, control system, first wall and divertor laid out together from the beginning.
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DIII-D’s campaign therefore answers a practical question rather than ending the design debate: researchers can create and study the required exhaust region on an existing tokamak. The remaining work is to show that the shape can be controlled across a broad operating range and that a purpose-built divertor can survive sustained, reactor-relevant power.
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