Almost
Limitless?
“The dream of almost limitless clean energy from nuclear fusion is close to being realized.”
The question: is Helion’s D-He3 fuel cycle more sustainable long-term than the D-T tokamak’s, in terms of fuel supply?
Big milestones.
Bigger engineering problem.
Fusion fans have always said limitless clean energy is “right around the corner”. Recent records are real steps forward, but they only show that fusion works at small scale. Turning it into commercial power is the real challenge.
of fusion energy from one pulse in the JET tokamak.
plasma temperature reached by Helion’s private fusion machine (GeekWire, 2026).
Follow the logic
“Almost limitless” means the fuel must not run out
If the fuel runs out, the energy can’t be limitless. So the fuel supply decides whether the claim is true.
Deuterium is the easy part
Both designs use deuterium, which is plentiful in seawater. It isn’t the bottleneck.
So it comes down to what deuterium fuses with
Tokamaks (ITER) fuse it with tritium, which is scarce and radioactive. Helion fuses it with helium-3, which is also scarce on Earth.
Fuel supply decides how many plants, and how soon
Choosing the fuel with a reliable long-term supply is central to commercialising fusion. Helion is already building a real plant (Orion) and aims to supply Microsoft by 2028.
Smash some nuclei.
Fusion squeezes two light nuclei together until they merge, releasing a huge amount of energy. Choose a reaction and fire it. Then switch on the magnetic field and watch what happens to the neutral neutrons compared with the charged particles.
A donut vs. a plasma cannon.
This report compares two very different designs: ITER’s tokamak, which holds a steady ring of 150-million-°C plasma in magnetic fields, and Helion’s pulsed machine, which fires two plasma rings into each other, compresses them, and takes electricity straight from the expanding plasma.
Follow the fuel.
Each glowing dot is fuel moving through the cycle. Click any box to see what happens there. Look out for the red boxes: they mark the places where each cycle is weak.
D-T tokamak
Helion D-He3
The plot twist: the same atom, two fates
Tritium decays into helium-3 with a 12.3-year half-life. For a D-T plant, that decay is fuel being lost (about 5.5% every year). For Helion, the very same decay is a slow way of making fuel. The weakness of one cycle is a (slow) supply route for the other.
The world’s tritium fits in a jar.
Almost all of the world’s tritium comes from Canadian (CANDU) reactors. Abdou et al. (2020) modelled what happens when those reactors retire and ITER starts burning 0.9 kg per year for 16 years, while every gram decays at 5.5% per year. Drag through time.
Tritium inventory (kg)
How many power plants could the world start?
Abdou: a 3 GW fusion plant burns about 0.46 kg of tritium per day and needs a start-up stockpile of under 5 kg (good case) up to 11 kg (poor case). Start-up inventory scales linearly with fusion power.
What the graph shows
The inventory peaks around 2027, then falls once the CANDU reactors retire (2027–2030). When ITER starts burning, the red line drops steeply to about 5 kg in the early 2050s, just when DEMO and the first commercial plants were expected to start. In the poor case, that isn’t enough to start even one plant.
“There is a linear dependency between the tritium start-up inventory and the fusion power.” (Abdou et al., 2020)
Catch the energy, or chase it?
80% of D-T’s energy comes out as a 14.1 MeV neutron. Neutrons have no charge, so magnets can’t catch them. They slam into the reactor walls and make them radioactive, and their energy can only be used as heat. D-He3 puts its energy into charged particles (a 14.7 MeV proton and a 3.6 MeV alpha), which magnetic fields can grab directly.
From fusion energy to electricity
*Helion’s figure is self-reported (Kirtley & Milroy, 2023). The fuel numbers are simple estimates: they assume every fuel atom fuses and use the report’s 0.46 kg/day per 3 GW for D-T.
Turn up the heat.
The reactivity ⟨σv⟩ measures how fast a fuel fuses at a given temperature: higher means more reactions per second. Plasma physicists measure temperature in keV, where 1 keV ≈ 11.6 million °C. Slide the temperature and watch the plasma: pink flashes are D-T fusions and cyan flashes are D-He3 fusions.
Reactivity vs temperature
From the graph: D-T reacts fastest at about 80 keV and D-He3 at about 250 keV, roughly 3.2× hotter, and even at its peak D-He3 reacts more slowly. Helion’s engineers say real D-He3 machines need at least 10–20 keV, while D-T reaches net gain at 3–10 keV.
“For a high-beta condition, D–T breakeven is possible above 3 keV, with Qsci > 10 and likely ignition at 10 keV. D–D fusion, even for high-beta conditions, does not break even.” (Kirtley & Milroy, 2023)
Polaris reached 150 million °C (≈13 keV) in 2026 with measurable D-T fusion. That is inside the 10–20 keV window, but it doesn’t prove that continuous, pulsed D-He3 power works yet.
Put it on the scale.
Click each piece of evidence to drop it on the side it supports. The number shows how much weight the report gives it. Which fuel cycle is more sustainable long-term?
Weights reflect how strongly the report leans on each point. Fuel supply problems weigh most, because that is what the research question asks about.
D-He3 is more sustainable long-term. Conditionally.
D-T’s fatal problem is supply: there isn’t enough tritium to start commercial plants. About 5 kg is projected for the 2050s, against up to 11 kg needed per plant, and it decays 5.5% a year whether it’s used or not. Helion’s D-He3 avoids tritium breeding, lithium blankets and beryllium, and could burn less fuel per unit of electricity. But that lean only holds if:
10–20 keV, with continuous pulsed D-He3 operation, which hasn’t been proven yet.
The key source is co-authored by Helion’s CEO, and little independent data has been published.
Judge the sources.
Abdou et al. (2020) · Nuclear Fusion (IAEA)
RELIABILITYKirtley & Milroy (2023) · J. Fusion Energy
RELIABILITYImprovements
Bring in more perspectives, especially physicists who critique Helion’s claims. Their counter-arguments would test Helion’s self-reported numbers and either confirm or weaken this investigation’s conditional lean towards D-He3.
Extensions
Compare the radioactive by-products fairly. D-T neutrons activate reactor materials, while D-He3 makes far fewer neutrons. Estimating the long-term effect would show which fuel is closer to being truly “clean”, which is the other half of the original claim.
Almost limitless? Not yet.
D-T’s tritium can’t support a commercial fleet without big improvements in breeding and processing. D-He3 is the more sustainable fuel cycle on paper, but it still has to show that it works.
- Abdou, M., Riva, M., Ying, A., Day, C., Loarte, A., Baylor, L. R., Humrickhouse, P., Fuerst, T. F., & Cho, S. (2020). Physics and technology considerations for the deuterium–tritium fuel cycle and conditions for tritium fuel self sufficiency. Nuclear Fusion, 61(1), 013001. https://doi.org/10.1088/1741-4326/abbf35
- Kirtley, D., & Milroy, R. (2023). Fundamental scaling of adiabatic compression of field reversed configuration thermonuclear fusion plasmas. Journal of Fusion Energy, 42(2). https://doi.org/10.1007/s10894-023-00367-7
- Kirtley, D. D. (2026, February 13). Helion achieves new industry-first fusion energy milestones, accelerating path to commercial fusion. Helion. helionenergy.com
- Also cited in the report text: GeekWire (2026); Federation of American Scientists (2024); Physics World (2024).