Everett, Washington-based fusion energy startup Helion announced Friday that it has hit an important milestone in its quest for fusion power. The plasmas inside the company’s Polaris prototype reactor have reached 150 million degrees Celsius, three-quarters of the way to what the company thinks is necessary to operate a commercial fusion power plant.

“We’re obviously excited to be in this space,” David Kirtley, co-founder and CEO of Helion, told TechCrunch.

Polaris also operates with deuterium-tritium fuel — a mixture of two isotopes of hydrogen — which Kirtley said makes Helion the first fusion company to do so. “We found that the fusion power output increases significantly as expected in the form of heat,” he said.

The startup is locked in a race with several other companies seeking to commercialize fusion power, which could be an unlimited source of clean energy.

That potential has investors rushing to bet on the technology. This week, Inertia Enterprises announced a $450 million Series A round that includes Bessemer and GV. In January, Type One Energy told TechCrunch that it was in in between raising $250 millionwhile last summer Commonwealth Fusion Systems was raised $863 million from investors including Google and Nvidia. Helion himself stepped up $425 million last year from a group that included Sam Altman, Mithril, Lightspeed, and SoftBank.

While most other fusion startups aim for the early 2030s to put electricity on the grid, Helion has a contract with Microsoft to sell this electricity starting in 2028, although that power will come from a larger commercial reactor called Orion that the company is currently building, not Polaris.

Each fusion startup has its own milestones based on its reactor design. Commonwealth Fusion Systems, for example, must heat its plasmas to more than 100 million degrees C inside its tokamak, a doughnut-shaped device that uses powerful magnets to contain the plasma.

The Helion reactor is different, requiring plasmas about twice as hot to function as intended.

The company’s reactor design is a so-called field-reversed configuration. The inner chamber looks like an hourglass, and at the wide ends, the fuel is injected and forms plasmas. The magnets then accelerate the plasmas toward each other. When they first come together, they are at 10 million to 20 million degrees C. Powerful magnets then press on the fused ball, raising the temperature to 150 million degrees C. All this happens in less than a millisecond.

Instead of extracting energy from fusion reactions in the form of heat, Helion uses its own magnetic field in the fusion reaction to generate electricity. Each pulse pushes back against the reactor’s own magnets, triggering an electric current that can be drawn. By harvesting electricity directly from fusion reactions, the company hopes to be more efficient than its competitors.

Last year, Kirtley said Helion refined some of the reactor’s circuits to increase the amount of electricity they recovered.

While the company uses deuterium-tritium fuel now, down the road it plans to use deuterium-helium-3. Most fusion companies plan to use deuterium-tritium and capture the energy as heat. Helion’s fuel of choice, deuterium-helium-3, produces a large number of charged particles, which strongly push against the magnetic fields that confine the plasma, making it better suited for Helion’s method of direct electricity generation.

Helion’s ultimate goal is to create plasmas that hit 200 million degrees C, higher than other companies’ targets, a function of its reactor design and fuel choice. “We believe that at 200 million degrees, that’s where you get to the sweet spot where you want to operate a power plant,” Kirtley said.

When asked if Helion had reached scientific breakeven — the point at which a fusion reaction produces more energy than is needed to start it — Kirtley demurred. “We’re focusing on the electrical piece, which makes the electricity, rather than the pure scientific milestones.”

Helium-3 is common on the Moon, but not on Earth, so Helion must make its own fuel. Initially, it will fuse deuterium nuclei to produce the first batches. In regular operation, while the main source of power is deuterium-helium-3 fusion, some of the reactions are still deuterium-on-deuterium, producing helium-3 that is pure and reused by the company.

Work has begun to refine the fuel cycle. “It was a pleasant surprise because a lot of that technology is easier to do than we expected,” Kirtley said. Helion was able to produce helium-3 “at a very high efficiency in terms of throughput and purity,” he added.

While Helion is currently the only fusion startup that uses helium-3 in its fuel, Kirtley said he thinks other companies will in the future, indicating that he is open to selling it to them. “Some people – when they come and recognize that they want to do this method of direct recovery of electricity and see the efficiency obtained from it – want to use helium-3 fuel as well,” he said.

Along with its Polaris experiments, Helion is also building Orion, a 50-megawatt fusion reactor it needs to fulfill the Microsoft contract “Our ultimate goal is not to build and deliver Polaris,” Kirtley said. “That’s a step on the road to scaled-up power plants.”